Android——ViewPager缓存(预加载)机制及如何禁止预加载

一、来源
android.support.v4.view：这是谷歌官方给我们提供的一个兼容低版本安卓设备的软件包，里面包囊了只有在安卓3.0以上可以使用的api。而viewpager就是其中之一利用它，我们可以做很多事情，从最简单的导航，到页面菜单等等。那如何使用它呢，与ListView类似，我们也需要一个适配器，他就是PagerAdapter。

二、缓存机制
ViewPager切换页面时默认情况下非相邻的页面会被销毁掉(ViewPager默认缓存相邻的页面以便快速切换）。

ViewPager懒加载机制相关源代码：

核心算法：

从上面的源代码来看,不管limit设置什么值,至少会预先加载下一个Fragment。（DEFAULT_OFFSCREEN_PAGES = 1；// 默认会加载下一个fragment，缓存相邻的2个fragment）

通过设置limit来预先加载N个页面，适用于如音乐播放时，自动加载歌词。

三、如何禁止ViewPager预加载机制
某些情况下，涉及到网络访问、数据图片资源量大和布局比较复杂，如果默认使用ViewPager缓存机制，会导致内存消耗太大。Google官方提供的ViewPager无法禁止，只能借助于android开源项目，LazyViewPager 是一个比较好的选择方案，核心代码如下：

以下是LazyViewPager 的完整源码：

/* * Copyright (C) 2011 The Android Open Source Project * * Licensed under the Apache License, Version 2.0 (the "License"); * you may not use this file except in compliance with the License. * You may obtain a copy of the License at * * http://www.apache.org/licenses/LICENSE-2.0 * * Unless required by applicable law or agreed to in writing, software * distributed under the License is distributed on an "AS IS" BASIS, * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. * See the License for the specific language governing permissions and * limitations under the License. */

package com.test.view;

import java.util.ArrayList;
import java.util.Collections;
import java.util.Comparator;

import android.content.Context;
import android.database.DataSetObserver;
import android.graphics.Canvas;
import android.graphics.Rect;
import android.graphics.drawable.Drawable;
import android.os.Parcel;
import android.os.Parcelable;
import android.os.SystemClock;
import android.support.v4.os.ParcelableCompat;
import android.support.v4.os.ParcelableCompatCreatorCallbacks;
import android.support.v4.view.KeyEventCompat;
import android.support.v4.view.MotionEventCompat;
import android.support.v4.view.PagerAdapter;
import android.support.v4.view.VelocityTrackerCompat;
import android.support.v4.view.ViewCompat;
import android.support.v4.view.ViewConfigurationCompat;
import android.support.v4.widget.EdgeEffectCompat;
import android.util.AttributeSet;
import android.util.Log;
import android.view.FocusFinder;
import android.view.KeyEvent;
import android.view.MotionEvent;
import android.view.SoundEffectConstants;
import android.view.VelocityTracker;
import android.view.View;
import android.view.ViewConfiguration;
import android.view.ViewGroup;
import android.view.ViewParent;
import android.view.accessibility.AccessibilityEvent;
import android.view.animation.Interpolator;
import android.widget.Scroller;

/** * Layout manager that allows the user to flip left and right through pages of * data. You supply an implementation of a * {@link android.support.v4.view.PagerAdapter} to generate the pages that the * view shows. * * <p> * Note this class is currently under early design and development. The API will * likely change in later updates of the compatibility library, requiring * changes to the source code of apps when they are compiled against the newer * version. * </p> */
public class LazyViewPager extends ViewGroup {
    private static final String TAG = "LazyViewPager";
    private static final boolean DEBUG = false;

    private static final boolean USE_CACHE = false;

    private static final int DEFAULT_OFFSCREEN_PAGES = 0;// 默认的加载页面,为0表示不预加载下一个界面
    private static final int MAX_SETTLE_DURATION = 600; // ms

    static class ItemInfo {
        Object object;
        int position;
        boolean scrolling;
    }

    private static final Comparator<ItemInfo> COMPARATOR = new Comparator<ItemInfo>() {
        @Override
        public int compare(ItemInfo lhs, ItemInfo rhs) {
            return lhs.position - rhs.position;
        }
    };

    private static final Interpolator sInterpolator = new Interpolator() {
        public float getInterpolation(float t) {
            // _o(t) = t * t * ((tension + 1) * t + tension)
            // o(t) = _o(t - 1) + 1
            t -= 1.0f;
            return t * t * t + 1.0f;
        }
    };

    private final ArrayList<ItemInfo> mItems = new ArrayList<ItemInfo>();

    private PagerAdapter mAdapter;
    private int mCurItem; // Index of currently displayed page.
    private int mRestoredCurItem = -1;
    private Parcelable mRestoredAdapterState = null;
    private ClassLoader mRestoredClassLoader = null;
    private Scroller mScroller;
    private PagerObserver mObserver;

    private int mPageMargin;
    private Drawable mMarginDrawable;

    private int mChildWidthMeasureSpec;
    private int mChildHeightMeasureSpec;
    private boolean mInLayout;

    private boolean mScrollingCacheEnabled;

    private boolean mPopulatePending;
    private boolean mScrolling;
    private int mOffscreenPageLimit = DEFAULT_OFFSCREEN_PAGES;

    private boolean mIsBeingDragged;
    private boolean mIsUnableToDrag;
    private int mTouchSlop;
    private float mInitialMotionX;
    /** * Position of the last motion event. */
    private float mLastMotionX;
    private float mLastMotionY;
    /** * ID of the active pointer. This is used to retain consistency during * drags/flings if multiple pointers are used. */
    private int mActivePointerId = INVALID_POINTER;
    /** * Sentinel value for no current active pointer. Used by * {@link #mActivePointerId}. */
    private static final int INVALID_POINTER = -1;

    /** * Determines speed during touch scrolling */
    private VelocityTracker mVelocityTracker;
    private int mMinimumVelocity;
    private int mMaximumVelocity;
    private float mBaseLineFlingVelocity;
    private float mFlingVelocityInfluence;

    private boolean mFakeDragging;
    private long mFakeDragBeginTime;

    private EdgeEffectCompat mLeftEdge;
    private EdgeEffectCompat mRightEdge;

    private boolean mFirstLayout = true;

    private OnPageChangeListener mOnPageChangeListener;

    /** * Indicates that the pager is in an idle, settled state. The current page * is fully in view and no animation is in progress. */
    public static final int SCROLL_STATE_IDLE = 0;

    /** * Indicates that the pager is currently being dragged by the user. */
    public static final int SCROLL_STATE_DRAGGING = 1;

    /** * Indicates that the pager is in the process of settling to a final * position. */
    public static final int SCROLL_STATE_SETTLING = 2;

    private int mScrollState = SCROLL_STATE_IDLE;

    /** * Callback interface for responding to changing state of the selected page. */
    public interface OnPageChangeListener {

        /** * This method will be invoked when the current page is scrolled, either * as part of a programmatically initiated smooth scroll or a user * initiated touch scroll. * * @param position * Position index of the first page currently being * displayed. Page position+1 will be visible if * positionOffset is nonzero. * @param positionOffset * Value from [0, 1) indicating the offset from the page at * position. * @param positionOffsetPixels * Value in pixels indicating the offset from position. */
        public void onPageScrolled(int position, float positionOffset,
                int positionOffsetPixels);

        /** * This method will be invoked when a new page becomes selected. * Animation is not necessarily complete. * * @param position * Position index of the new selected page. */
        public void onPageSelected(int position);

