ViewDragHelper完全解析以及对自定义ViewGroup的作用

在自定义ViewGroup中，很多效果都包含用户手指去拖动其内部的某个View(eg:侧滑菜单等)，针对具体的需要去写好onInterceptTouchEvent和onTouchEvent这两个方法是一件很不容易的事，需要自己去处理：多手指的处理、加速度检测等等。 

好在官方在v4的支持包中提供了ViewDragHelper这样一个类来帮助我们方便的编写自定义ViewGroup。

 ViewDragHelper dragger = ViewDragHelper.create(this, 1f, new ViewDragHelper.Callback() {
            @Override
            public boolean tryCaptureView(View child, int pointerId) {
                return false;
            }

            @Override
            public int clampViewPositionHorizontal(View child, int left, int dx) {
                return super.clampViewPositionHorizontal(child, left, dx);
            }

            @Override
            public int clampViewPositionVertical(View child, int top, int dy) {
                return super.clampViewPositionVertical(child, top, dy);
            }
            // 手指释放时可以自动回去
            @Override
            public void onViewReleased(View releasedChild, float xvel, float yvel) {
                super.onViewReleased(releasedChild, xvel, yvel);
            }
            // 边界拖动时回调
            @Override
            public void onEdgeDragStarted(int edgeFlags, int pointerId) {
                super.onEdgeDragStarted(edgeFlags, pointerId);
            }
            /* 主要是因为，如果子View不消耗事件，那么整个手势（DOWN-MOVE*-UP）都是直接进入onTouchEvent，
            在onTouchEvent的DOWN的时候就确定了captureView。如果消耗事件，那么就会先走onInterceptTouchEvent方法，判断是否可以捕获，
            而在判断的过程中会去判断另外两个回调的方法：getViewHorizontalDragRange和getViewVerticalDragRange，只有这两个方法返回大于0的值才能正常的捕获。*/
            @Override
            public int getViewHorizontalDragRange(View child) {
                return super.getViewHorizontalDragRange(child);
            }
            @Override
            public int getViewVerticalDragRange(View child) {
                return super.getViewVerticalDragRange(child);
            }
            // 当ViewDragHelper状态发生变化时回调
            @Override
            public void onViewDragStateChanged(int state) {
                super.onViewDragStateChanged(state);
            }
            // 当捕获的View的位置改变时回调
            @Override
            public void onViewPositionChanged(View changedView, int left, int top, int dx, int dy) {
                super.onViewPositionChanged(changedView, left, top, dx, dy);
            }
            // 当captureview被捕获时回调
            @Override
            public void onViewCaptured(View capturedChild, int activePointerId) {
                super.onViewCaptured(capturedChild, activePointerId);
            }

            @Override
            public int getOrderedChildIndex(int index) {
                return super.getOrderedChildIndex(index);
            }
        });

ViewDragHelper还能做以下的一些操作：

• 边界检测、加速度检测(eg：DrawerLayout边界触发拉出)
• 回调Drag Release（eg：DrawerLayout部分，手指抬起，自动展开/收缩）
• 移动到某个指定的位置(eg:点击Button，展开/关闭Drawerlayout)

那么我们接下来对我们最基本的例子进行改造，包含上述的几个操作。

package com.zhy.learn.view;

import android.content.Context;
import android.graphics.Point;
import android.support.v4.widget.ViewDragHelper;
import android.util.AttributeSet;
import android.view.MotionEvent;
import android.view.View;
import android.widget.LinearLayout;

/**
 * Created by zhy on 15/6/3.
 */
public class VDHLayout extends LinearLayout
{
    private ViewDragHelper mDragger;

    private View mDragView;
    private View mAutoBackView;
    private View mEdgeTrackerView;

    private Point mAutoBackOriginPos = new Point();

    public VDHLayout(Context context, AttributeSet attrs)
    {
        super(context, attrs);
        mDragger = ViewDragHelper.create(this, 1.0f, new ViewDragHelper.Callback()
        {
            @Override
            public boolean tryCaptureView(View child, int pointerId)
            {
                //mEdgeTrackerView禁止直接移动
                return child == mDragView || child == mAutoBackView;
            }

            @Override
            public int clampViewPositionHorizontal(View child, int left, int dx)
            {
                return left;
            }

            @Override
            public int clampViewPositionVertical(View child, int top, int dy)
            {
                return top;
            }


            //手指释放的时候回调
            @Override
            public void onViewReleased(View releasedChild, float xvel, float yvel)
            {
                //mAutoBackView手指释放时可以自动回去
                if (releasedChild == mAutoBackView)
                {
                    mDragger.settleCapturedViewAt(mAutoBackOriginPos.x, mAutoBackOriginPos.y);
                    invalidate();
                }
            }

            //在边界拖动时回调
            @Override
            public void onEdgeDragStarted(int edgeFlags, int pointerId)
            {
                mDragger.captureChildView(mEdgeTrackerView, pointerId);
            }
        });
        mDragger.setEdgeTrackingEnabled(ViewDragHelper.EDGE_LEFT);
    }


    @Override
    public boolean onInterceptTouchEvent(MotionEvent event)
    {
        return mDragger.shouldInterceptTouchEvent(event);
    }

    @Override
    public boolean onTouchEvent(MotionEvent event)
    {
        mDragger.processTouchEvent(event);
        return true;
    }

    @Override
    public void computeScroll()
    {
        if(mDragger.continueSettling(true))
        {
            invalidate();
        }
    }

    @Override
    protected void onLayout(boolean changed, int l, int t, int r, int b)
    {
        super.onLayout(changed, l, t, r, b);

        mAutoBackOriginPos.x = mAutoBackView.getLeft();
        mAutoBackOriginPos.y = mAutoBackView.getTop();
    }

    @Override
    protected void onFinishInflate()
    {
        super.onFinishInflate();

        mDragView = getChildAt(0);
        mAutoBackView = getChildAt(1);
        mEdgeTrackerView = getChildAt(2);
    }
}

http://blog.youkuaiyun.com/lmj623565791/article/details/46858663#t4