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, 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=""
  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>