        /** * Called when the scroll state changes. Useful for discovering when the * user begins dragging, when the pager is automatically settling to the * current page, or when it is fully stopped/idle. * * @param state * The new scroll state. * @see android.support.v4.view.ViewPager#SCROLL_STATE_IDLE * @see android.support.v4.view.ViewPager#SCROLL_STATE_DRAGGING * @see android.support.v4.view.ViewPager#SCROLL_STATE_SETTLING */
        public void onPageScrollStateChanged(int state);
    }

    /** * Simple implementation of the * {@link com.bxg.news.view.LazyViewPager.OnPageChangeListener} * interface with stub implementations of each method. Extend this if you do * not intend to override every method of * {@link com.bxg.news.view.LazyViewPager.OnPageChangeListener}. */
    public static class SimpleOnPageChangeListener implements OnPageChangeListener {
        @Override
        public void onPageScrolled(int position, float positionOffset,
                int positionOffsetPixels) {
            // This space for rent
        }

        @Override
        public void onPageSelected(int position) {
            // This space for rent
        }

        @Override
        public void onPageScrollStateChanged(int state) {
            // This space for rent
        }
    }

    public LazyViewPager(Context context) {
        super(context);
        initViewPager();
    }

    public LazyViewPager(Context context, AttributeSet attrs) {
        super(context, attrs);
        initViewPager();
    }

    void initViewPager() {
        setWillNotDraw(false);
        setDescendantFocusability(FOCUS_AFTER_DESCENDANTS);
        setFocusable(true);
        final Context context = getContext();
        mScroller = new Scroller(context, sInterpolator);
        final ViewConfiguration configuration = ViewConfiguration.get(context);
        mTouchSlop = ViewConfigurationCompat
                .getScaledPagingTouchSlop(configuration);
        mMinimumVelocity = configuration.getScaledMinimumFlingVelocity();
        mMaximumVelocity = configuration.getScaledMaximumFlingVelocity();
        mLeftEdge = new EdgeEffectCompat(context);
        mRightEdge = new EdgeEffectCompat(context);

        float density = context.getResources().getDisplayMetrics().density;
        mBaseLineFlingVelocity = 2500.0f * density;
        mFlingVelocityInfluence = 0.4f;
    }

    private void setScrollState(int newState) {
        if (mScrollState == newState) {
            return;
        }

        mScrollState = newState;
        if (mOnPageChangeListener != null) {
            mOnPageChangeListener.onPageScrollStateChanged(newState);
        }
    }

    public void setAdapter(PagerAdapter adapter) {
        if (mAdapter != null) {
            mAdapter.unregisterDataSetObserver(mObserver);
            mAdapter.startUpdate(this);
            for (int i = 0; i < mItems.size(); i++) {
                final ItemInfo ii = mItems.get(i);
                mAdapter.destroyItem(this, ii.position, ii.object);
            }
            mAdapter.finishUpdate(this);
            mItems.clear();
            removeAllViews();
            mCurItem = 0;
            scrollTo(0, 0);
        }

        mAdapter = adapter;

        if (mAdapter != null) {
            if (mObserver == null) {
                mObserver = new PagerObserver();
            }
            mAdapter.registerDataSetObserver(mObserver);
            mPopulatePending = false;
            if (mRestoredCurItem >= 0) {
                mAdapter.restoreState(mRestoredAdapterState,
                        mRestoredClassLoader);
                setCurrentItemInternal(mRestoredCurItem, false, true);
                mRestoredCurItem = -1;
                mRestoredAdapterState = null;
                mRestoredClassLoader = null;
            } else {
                populate();
            }
        }
    }

    public PagerAdapter getAdapter() {
        return mAdapter;
    }

    /** * Set the currently selected page. If the ViewPager has already been * through its first layout there will be a smooth animated transition * between the current item and the specified item. * * @param item * Item index to select */
    public void setCurrentItem(int item) {
        mPopulatePending = false;
        setCurrentItemInternal(item, !mFirstLayout, false);
    }

    /** * Set the currently selected page. * * @param item * Item index to select * @param smoothScroll * True to smoothly scroll to the new item, false to transition * immediately */
    public void setCurrentItem(int item, boolean smoothScroll) {
        mPopulatePending = false;
        setCurrentItemInternal(item, smoothScroll, false);
    }

    public int getCurrentItem() {
        return mCurItem;
    }

    void setCurrentItemInternal(int item, boolean smoothScroll, boolean always) {
        setCurrentItemInternal(item, smoothScroll, always, 0);
    }

    void setCurrentItemInternal(int item, boolean smoothScroll, boolean always,
            int velocity) {
        if (mAdapter == null || mAdapter.getCount() <= 0) {
            setScrollingCacheEnabled(false);
            return;
        }
        if (!always && mCurItem == item && mItems.size() != 0) {
            setScrollingCacheEnabled(false);
            return;
        }
        if (item < 0) {
            item = 0;
        } else if (item >= mAdapter.getCount()) {
            item = mAdapter.getCount() - 1;
        }
        final int pageLimit = mOffscreenPageLimit;
        if (item > (mCurItem + pageLimit) || item < (mCurItem - pageLimit)) {
            // We are doing a jump by more than one page. To avoid
            // glitches, we want to keep all current pages in the view
            // until the scroll ends.
            for (int i = 0; i < mItems.size(); i++) {
                mItems.get(i).scrolling = true;
            }
        }

        final boolean dispatchSelected = mCurItem != item;
        mCurItem = item;
        populate();
        final int destX = (getWidth() + mPageMargin) * item;
        if (smoothScroll) {
            smoothScrollTo(destX, 0, velocity);
            if (dispatchSelected && mOnPageChangeListener != null) {
                mOnPageChangeListener.onPageSelected(item);
            }
        } else {
            if (dispatchSelected && mOnPageChangeListener != null) {
                mOnPageChangeListener.onPageSelected(item);
            }
            completeScroll();
            scrollTo(destX, 0);
        }
    }

    public void setOnPageChangeListener(OnPageChangeListener listener) {
        mOnPageChangeListener = listener;
    }

    /** * Returns the number of pages that will be retained to either side of the * current page in the view hierarchy in an idle state. Defaults to 1. * * @return How many pages will be kept offscreen on either side * @see #setOffscreenPageLimit(int) */
    public int getOffscreenPageLimit() {
        return mOffscreenPageLimit;
    }

    /** * Set the number of pages that should be retained to either side of the * current page in the view hierarchy in an idle state. Pages beyond this * limit will be recreated from the adapter when needed. * * <p> * This is offered as an optimization. If you know in advance the number of * pages you will need to support or have lazy-loading mechanisms in place * on your pages, tweaking this setting can have benefits in perceived * smoothness of paging animations and interaction. If you have a small * number of pages (3-4) that you can keep active all at once, less time * will be spent in layout for newly created view subtrees as the user pages * back and forth. * </p> * * <p> * You should keep this limit low, especially if your pages have complex * layouts. This setting defaults to 1. * </p> * * @param limit * How many pages will be kept offscreen in an idle state. */
    public void setOffscreenPageLimit(int limit) {
        if (limit < DEFAULT_OFFSCREEN_PAGES) {
            Log.w(TAG, "Requested offscreen page limit " + limit
                    + " too small; defaulting to " + DEFAULT_OFFSCREEN_PAGES);
            limit = DEFAULT_OFFSCREEN_PAGES;
        }
        if (limit != mOffscreenPageLimit) {
            mOffscreenPageLimit = limit;
            populate();
        }
    }

