Android常用三栏式滑动/滚动视图(View)的设计与实现(2)

package com.xxxxx.ui;

import android.content.Context;
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  
 */      
public class ScrollLayout extends ViewGroup {      
    private static final String TAG = "ScrollLayout";      
    private Scroller mScroller = null;      
    private VelocityTracker mVelocityTracker = null;      
          
    private int mCurScreen = 0;      
    private int mDefaultScreen = 1;      
          
    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 = 0;      
    private float mLastMotionX = 0;      
    private float mLastMotionY = 0;     
    
    public ScrollLayout(Context context, AttributeSet attrs) {      
        this(context, attrs, 0);
    }
   
    public ScrollLayout(Context context, AttributeSet attrs, int defStyle) {      
        super(context, attrs, defStyle);
        
        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 can 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() {      
        if (mScroller.computeScrollOffset()) {      
            scrollTo(mScroller.getCurrX(), mScroller.getCurrY());      
            postInvalidate();      
        }      
    }   
    
    @Override      
    public boolean onTouchEvent(MotionEvent event) {
        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) {       
        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;      
    }      
          
}