Android功能总结：仿照Launcher的Workspace实现左右滑动切换

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) {    
        super(context);    
        // 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>