    /** * Set the margin between pages. * * @param marginPixels * Distance between adjacent pages in pixels * @see #getPageMargin() * @see #setPageMarginDrawable(android.graphics.drawable.Drawable) * @see #setPageMarginDrawable(int) */
    public void setPageMargin(int marginPixels) {
        final int oldMargin = mPageMargin;
        mPageMargin = marginPixels;

        final int width = getWidth();
        recomputeScrollPosition(width, width, marginPixels, oldMargin);

        requestLayout();
    }

    /** * Return the margin between pages. * * @return The size of the margin in pixels */
    public int getPageMargin() {
        return mPageMargin;
    }

    /** * Set a drawable that will be used to fill the margin between pages. * * @param d * Drawable to display between pages */
    public void setPageMarginDrawable(Drawable d) {
        mMarginDrawable = d;
        if (d != null)
            refreshDrawableState();
        setWillNotDraw(d == null);
        invalidate();
    }

    /** * Set a drawable that will be used to fill the margin between pages. * * @param resId * Resource ID of a drawable to display between pages */
    public void setPageMarginDrawable(int resId) {
        setPageMarginDrawable(getContext().getResources().getDrawable(resId));
    }

    @Override
    protected boolean verifyDrawable(Drawable who) {
        return super.verifyDrawable(who) || who == mMarginDrawable;
    }

    @Override
    protected void drawableStateChanged() {
        super.drawableStateChanged();
        final Drawable d = mMarginDrawable;
        if (d != null && d.isStateful()) {
            d.setState(getDrawableState());
        }
    }

    // We want the duration of the page snap animation to be influenced by the
    // distance that
    // the screen has to travel, however, we don't want this duration to be
    // effected in a
    // purely linear fashion. Instead, we use this method to moderate the effect
    // that the distance
    // of travel has on the overall snap duration.
    float distanceInfluenceForSnapDuration(float f) {
        f -= 0.5f; // center the values about 0.
        f *= 0.3f * Math.PI / 2.0f;
        return (float) Math.sin(f);
    }

    /** * Like {@link android.view.View#scrollBy}, but scroll smoothly instead of * immediately. * * @param x * the number of pixels to scroll by on the X axis * @param y * the number of pixels to scroll by on the Y axis */
    void smoothScrollTo(int x, int y) {
        smoothScrollTo(x, y, 0);
    }

    /** * Like {@link android.view.View#scrollBy}, but scroll smoothly instead of * immediately. * * @param x * the number of pixels to scroll by on the X axis * @param y * the number of pixels to scroll by on the Y axis * @param velocity * the velocity associated with a fling, if applicable. (0 * otherwise) */
    void smoothScrollTo(int x, int y, int velocity) {
        if (getChildCount() == 0) {
            // Nothing to do.
            setScrollingCacheEnabled(false);
            return;
        }
        int sx = getScrollX();
        int sy = getScrollY();
        int dx = x - sx;
        int dy = y - sy;
        if (dx == 0 && dy == 0) {
            completeScroll();
            setScrollState(SCROLL_STATE_IDLE);
            return;
        }

        setScrollingCacheEnabled(true);
        mScrolling = true;
        setScrollState(SCROLL_STATE_SETTLING);

        final float pageDelta = (float) Math.abs(dx)
                / (getWidth() + mPageMargin);
        int duration = (int) (pageDelta * 100);

        velocity = Math.abs(velocity);
        if (velocity > 0) {
            duration += (duration / (velocity / mBaseLineFlingVelocity))
                    * mFlingVelocityInfluence;
        } else {
            duration += 100;
        }
        duration = Math.min(duration, MAX_SETTLE_DURATION);

        mScroller.startScroll(sx, sy, dx, dy, duration);
        invalidate();
    }

    void addNewItem(int position, int index) {
        ItemInfo ii = new ItemInfo();
        ii.position = position;
        ii.object = mAdapter.instantiateItem(this, position);
        if (index < 0) {
            mItems.add(ii);
        } else {
            mItems.add(index, ii);
        }
    }

    void dataSetChanged() {
        // This method only gets called if our observer is attached, so mAdapter
        // is non-null.

        boolean needPopulate = mItems.size() < 3
                && mItems.size() < mAdapter.getCount();
        int newCurrItem = -1;

        for (int i = 0; i < mItems.size(); i++) {
            final ItemInfo ii = mItems.get(i);
            final int newPos = mAdapter.getItemPosition(ii.object);

            if (newPos == PagerAdapter.POSITION_UNCHANGED) {
                continue;
            }

            if (newPos == PagerAdapter.POSITION_NONE) {
                mItems.remove(i);
                i--;
                mAdapter.destroyItem(this, ii.position, ii.object);
                needPopulate = true;

                if (mCurItem == ii.position) {
                    // Keep the current item in the valid range
                    newCurrItem = Math.max(0,
                            Math.min(mCurItem, mAdapter.getCount() - 1));
                }
                continue;
            }

            if (ii.position != newPos) {
                if (ii.position == mCurItem) {
                    // Our current item changed position. Follow it.
                    newCurrItem = newPos;
                }

                ii.position = newPos;
                needPopulate = true;
            }
        }

        Collections.sort(mItems, COMPARATOR);

        if (newCurrItem >= 0) {
            // TODO This currently causes a jump.
            setCurrentItemInternal(newCurrItem, false, true);
            needPopulate = true;
        }
        if (needPopulate) {
            populate();
            requestLayout();
        }
    }

    void populate() {
        if (mAdapter == null) {
            return;
        }

        // Bail now if we are waiting to populate. This is to hold off
        // on creating views from the time the user releases their finger to
        // fling to a new position until we have finished the scroll to
        // that position, avoiding glitches from happening at that point.
        if (mPopulatePending) {
            if (DEBUG)
                Log.i(TAG, "populate is pending, skipping for now...");
            return;
        }

        // Also, don't populate until we are attached to a window. This is to
        // avoid trying to populate before we have restored our view hierarchy
        // state and conflicting with what is restored.
        if (getWindowToken() == null) {
            return;
        }

        mAdapter.startUpdate(this);

        final int pageLimit = mOffscreenPageLimit;
        final int startPos = Math.max(0, mCurItem - pageLimit);
        final int N = mAdapter.getCount();
        final int endPos = Math.min(N - 1, mCurItem + pageLimit);

        if (DEBUG)
            Log.v(TAG, "populating: startPos=" + startPos + " endPos=" + endPos);

        // Add and remove pages in the existing list.
        int lastPos = -1;
        for (int i = 0; i < mItems.size(); i++) {
            ItemInfo ii = mItems.get(i);
            if ((ii.position < startPos || ii.position > endPos)
                    && !ii.scrolling) {
                if (DEBUG)
                    Log.i(TAG, "removing: " + ii.position + " @ " + i);
                mItems.remove(i);
                i--;
                mAdapter.destroyItem(this, ii.position, ii.object);
            } else if (lastPos < endPos && ii.position > startPos) {
                // The next item is outside of our range, but we have a gap
                // between it and the last item where we want to have a page
                // shown. Fill in the gap.
                lastPos++;
                if (lastPos < startPos) {
                    lastPos = startPos;
                }
                while (lastPos <= endPos && lastPos < ii.position) {
                    if (DEBUG)
                        Log.i(TAG, "inserting: " + lastPos + " @ " + i);
                    addNewItem(lastPos, i);
                    lastPos++;
                    i++;
                }
            }
            lastPos = ii.position;
        }

