提取Launcher中的WorkSapce，可以左右滑动切换屏幕页面的类

对于Launcher的桌面滑动大家应该都比较熟悉了，最好的体验应该是可以随着手指的滑动而显示不同位置的桌面，

比一般用ViewFlinger+动画所实现的手势切换页面感觉良好多了~~~~

分析了一下Launcher中的WorkSpace，里面有太多的代码我们用不上了（拖拽，长按，，，），把里面的冗余代码去掉得到实现滑动切换屏幕所必需的。。。。

新建一个ScrollLayout类，继承自ViewGroup。

重写onMeasure和onLayout两个方法：

其中onMeasure方法中，得到ScrollLayout的布局方式（一般使用FILL_PARENT），然后再枚举其中所有的子view，设置它们的布局（FILL_PARENT），这样在ScrollLayout之中的每一个子view即为充满屏幕可以滑动显示的其中一页。

在onLayout方法中，横向画出每一个子view，这样所得到的view的高与屏幕高一致，宽度为getChildCount()-1个屏幕宽度的view。

添加一个Scroller来平滑过渡各个页面之间的切换，

重写onInterceptTouchEvent和onTouchEvent来响应手指按下划动时所需要捕获的消息，例如划动的速度，划动的距离等。再配合使用scrollBy (int x, int y)方法得到慢速滑动小距离的时候，所需要显示的内容。最后当手指起来时，根据划动的速度与跨度来判断是向左滑动一页还是向右滑动一页，确保每次用户操作结束之后显示的都是整体的一个子view.

ScrollLayout源码：

 package com.yao_guet.test;
 import Android.content.Context;
 import android.graphics.Canvas;
 import android.util.AttributeSet;
 import android.util.Log;
 import android.view.MotionEvent;
 import android.view.VelocityTracker;
 import android.view.View;
 import android.view.ViewConfiguration;
 import android.view.ViewGroup;
 import android.widget.Scroller;
 /**
  * 仿Launcher中的WorkSapce，可以左右滑动切换屏幕的类
  * @author Yao.GUET
  * blog: http://blog.csdn.NET/Yao_GUET
  * date: 2011-05-04
  */
 public class ScrollLayout extends ViewGroup {
     private static final String TAG = "ScrollLayout";
     private Scroller mScroller;
     private VelocityTracker mVelocityTracker;

     private int mCurScreen;
     private int mDefaultScreen = 0;

     private static final int TOUCH_STATE_REST = 0;
     private static final int TOUCH_STATE_SCROLLING = 1;

     private static final int SNAP_VELOCITY = 600;

     private int mTouchState = TOUCH_STATE_REST;
     private int mTouchSlop;
     private float mLastMotionX;
     private float mLastMotionY;
     public ScrollLayout(Context context, AttributeSet attrs) {
         this(context, attrs, 0);
         // TODO Auto-generated constructor stub
     }
     public ScrollLayout(Context context, AttributeSet attrs, int defStyle) {
         super(context, attrs, defStyle);
         // TODO Auto-generated constructor stub
         mScroller = new Scroller(context);

         mCurScreen = mDefaultScreen;
         mTouchSlop = ViewConfiguration.get(getContext()).getScaledTouchSlop();
     }
     @Override
     protected void onLayout(boolean changed, int l, int t, int r, int b) {
         // TODO Auto-generated method stub
         if (changed) {
             int childLeft = 0;
             final int childCount = getChildCount();

             for (int i=0; i<childCount; i++) {
                 final View childView = getChildAt(i);
                 if (childView.getVisibility() != View.GONE) {
                     final int childWidth = childView.getMeasuredWidth();
                     childView.layout(childLeft, 0,
                             childLeft+childWidth, childView.getMeasuredHeight());
                     childLeft += childWidth;
                 }
             }
         }
     }
     @Override
     protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
         Log.e(TAG, "onMeasure");
         super.onMeasure(widthMeasureSpec, heightMeasureSpec);

         final int width = MeasureSpec.getSize(widthMeasureSpec);
         final int widthMode = MeasureSpec.getMode(widthMeasureSpec);
         if (widthMode != MeasureSpec.EXACTLY) {
             throw new IllegalStateException("ScrollLayout only canmCurScreen run at EXACTLY mode!");
         }

         final int heightMode = MeasureSpec.getMode(heightMeasureSpec);
         if (heightMode != MeasureSpec.EXACTLY) {
             throw new IllegalStateException("ScrollLayout only can run at EXACTLY mode!");
         }

         // The children are given the same width and height as the scrollLayout
         final int count = getChildCount();
         for (int i = 0; i < count; i++) {
             getChildAt(i).measure(widthMeasureSpec, heightMeasureSpec);
         }
         // Log.e(TAG, "moving to screen "+mCurScreen);
         scrollTo(mCurScreen * width, 0);
     }

