ViewPager之间的切换速度

突然遇到脑残的设计说viewpager的切换速度过快，要来个满动作，开始一听，这脑残设计，其实还是比较简单的

我们先来看段ViewPager的源码：

void initViewPager() {
        setWillNotDraw(false);
        setDescendantFocusability(FOCUS_AFTER_DESCENDANTS);
        setFocusable(true);
        final Context context = getContext();
        mScroller = new Scroller(context, sInterpolator);
       .....
    }

这个Scroller就是控制ViewPager滚动的， Android里Scroller类是为了实现View平滑滚动的一个Helper类

来看一下源码中事件处理:
<pre name="code" class="java">  @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!");
            resetTouch();
            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 - mInitialMotionY);
                if (DEBUG) Log.v(TAG, "Moved x to " + x + "," + y + " diff=" + xDiff + "," + yDiff);

                if (dx != 0 && !isGutterDrag(mLastMotionX, dx) &&
                        canScroll(this, false, (int) dx, (int) x, (int) y)) {
                    // Nested view has scrollable area under this point. Let it be handled there.
                    mLastMotionX = x;
                    mLastMotionY = y;
                    mIsUnableToDrag = true;
                    return false;
                }
                if (xDiff > mTouchSlop && xDiff * 0.5f > yDiff) {
                    if (DEBUG) Log.v(TAG, "Starting drag!");
                    mIsBeingDragged = true;
                    requestParentDisallowInterceptTouchEvent(true);
                    setScrollState(SCROLL_STATE_DRAGGING);
                    mLastMotionX = dx > 0 ? mInitialMotionX + mTouchSlop :
                            mInitialMotionX - mTouchSlop;
                    mLastMotionY = y;
                    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;
                }
                if (mIsBeingDragged) {
                    // Scroll to follow the motion event
                    if (performDrag(x)) {
                        ViewCompat.postInvalidateOnAnimation(this);
                    }
                }
                break;
            }

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

                mScroller.computeScrollOffset();
                if (mScrollState == SCROLL_STATE_SETTLING &&
                        Math.abs(mScroller.getFinalX() - mScroller.getCurrX()) > mCloseEnough) {
                    // Let the user 'catch' the pager as it animates.
                    mScroller.abortAnimation();
                    mPopulatePending = false;
                    populate();
                    mIsBeingDragged = true;
                    requestParentDisallowInterceptTouchEvent(true);
                    setScrollState(SCROLL_STATE_DRAGGING);
                } else {
                    completeScroll(false);
                    mIsBeingDragged = false;
                }

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

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

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

        /*
         * The only time we want to intercept motion events is if we are in the
         * drag mode.
         */
        return mIsBeingDragged;
    }
最终控制速度的体现都是mScroller

所以我们需要自定义Scroller

package com.example.viewpagerresearch;

import android.content.Context;
import android.view.animation.Interpolator;
import android.widget.Scroller;

/**
 * @author xuanyouwu
 * @email [email protected]
 * @time 2016-01-12 15:17
 */
public class MyScroller extends Scroller {

    private long mDuration;

    public MyScroller(Context context) {
        super(context);
    }

    public MyScroller(Context context, Interpolator interpolator) {
        super(context, interpolator);
    }

    public MyScroller(Context context, Interpolator interpolator, boolean flywheel) {
        super(context, interpolator, flywheel);
    }

    public long getmDuration() {
        return mDuration;
    }

    public void setmDuration(long mDuration) {
        this.mDuration = mDuration;
    }

    @Override
    public void startScroll(int startX, int startY, int dx, int dy) {
        //super.startScroll(startX, startY, dx, dy);
        super.startScroll(startX, startY, dx, dy, this.getDuration());
    }

    @Override
    public void startScroll(int startX, int startY, int dx, int dy, int duration) {
        // super.startScroll(startX, startY, dx, dy, duration);
        super.startScroll(startX, startY, dx, dy, this.getDuration());
    }
}

应用:

由于Viewpager中的mScroll是私有的，所以我们需要考虑反射

package com.example.viewpagerresearch;

import android.support.v4.view.ViewPager;
import android.view.animation.AccelerateInterpolator;

import java.lang.reflect.Field;

/**
 * @author xuanyouwu
 * @email [email protected]
 * @time 2016-01-12 16:54
 */
public class ScrollUtils {

    /**
     * 例子  如匀加速
     *
     * @param viewPager
     * @param speed
     */
    public static void setViewPagerScrollSpeed(ViewPager viewPager, int speed) {
        try {
            Field field = ViewPager.class.getDeclaredField("mScroller");
            field.setAccessible(true);
            // AccelerateInterpolator 是匀加速插值器
            MyScroller viewPagerScroller = new MyScroller(viewPager.getContext(), new AccelerateInterpolator());
            field.set(viewPager, viewPagerScroller);
            viewPagerScroller.setmDuration(speed);
        } catch (NoSuchFieldException e) {
            e.printStackTrace();
        } catch (IllegalAccessException e) {
            e.printStackTrace();
        }
    }
}
ok 调用
 ScrollUtils.setViewPagerScrollSpeed(pager,1000);