        // Add any new pages we need at the end.
        lastPos = mItems.size() > 0 ? mItems.get(mItems.size() - 1).position
                : -1;
        if (lastPos < endPos) {
            lastPos++;
            lastPos = lastPos > startPos ? lastPos : startPos;
            while (lastPos <= endPos) {
                if (DEBUG)
                    Log.i(TAG, "appending: " + lastPos);
                addNewItem(lastPos, -1);
                lastPos++;
            }
        }

        if (DEBUG) {
            Log.i(TAG, "Current page list:");
            for (int i = 0; i < mItems.size(); i++) {
                Log.i(TAG, "#" + i + ": page " + mItems.get(i).position);
            }
        }

        ItemInfo curItem = null;
        for (int i = 0; i < mItems.size(); i++) {
            if (mItems.get(i).position == mCurItem) {
                curItem = mItems.get(i);
                break;
            }
        }
        mAdapter.setPrimaryItem(this, mCurItem,
                curItem != null ? curItem.object : null);

        mAdapter.finishUpdate(this);

        if (hasFocus()) {
            View currentFocused = findFocus();
            ItemInfo ii = currentFocused != null ? infoForAnyChild(currentFocused)
                    : null;
            if (ii == null || ii.position != mCurItem) {
                for (int i = 0; i < getChildCount(); i++) {
                    View child = getChildAt(i);
                    ii = infoForChild(child);
                    if (ii != null && ii.position == mCurItem) {
                        if (child.requestFocus(FOCUS_FORWARD)) {
                            break;
                        }
                    }
                }
            }
        }
    }

    public static class SavedState extends BaseSavedState {
        int position;
        Parcelable adapterState;
        ClassLoader loader;

        public SavedState(Parcelable superState) {
            super(superState);
        }

        @Override
        public void writeToParcel(Parcel out, int flags) {
            super.writeToParcel(out, flags);
            out.writeInt(position);
            out.writeParcelable(adapterState, flags);
        }

        @Override
        public String toString() {
            return "FragmentPager.SavedState{"
                    + Integer.toHexString(System.identityHashCode(this))
                    + " position=" + position + "}";
        }

        public static final Creator<SavedState> CREATOR = ParcelableCompat
                .newCreator(new ParcelableCompatCreatorCallbacks<SavedState>() {
                    @Override
                    public SavedState createFromParcel(Parcel in,
                            ClassLoader loader) {
                        return new SavedState(in, loader);
                    }

                    @Override
                    public SavedState[] newArray(int size) {
                        return new SavedState[size];
                    }
                });

        SavedState(Parcel in, ClassLoader loader) {
            super(in);
            if (loader == null) {
                loader = getClass().getClassLoader();
            }
            position = in.readInt();
            adapterState = in.readParcelable(loader);
            this.loader = loader;
        }
    }

    @Override
    public Parcelable onSaveInstanceState() {
        Parcelable superState = super.onSaveInstanceState();
        SavedState ss = new SavedState(superState);
        ss.position = mCurItem;
        if (mAdapter != null) {
            ss.adapterState = mAdapter.saveState();
        }
        return ss;
    }

    @Override
    public void onRestoreInstanceState(Parcelable state) {
        if (!(state instanceof SavedState)) {
            super.onRestoreInstanceState(state);
            return;
        }

        SavedState ss = (SavedState) state;
        super.onRestoreInstanceState(ss.getSuperState());

        if (mAdapter != null) {
            mAdapter.restoreState(ss.adapterState, ss.loader);
            setCurrentItemInternal(ss.position, false, true);
        } else {
            mRestoredCurItem = ss.position;
            mRestoredAdapterState = ss.adapterState;
            mRestoredClassLoader = ss.loader;
        }
    }

    @Override
    public void addView(View child, int index, LayoutParams params) {
        if (mInLayout) {
            addViewInLayout(child, index, params);
            child.measure(mChildWidthMeasureSpec, mChildHeightMeasureSpec);
        } else {
            super.addView(child, index, params);
        }

        if (USE_CACHE) {
            if (child.getVisibility() != GONE) {
                child.setDrawingCacheEnabled(mScrollingCacheEnabled);
            } else {
                child.setDrawingCacheEnabled(false);
            }
        }
    }

    ItemInfo infoForChild(View child) {
        for (int i = 0; i < mItems.size(); i++) {
            ItemInfo ii = mItems.get(i);
            if (mAdapter.isViewFromObject(child, ii.object)) {
                return ii;
            }
        }
        return null;
    }

    ItemInfo infoForAnyChild(View child) {
        ViewParent parent;
        while ((parent = child.getParent()) != this) {
            if (parent == null || !(parent instanceof View)) {
                return null;
            }
            child = (View) parent;
        }
        return infoForChild(child);
    }

    @Override
    protected void onAttachedToWindow() {
        super.onAttachedToWindow();
        mFirstLayout = true;
    }

    @Override
    protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
        // For simple implementation, or internal size is always 0.
        // We depend on the container to specify the layout size of
        // our view. We can't really know what it is since we will be
        // adding and removing different arbitrary views and do not
        // want the layout to change as this happens.
        setMeasuredDimension(getDefaultSize(0, widthMeasureSpec),
                getDefaultSize(0, heightMeasureSpec));

        // Children are just made to fill our space.
        mChildWidthMeasureSpec = MeasureSpec.makeMeasureSpec(getMeasuredWidth()
                - getPaddingLeft() - getPaddingRight(), MeasureSpec.EXACTLY);
        mChildHeightMeasureSpec = MeasureSpec.makeMeasureSpec(
                getMeasuredHeight() - getPaddingTop() - getPaddingBottom(),
                MeasureSpec.EXACTLY);

        // Make sure we have created all fragments that we need to have shown.
        mInLayout = true;
        populate();
        mInLayout = false;

        // Make sure all children have been properly measured.
        final int size = getChildCount();
        for (int i = 0; i < size; ++i) {
            final View child = getChildAt(i);
            if (child.getVisibility() != GONE) {
                if (DEBUG)
                    Log.v(TAG, "Measuring #" + i + " " + child + ": "
                            + mChildWidthMeasureSpec);
                child.measure(mChildWidthMeasureSpec, mChildHeightMeasureSpec);
            }
        }
    }

    @Override
    protected void onSizeChanged(int w, int h, int oldw, int oldh) {
        super.onSizeChanged(w, h, oldw, oldh);

        // Make sure scroll position is set correctly.
        if (w != oldw) {
            recomputeScrollPosition(w, oldw, mPageMargin, mPageMargin);
        }
    }

    private void recomputeScrollPosition(int width, int oldWidth, int margin,
            int oldMargin) {
        final int widthWithMargin = width + margin;
        if (oldWidth > 0) {
            final int oldScrollPos = getScrollX();
            final int oldwwm = oldWidth + oldMargin;
            final int oldScrollItem = oldScrollPos / oldwwm;
            final float scrollOffset = (float) (oldScrollPos % oldwwm) / oldwwm;
            final int scrollPos = (int) ((oldScrollItem + scrollOffset) * widthWithMargin);
            scrollTo(scrollPos, getScrollY());
            if (!mScroller.isFinished()) {
                // We now return to your regularly scheduled scroll, already in
                // progress.
                final int newDuration = mScroller.getDuration()
                        - mScroller.timePassed();
                mScroller.startScroll(scrollPos, 0, mCurItem * widthWithMargin,
                        0, newDuration);
            }
        } else {
            int scrollPos = mCurItem * widthWithMargin;
            if (scrollPos != getScrollX()) {
                completeScroll();
                scrollTo(scrollPos, getScrollY());
            }
        }
    }