     /**
      * According to the position of current layout
      * scroll to the destination page.
      */
     public void snapToDestination() {
         final int screenWidth = getWidth();
         final int destScreen = (getScrollX()+ screenWidth/2)/screenWidth;
         snapToScreen(destScreen);
     }

     public void snapToScreen(int whichScreen) {
         // get the valid layout page
         whichScreen = Math.max(0, Math.min(whichScreen, getChildCount()-1));
         if (getScrollX() != (whichScreen*getWidth())) {

             final int delta = whichScreen*getWidth()-getScrollX();
             mScroller.startScroll(getScrollX(), 0,
                     delta, 0, Math.abs(delta)*2);
             mCurScreen = whichScreen;
             invalidate();       // Redraw the layout
         }
     }

     public void setToScreen(int whichScreen) {
         whichScreen = Math.max(0, Math.min(whichScreen, getChildCount()-1));
         mCurScreen = whichScreen;
         scrollTo(whichScreen*getWidth(), 0);
     }

     public int getCurScreen() {
         return mCurScreen;
     }

     @Override
     public void computeScroll() {
         // TODO Auto-generated method stub
         if (mScroller.computeScrollOffset()) {
             scrollTo(mScroller.getCurrX(), mScroller.getCurrY());
             postInvalidate();
         }
     }
     @Override
     public boolean onTouchEvent(MotionEvent event) {
         // TODO Auto-generated method stub

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

         final int action = event.getAction();
         final float x = event.getX();
         final float y = event.getY();

         switch (action) {
         case MotionEvent.ACTION_DOWN:
             Log.e(TAG, "event down!");
             if (!mScroller.isFinished()){
                 mScroller.abortAnimation();
             }
             mLastMotionX = x;
             break;

         case MotionEvent.ACTION_MOVE:
             int deltaX = (int)(mLastMotionX - x);
             mLastMotionX = x;

             scrollBy(deltaX, 0);
             break;

         case MotionEvent.ACTION_UP:
             Log.e(TAG, "event : up");
             // if (mTouchState == TOUCH_STATE_SCROLLING) {
             final VelocityTracker velocityTracker = mVelocityTracker;
             velocityTracker.computeCurrentVelocity(1000);
             int velocityX = (int) velocityTracker.getXVelocity();
             Log.e(TAG, "velocityX:"+velocityX);

             if (velocityX > SNAP_VELOCITY && mCurScreen > 0) {
                 // Fling enough to move left
                 Log.e(TAG, "snap left");
                 snapToScreen(mCurScreen - 1);
             } else if (velocityX < -SNAP_VELOCITY
                     && mCurScreen < getChildCount() - 1) {
                 // Fling enough to move right
                 Log.e(TAG, "snap right");
                 snapToScreen(mCurScreen + 1);
             } else {
                 snapToDestination();
             }
             if (mVelocityTracker != null) {
                 mVelocityTracker.recycle();
                 mVelocityTracker = null;
             }
             // }
             mTouchState = TOUCH_STATE_REST;
             break;
         case MotionEvent.ACTION_CANCEL:
             mTouchState = TOUCH_STATE_REST;
             break;
         }

         return true;
     }
     @Override
     public boolean onInterceptTouchEvent(MotionEvent ev) {
         // TODO Auto-generated method stub
         Log.e(TAG, "onInterceptTouchEvent-slop:"+mTouchSlop);

         final int action = ev.getAction();
         if ((action == MotionEvent.ACTION_MOVE) &&
                 (mTouchState != TOUCH_STATE_REST)) {
             return true;
         }

         final float x = ev.getX();
         final float y = ev.getY();

         switch (action) {
         case MotionEvent.ACTION_MOVE:
             final int xDiff = (int)Math.abs(mLastMotionX-x);
             if (xDiff>mTouchSlop) {
                 mTouchState = TOUCH_STATE_SCROLLING;

             }
             break;

         case MotionEvent.ACTION_DOWN:
             mLastMotionX = x;
             mLastMotionY = y;
             mTouchState = mScroller.isFinished()? TOUCH_STATE_REST : TOUCH_STATE_SCROLLING;
             break;

         case MotionEvent.ACTION_CANCEL:
         case MotionEvent.ACTION_UP:
             mTouchState = TOUCH_STATE_REST;
             break;
         }

         return mTouchState != TOUCH_STATE_REST;
     }

 }

 

测试程序布局：

 <?xml version="1.0" encoding="utf-8"?>
 <com.yao_guet.test.ScrollLayout
   xmlns:android="http://schemas.android.com/apk/res/android"
   android:id="@+id/ScrollLayoutTest"
   android:layout_width="fill_parent"
   android:layout_height="fill_parent">
 <LinearLayout
   android:background="#FF00"
   android:layout_width="fill_parent"
   android:layout_height="fill_parent"></LinearLayout>

 <FrameLayout
   android:background="#F0F0"
   android:layout_width="fill_parent"
   android:layout_height="fill_parent"></FrameLayout>

 <FrameLayout
   android:background="#F00F"
   android:layout_width="fill_parent"
   android:layout_height="fill_parent">
   </FrameLayout>

 <LinearLayout
   android:background="#FF00"
   android:layout_width="fill_parent"
   android:layout_height="fill_parent">
   <Button
     android:layout_width="wrap_content"
     android:layout_height="wrap_content"
     android:text="Button1" />
   </LinearLayout>
 <LinearLayout
   android:layout_width="wrap_content"
   android:layout_height="wrap_content">
   <Button
     android:layout_width="wrap_content"
     android:layout_height="wrap_content"
     android:text="Button2" />
   </LinearLayout>
 </com.yao_guet.test.ScrollLayout>