    @Override
    protected void onLayout(boolean changed, int l, int t, int r, int b) {
        mInLayout = true;
        populate();
        mInLayout = false;

        final int count = getChildCount();
        final int width = r - l;

        for (int i = 0; i < count; i++) {
            View child = getChildAt(i);
            ItemInfo ii;
            if (child.getVisibility() != GONE
                    && (ii = infoForChild(child)) != null) {
                int loff = (width + mPageMargin) * ii.position;
                int childLeft = getPaddingLeft() + loff;
                int childTop = getPaddingTop();
                if (DEBUG)
                    Log.v(TAG,
                            "Positioning #" + i + " " + child + " f="
                                    + ii.object + ":" + childLeft + ","
                                    + childTop + " " + child.getMeasuredWidth()
                                    + "x" + child.getMeasuredHeight());
                child.layout(childLeft, childTop,
                        childLeft + child.getMeasuredWidth(),
                        childTop + child.getMeasuredHeight());
            }
        }
        mFirstLayout = false;
    }

    @Override
    public void computeScroll() {
        if (DEBUG)
            Log.i(TAG, "computeScroll: finished=" + mScroller.isFinished());
        if (!mScroller.isFinished()) {
            if (mScroller.computeScrollOffset()) {
                if (DEBUG)
                    Log.i(TAG, "computeScroll: still scrolling");
                int oldX = getScrollX();
                int oldY = getScrollY();
                int x = mScroller.getCurrX();
                int y = mScroller.getCurrY();

                if (oldX != x || oldY != y) {
                    scrollTo(x, y);
                }

                if (mOnPageChangeListener != null) {
                    final int widthWithMargin = getWidth() + mPageMargin;
                    final int position = x / widthWithMargin;
                    final int offsetPixels = x % widthWithMargin;
                    final float offset = (float) offsetPixels / widthWithMargin;
                    mOnPageChangeListener.onPageScrolled(position, offset,
                            offsetPixels);
                }

                // Keep on drawing until the animation has finished.
                invalidate();
                return;
            }
        }

        // Done with scroll, clean up state.
        completeScroll();
    }

    private void completeScroll() {
        boolean needPopulate = mScrolling;
        if (needPopulate) {
            // Done with scroll, no longer want to cache view drawing.
            setScrollingCacheEnabled(false);
            mScroller.abortAnimation();
            int oldX = getScrollX();
            int oldY = getScrollY();
            int x = mScroller.getCurrX();
            int y = mScroller.getCurrY();
            if (oldX != x || oldY != y) {
                scrollTo(x, y);
            }
            setScrollState(SCROLL_STATE_IDLE);
        }
        mPopulatePending = false;
        mScrolling = false;
        for (int i = 0; i < mItems.size(); i++) {
            ItemInfo ii = mItems.get(i);
            if (ii.scrolling) {
                needPopulate = true;
                ii.scrolling = false;
            }
        }
        if (needPopulate) {
            populate();
        }
    }

    @Override
    public boolean onInterceptTouchEvent(MotionEvent ev) {
        /* * This method JUST determines whether we want to intercept the motion. * If we return true, onMotionEvent will be called and we do the actual * scrolling there. */

        final int action = ev.getAction() & MotionEventCompat.ACTION_MASK;

        // Always take care of the touch gesture being complete.
        if (action == MotionEvent.ACTION_CANCEL
                || action == MotionEvent.ACTION_UP) {
            // Release the drag.
            if (DEBUG)
                Log.v(TAG, "Intercept done!");
            mIsBeingDragged = false;
            mIsUnableToDrag = false;
            mActivePointerId = INVALID_POINTER;
            return false;
        }

        // Nothing more to do here if we have decided whether or not we
        // are dragging.
        if (action != MotionEvent.ACTION_DOWN) {
            if (mIsBeingDragged) {
                if (DEBUG)
                    Log.v(TAG, "Intercept returning true!");
                return true;
            }
            if (mIsUnableToDrag) {
                if (DEBUG)
                    Log.v(TAG, "Intercept returning false!");
                return false;
            }
        }

        switch (action) {
        case MotionEvent.ACTION_MOVE: {
            /* * mIsBeingDragged == false, otherwise the shortcut would have * caught it. Check whether the user has moved far enough from his * original down touch. */

            /* * Locally do absolute value. mLastMotionY is set to the y value of * the down event. */
            final int activePointerId = mActivePointerId;
            if (activePointerId == INVALID_POINTER) {
                // If we don't have a valid id, the touch down wasn't on
                // content.
                break;
            }

            final int pointerIndex = MotionEventCompat.findPointerIndex(ev,
                    activePointerId);
            final float x = MotionEventCompat.getX(ev, pointerIndex);
            final float dx = x - mLastMotionX;
            final float xDiff = Math.abs(dx);
            final float y = MotionEventCompat.getY(ev, pointerIndex);
            final float yDiff = Math.abs(y - mLastMotionY);
            final int scrollX = getScrollX();
            final boolean atEdge = (dx > 0 && scrollX == 0)
                    || (dx < 0 && mAdapter != null && scrollX >= (mAdapter
                            .getCount() - 1) * getWidth() - 1);
            if (DEBUG)
                Log.v(TAG, "Moved x to " + x + "," + y + " diff=" + xDiff + ","
                        + yDiff);

            if (canScroll(this, false, (int) dx, (int) x, (int) y)) {
                // Nested view has scrollable area under this point. Let it be
                // handled there.
                mInitialMotionX = mLastMotionX = x;
                mLastMotionY = y;
                return false;
            }
            if (xDiff > mTouchSlop && xDiff > yDiff) {
                if (DEBUG)
                    Log.v(TAG, "Starting drag!");
                mIsBeingDragged = true;
                setScrollState(SCROLL_STATE_DRAGGING);
                mLastMotionX = x;
                setScrollingCacheEnabled(true);
            } else {
                if (yDiff > mTouchSlop) {
                    // The finger has moved enough in the vertical
                    // direction to be counted as a drag... abort
                    // any attempt to drag horizontally, to work correctly
                    // with children that have scrolling containers.
                    if (DEBUG)
                        Log.v(TAG, "Starting unable to drag!");
                    mIsUnableToDrag = true;
                }
            }
            break;
        }

        case MotionEvent.ACTION_DOWN: {
            /* * Remember location of down touch. ACTION_DOWN always refers to * pointer index 0. */
            mLastMotionX = mInitialMotionX = ev.getX();
            mLastMotionY = ev.getY();
            mActivePointerId = MotionEventCompat.getPointerId(ev, 0);

            if (mScrollState == SCROLL_STATE_SETTLING) {
                // Let the user 'catch' the pager as it animates.
                mIsBeingDragged = true;
                mIsUnableToDrag = false;
                setScrollState(SCROLL_STATE_DRAGGING);
            } else {
                completeScroll();
                mIsBeingDragged = false;
                mIsUnableToDrag = false;
            }

            if (DEBUG)
                Log.v(TAG, "Down at " + mLastMotionX + "," + mLastMotionY
                        + " mIsBeingDragged=" + mIsBeingDragged
                        + "mIsUnableToDrag=" + mIsUnableToDrag);
            break;
        }

        case MotionEventCompat.ACTION_POINTER_UP:
            onSecondaryPointerUp(ev);
            break;
        }

        /* * The only time we want to intercept motion events is if we are in the * drag mode. */
        return mIsBeingDragged;
    }

    @Override
    public boolean onTouchEvent(MotionEvent ev) {
        if (mFakeDragging) {
            // A fake drag is in progress already, ignore this real one
            // but still eat the touch events.
            // (It is likely that the user is multi-touching the screen.)
            return true;
        }

        if (ev.getAction() == MotionEvent.ACTION_DOWN && ev.getEdgeFlags() != 0) {
            // Don't handle edge touches immediately -- they may actually belong
            // to one of our
            // descendants.
            return false;
        }

        if (mAdapter == null || mAdapter.getCount() == 0) {
            // Nothing to present or scroll; nothing to touch.
            return false;
        }

        if (mVelocityTracker == null) {
            mVelocityTracker = VelocityTracker.obtain();
        }
        mVelocityTracker.addMovement(ev);

        final int action = ev.getAction();
        boolean needsInvalidate = false;

        switch (action & MotionEventCompat.ACTION_MASK) {
        case MotionEvent.ACTION_DOWN: {
            /* * If being flinged and user touches, stop the fling. isFinished * will be false if being flinged. */
            completeScroll();

            // Remember where the motion event started
            mLastMotionX = mInitialMotionX = ev.getX();
            mActivePointerId = MotionEventCompat.getPointerId(ev, 0);
            break;
        }
        case MotionEvent.ACTION_MOVE:
            if (!mIsBeingDragged) {
                final int pointerIndex = MotionEventCompat.findPointerIndex(ev,
                        mActivePointerId);
                final float x = MotionEventCompat.getX(ev, pointerIndex);
                final float xDiff = Math.abs(x - mLastMotionX);
                final float y = MotionEventCompat.getY(ev, pointerIndex);
                final float yDiff = Math.abs(y - mLastMotionY);
                if (DEBUG)
                    Log.v(TAG, "Moved x to " + x + "," + y + " diff=" + xDiff
                            + "," + yDiff);
                if (xDiff > mTouchSlop && xDiff > yDiff) {
                    if (DEBUG)
                        Log.v(TAG, "Starting drag!");
                    mIsBeingDragged = true;
                    mLastMotionX = x;
                    setScrollState(SCROLL_STATE_DRAGGING);
                    setScrollingCacheEnabled(true);
                }
            }
            if (mIsBeingDragged) {
                // Scroll to follow the motion event
                final int activePointerIndex = MotionEventCompat
                        .findPointerIndex(ev, mActivePointerId);
                final float x = MotionEventCompat.getX(ev, activePointerIndex);
                final float deltaX = mLastMotionX - x;
                mLastMotionX = x;
                float oldScrollX = getScrollX();
                float scrollX = oldScrollX + deltaX;
                final int width = getWidth();
                final int widthWithMargin = width + mPageMargin;

                final int lastItemIndex = mAdapter.getCount() - 1;
                final float leftBound = Math.max(0, (mCurItem - 1)
                        * widthWithMargin);
                final float rightBound = Math.min(mCurItem + 1, lastItemIndex)
                        * widthWithMargin;
                if (scrollX < leftBound) {
                    if (leftBound == 0) {
                        float over = -scrollX;
                        needsInvalidate = mLeftEdge.onPull(over / width);
                    }
                    scrollX = leftBound;
                } else if (scrollX > rightBound) {
                    if (rightBound == lastItemIndex * widthWithMargin) {
                        float over = scrollX - rightBound;
                        needsInvalidate = mRightEdge.onPull(over / width);
                    }
                    scrollX = rightBound;
                }
                // Don't lose the rounded component
                mLastMotionX += scrollX - (int) scrollX;
                scrollTo((int) scrollX, getScrollY());
                if (mOnPageChangeListener != null) {
                    final int position = (int) scrollX / widthWithMargin;
                    final int positionOffsetPixels = (int) scrollX
                            % widthWithMargin;
                    final float positionOffset = (float) positionOffsetPixels
                            / widthWithMargin;
                    mOnPageChangeListener.onPageScrolled(position,
                            positionOffset, positionOffsetPixels);
                }
            }
            break;
        case MotionEvent.ACTION_UP:
            if (mIsBeingDragged) {
                final VelocityTracker velocityTracker = mVelocityTracker;
                velocityTracker.computeCurrentVelocity(1000, mMaximumVelocity);
                int initialVelocity = (int) VelocityTrackerCompat.getXVelocity(
                        velocityTracker, mActivePointerId);
                mPopulatePending = true;
                final int widthWithMargin = getWidth() + mPageMargin;
                final int scrollX = getScrollX();
                final int currentPage = scrollX / widthWithMargin;
                int nextPage = initialVelocity > 0 ? currentPage
                        : currentPage + 1;
                setCurrentItemInternal(nextPage, true, true, initialVelocity);

                mActivePointerId = INVALID_POINTER;
                endDrag();
                needsInvalidate = mLeftEdge.onRelease()
                        | mRightEdge.onRelease();
            }
            break;
        case MotionEvent.ACTION_CANCEL:
            if (mIsBeingDragged) {
                setCurrentItemInternal(mCurItem, true, true);
                mActivePointerId = INVALID_POINTER;
                endDrag();
                needsInvalidate = mLeftEdge.onRelease()
                        | mRightEdge.onRelease();
            }
            break;
        case MotionEventCompat.ACTION_POINTER_DOWN: {
            final int index = MotionEventCompat.getActionIndex(ev);
            final float x = MotionEventCompat.getX(ev, index);
            mLastMotionX = x;
            mActivePointerId = MotionEventCompat.getPointerId(ev, index);
            break;
        }
        case MotionEventCompat.ACTION_POINTER_UP:
            onSecondaryPointerUp(ev);
            mLastMotionX = MotionEventCompat.getX(ev,
                    MotionEventCompat.findPointerIndex(ev, mActivePointerId));
            break;
        }
        if (needsInvalidate) {
            invalidate();
        }
        return true;
    }

    @Override
    public void draw(Canvas canvas) {
        super.draw(canvas);
        boolean needsInvalidate = false;

        final int overScrollMode = ViewCompat.getOverScrollMode(this);
        if (overScrollMode == ViewCompat.OVER_SCROLL_ALWAYS
                || (overScrollMode == ViewCompat.OVER_SCROLL_IF_CONTENT_SCROLLS
                        && mAdapter != null && mAdapter.getCount() > 1)) {
            if (!mLeftEdge.isFinished()) {
                final int restoreCount = canvas.save();
                final int height = getHeight() - getPaddingTop()
                        - getPaddingBottom();

                canvas.rotate(270);
                canvas.translate(-height + getPaddingTop(), 0);
                mLeftEdge.setSize(height, getWidth());
                needsInvalidate |= mLeftEdge.draw(canvas);
                canvas.restoreToCount(restoreCount);
            }
            if (!mRightEdge.isFinished()) {
                final int restoreCount = canvas.save();
                final int width = getWidth();
                final int height = getHeight() - getPaddingTop()
                        - getPaddingBottom();
                final int itemCount = mAdapter != null ? mAdapter.getCount()
                        : 1;

                canvas.rotate(90);
                canvas.translate(-getPaddingTop(), -itemCount
                        * (width + mPageMargin) + mPageMargin);
                mRightEdge.setSize(height, width);
                needsInvalidate |= mRightEdge.draw(canvas);
                canvas.restoreToCount(restoreCount);
            }
        } else {
            mLeftEdge.finish();
            mRightEdge.finish();
        }

        if (needsInvalidate) {
            // Keep animating
            invalidate();
        }
    }

    @Override
    protected void onDraw(Canvas canvas) {
        super.onDraw(canvas);

        // Draw the margin drawable if needed.
        if (mPageMargin > 0 && mMarginDrawable != null) {
            final int scrollX = getScrollX();
            final int width = getWidth();
            final int offset = scrollX % (width + mPageMargin);
            if (offset != 0) {
                // Pages fit completely when settled; we only need to draw when
                // in between
                final int left = scrollX - offset + width;
                mMarginDrawable.setBounds(left, 0, left + mPageMargin,
                        getHeight());
                mMarginDrawable.draw(canvas);
            }
        }
    }

    /** * Start a fake drag of the pager. * * <p> * A fake drag can be useful if you want to synchronize the motion of the * ViewPager with the touch scrolling of another view, while still letting * the ViewPager control the snapping motion and fling behavior. (e.g. * parallax-scrolling tabs.) Call {@link #fakeDragBy(float)} to simulate the * actual drag motion. Call {@link #endFakeDrag()} to complete the fake drag * and fling as necessary. * * <p> * During a fake drag the ViewPager will ignore all touch events. If a real * drag is already in progress, this method will return false. * * @return true if the fake drag began successfully, false if it could not * be started. * * @see #fakeDragBy(float) * @see #endFakeDrag() */
    public boolean beginFakeDrag() {
        if (mIsBeingDragged) {
            return false;
        }
        mFakeDragging = true;
        setScrollState(SCROLL_STATE_DRAGGING);
        mInitialMotionX = mLastMotionX = 0;
        if (mVelocityTracker == null) {
            mVelocityTracker = VelocityTracker.obtain();
        } else {
            mVelocityTracker.clear();
        }
        final long time = SystemClock.uptimeMillis();
        final MotionEvent ev = MotionEvent.obtain(time, time,
                MotionEvent.ACTION_DOWN, 0, 0, 0);
        mVelocityTracker.addMovement(ev);
        ev.recycle();
        mFakeDragBeginTime = time;
        return true;
    }

    /** * End a fake drag of the pager. * * @see #beginFakeDrag() * @see #fakeDragBy(float) */
    public void endFakeDrag() {
        if (!mFakeDragging) {
            throw new IllegalStateException(
                    "No fake drag in progress. Call beginFakeDrag first.");
        }

        final VelocityTracker velocityTracker = mVelocityTracker;
        velocityTracker.computeCurrentVelocity(1000, mMaximumVelocity);
        int initialVelocity = (int) VelocityTrackerCompat.getYVelocity(
                velocityTracker, mActivePointerId);
        mPopulatePending = true;
        if ((Math.abs(initialVelocity) > mMinimumVelocity)
                || Math.abs(mInitialMotionX - mLastMotionX) >= (getWidth() / 3)) {
            if (mLastMotionX > mInitialMotionX) {
                setCurrentItemInternal(mCurItem - 1, true, true);
            } else {
                setCurrentItemInternal(mCurItem + 1, true, true);
            }
        } else {
            setCurrentItemInternal(mCurItem, true, true);
        }
        endDrag();

        mFakeDragging = false;
    }

    /** * Fake drag by an offset in pixels. You must have called * {@link #beginFakeDrag()} first. * * @param xOffset * Offset in pixels to drag by. * @see #beginFakeDrag() * @see #endFakeDrag() */
    public void fakeDragBy(float xOffset) {
        if (!mFakeDragging) {
            throw new IllegalStateException(
                    "No fake drag in progress. Call beginFakeDrag first.");
        }

        mLastMotionX += xOffset;
        float scrollX = getScrollX() - xOffset;
        final int width = getWidth();
        final int widthWithMargin = width + mPageMargin;

        final float leftBound = Math.max(0, (mCurItem - 1) * widthWithMargin);
        final float rightBound = Math
                .min(mCurItem + 1, mAdapter.getCount() - 1) * widthWithMargin;
        if (scrollX < leftBound) {
            scrollX = leftBound;
        } else if (scrollX > rightBound) {
            scrollX = rightBound;
        }
        // Don't lose the rounded component
        mLastMotionX += scrollX - (int) scrollX;
        scrollTo((int) scrollX, getScrollY());
        if (mOnPageChangeListener != null) {
            final int position = (int) scrollX / widthWithMargin;
            final int positionOffsetPixels = (int) scrollX % widthWithMargin;
            final float positionOffset = (float) positionOffsetPixels
                    / widthWithMargin;
            mOnPageChangeListener.onPageScrolled(position, positionOffset,
                    positionOffsetPixels);
        }

        // Synthesize an event for the VelocityTracker.
        final long time = SystemClock.uptimeMillis();
        final MotionEvent ev = MotionEvent.obtain(mFakeDragBeginTime, time,
                MotionEvent.ACTION_MOVE, mLastMotionX, 0, 0);
        mVelocityTracker.addMovement(ev);
        ev.recycle();
    }

    /** * Returns true if a fake drag is in progress. * * @return true if currently in a fake drag, false otherwise. * * @see #beginFakeDrag() * @see #fakeDragBy(float) * @see #endFakeDrag() */
    public boolean isFakeDragging() {
        return mFakeDragging;
    }

    private void onSecondaryPointerUp(MotionEvent ev) {
        final int pointerIndex = MotionEventCompat.getActionIndex(ev);
        final int pointerId = MotionEventCompat.getPointerId(ev, pointerIndex);
        if (pointerId == mActivePointerId) {
            // This was our active pointer going up. Choose a new
            // active pointer and adjust accordingly.
            final int newPointerIndex = pointerIndex == 0 ? 1 : 0;
            mLastMotionX = MotionEventCompat.getX(ev, newPointerIndex);
            mActivePointerId = MotionEventCompat.getPointerId(ev,
                    newPointerIndex);
            if (mVelocityTracker != null) {
                mVelocityTracker.clear();
            }
        }
    }

    private void endDrag() {
        mIsBeingDragged = false;
        mIsUnableToDrag = false;

        if (mVelocityTracker != null) {
            mVelocityTracker.recycle();
            mVelocityTracker = null;
        }
    }

    private void setScrollingCacheEnabled(boolean enabled) {
        if (mScrollingCacheEnabled != enabled) {
            mScrollingCacheEnabled = enabled;
            if (USE_CACHE) {
                final int size = getChildCount();
                for (int i = 0; i < size; ++i) {
                    final View child = getChildAt(i);
                    if (child.getVisibility() != GONE) {
                        child.setDrawingCacheEnabled(enabled);
                    }
                }
            }
        }
    }

    /** * Tests scrollability within child views of v given a delta of dx. * * @param v * View to test for horizontal scrollability * @param checkV * Whether the view v passed should itself be checked for * scrollability (true), or just its children (false). * @param dx * Delta scrolled in pixels * @param x * X coordinate of the active touch point * @param y * Y coordinate of the active touch point * @return true if child views of v can be scrolled by delta of dx. */
    protected boolean canScroll(View v, boolean checkV, int dx, int x, int y) {
        if (v instanceof ViewGroup) {
            final ViewGroup group = (ViewGroup) v;
            final int scrollX = v.getScrollX();
            final int scrollY = v.getScrollY();
            final int count = group.getChildCount();
            // Count backwards - let topmost views consume scroll distance
            // first.
            for (int i = count - 1; i >= 0; i--) {
                // TODO: Add versioned support here for transformed views.
                // This will not work for transformed views in Honeycomb+
                final View child = group.getChildAt(i);
                if (x + scrollX >= child.getLeft()
                        && x + scrollX < child.getRight()
                        && y + scrollY >= child.getTop()
                        && y + scrollY < child.getBottom()
                        && canScroll(child, true, dx,
                                x + scrollX - child.getLeft(), y + scrollY
                                        - child.getTop())) {
                    return true;
                }
            }
        }

        return checkV && ViewCompat.canScrollHorizontally(v, -dx);
    }

    @Override
    public boolean dispatchKeyEvent(KeyEvent event) {
        // Let the focused view and/or our descendants get the key first
        return super.dispatchKeyEvent(event) || executeKeyEvent(event);
    }

    /** * You can call this function yourself to have the scroll view perform * scrolling from a key event, just as if the event had been dispatched to * it by the view hierarchy. * * @param event * The key event to execute. * @return Return true if the event was handled, else false. */
    public boolean executeKeyEvent(KeyEvent event) {
        boolean handled = false;
        if (event.getAction() == KeyEvent.ACTION_DOWN) {
            switch (event.getKeyCode()) {
            case KeyEvent.KEYCODE_DPAD_LEFT:
                handled = arrowScroll(FOCUS_LEFT);
                break;
            case KeyEvent.KEYCODE_DPAD_RIGHT:
                handled = arrowScroll(FOCUS_RIGHT);
                break;
            case KeyEvent.KEYCODE_TAB:
                if (KeyEventCompat.hasNoModifiers(event)) {
                    handled = arrowScroll(FOCUS_FORWARD);
                } else if (KeyEventCompat.hasModifiers(event,
                        KeyEvent.META_SHIFT_ON)) {
                    handled = arrowScroll(FOCUS_BACKWARD);
                }
                break;
            }
        }
        return handled;
    }

    public boolean arrowScroll(int direction) {
        View currentFocused = findFocus();
        if (currentFocused == this)
            currentFocused = null;

        boolean handled = false;

        View nextFocused = FocusFinder.getInstance().findNextFocus(this,
                currentFocused, direction);
        if (nextFocused != null && nextFocused != currentFocused) {
            if (direction == View.FOCUS_LEFT) {
                // If there is nothing to the left, or this is causing us to
                // jump to the right, then what we really want to do is page
                // left.
                if (currentFocused != null
                        && nextFocused.getLeft() >= currentFocused.getLeft()) {
                    handled = pageLeft();
                } else {
                    handled = nextFocused.requestFocus();
                }
            } else if (direction == View.FOCUS_RIGHT) {
                // If there is nothing to the right, or this is causing us to
                // jump to the left, then what we really want to do is page
                // right.
                if (currentFocused != null
                        && nextFocused.getLeft() <= currentFocused.getLeft()) {
                    handled = pageRight();
                } else {
                    handled = nextFocused.requestFocus();
                }
            }
        } else if (direction == FOCUS_LEFT || direction == FOCUS_BACKWARD) {
            // Trying to move left and nothing there; try to page.
            handled = pageLeft();
        } else if (direction == FOCUS_RIGHT || direction == FOCUS_FORWARD) {
            // Trying to move right and nothing there; try to page.
            handled = pageRight();
        }
        if (handled) {
            playSoundEffect(SoundEffectConstants
                    .getContantForFocusDirection(direction));
        }
        return handled;
    }

    boolean pageLeft() {
        if (mCurItem > 0) {
            setCurrentItem(mCurItem - 1, true);
            return true;
        }
        return false;
    }

    boolean pageRight() {
        if (mAdapter != null && mCurItem < (mAdapter.getCount() - 1)) {
            setCurrentItem(mCurItem + 1, true);
            return true;
        }
        return false;
    }

    /** * We only want the current page that is being shown to be focusable. */
    @Override
    public void addFocusables(ArrayList<View> views, int direction,
            int focusableMode) {
        final int focusableCount = views.size();

        final int descendantFocusability = getDescendantFocusability();

        if (descendantFocusability != FOCUS_BLOCK_DESCENDANTS) {
            for (int i = 0; i < getChildCount(); i++) {
                final View child = getChildAt(i);
                if (child.getVisibility() == VISIBLE) {
                    ItemInfo ii = infoForChild(child);
                    if (ii != null && ii.position == mCurItem) {
                        child.addFocusables(views, direction, focusableMode);
                    }
                }
            }
        }

        // we add ourselves (if focusable) in all cases except for when we are
        // FOCUS_AFTER_DESCENDANTS and there are some descendants focusable.
        // this is
        // to avoid the focus search finding layouts when a more precise search
        // among the focusable children would be more interesting.
        if (descendantFocusability != FOCUS_AFTER_DESCENDANTS ||
        // No focusable descendants
                (focusableCount == views.size())) {
            // Note that we can't call the superclass here, because it will
            // add all views in. So we need to do the same thing View does.
            if (!isFocusable()) {
                return;
            }
            if ((focusableMode & FOCUSABLES_TOUCH_MODE) == FOCUSABLES_TOUCH_MODE
                    && isInTouchMode() && !isFocusableInTouchMode()) {
                return;
            }
            if (views != null) {
                views.add(this);
            }
        }
    }

    /** * We only want the current page that is being shown to be touchable. */
    @Override
    public void addTouchables(ArrayList<View> views) {
        // Note that we don't call super.addTouchables(), which means that
        // we don't call View.addTouchables(). This is okay because a ViewPager
        // is itself not touchable.
        for (int i = 0; i < getChildCount(); i++) {
            final View child = getChildAt(i);
            if (child.getVisibility() == VISIBLE) {
                ItemInfo ii = infoForChild(child);
                if (ii != null && ii.position == mCurItem) {
                    child.addTouchables(views);
                }
            }
        }
    }

    /** * We only want the current page that is being shown to be focusable. */
    @Override
    protected boolean onRequestFocusInDescendants(int direction,
            Rect previouslyFocusedRect) {
        int index;
        int increment;
        int end;
        int count = getChildCount();
        if ((direction & FOCUS_FORWARD) != 0) {
            index = 0;
            increment = 1;
            end = count;
        } else {
            index = count - 1;
            increment = -1;
            end = -1;
        }
        for (int i = index; i != end; i += increment) {
            View child = getChildAt(i);
            if (child.getVisibility() == VISIBLE) {
                ItemInfo ii = infoForChild(child);
                if (ii != null && ii.position == mCurItem) {
                    if (child.requestFocus(direction, previouslyFocusedRect)) {
                        return true;
                    }
                }
            }
        }
        return false;
    }

    @Override
    public boolean dispatchPopulateAccessibilityEvent(AccessibilityEvent event) {
        // ViewPagers should only report accessibility info for the current
        // page,
        // otherwise things get very confusing.

        // TODO: Should this note something about the paging container?

        final int childCount = getChildCount();
        for (int i = 0; i < childCount; i++) {
            final View child = getChildAt(i);
            if (child.getVisibility() == VISIBLE) {
                final ItemInfo ii = infoForChild(child);
                if (ii != null && ii.position == mCurItem
                        && child.dispatchPopulateAccessibilityEvent(event)) {
                    return true;
                }
            }
        }

        return false;
    }

    private class PagerObserver extends DataSetObserver {

        @Override
        public void onChanged() {
            dataSetChanged();
        }

        @Override
        public void onInvalidated() {
            dataSetChanged();
        }
    }
}

参考：http://blog.csdn.net/jys1115/article/details/41862019?utm_source=tuicool