Android应用ViewDragHelper详解及部分源码浅析

@Override

public void computeScroll() {

if (mTopViewDragHelper.continueSettling(true)) {

invalidate();

}

}

public void closeDrawer() {

if (mIsOpen) {

mTopViewDragHelper.smoothSlideViewTo(mDrawerView, mDrawerView.getLeft(), -mDrawerView.getHeight());

invalidate();

}

}

public void openDrawer() {

if (!mIsOpen) {

mTopViewDragHelper.smoothSlideViewTo(mDrawerView, mDrawerView.getLeft(), 0);

invalidate();

}

}

public boolean isDrawerOpened() {

return mIsOpen;

}

//Step3：重写onInterceptTouchEvent回调ViewDragHelper中对应的方法.

@Override

public boolean onInterceptTouchEvent(MotionEvent ev) {

return mTopViewDragHelper.shouldInterceptTouchEvent(ev);

}

//Step3：重写onTouchEvent回调ViewDragHelper中对应的方法.

@Override

public boolean onTouchEvent(MotionEvent event) {

mTopViewDragHelper.processTouchEvent(event);

return true;

}

@Override

protected LayoutParams generateDefaultLayoutParams() {

return new MarginLayoutParams(LayoutParams.MATCH_PARENT, LayoutParams.MATCH_PARENT);

}

@Override

protected LayoutParams generateLayoutParams(LayoutParams p) {

return new MarginLayoutParams§;

}

@Override

public LayoutParams generateLayoutParams(AttributeSet attrs) {

return new MarginLayoutParams(getContext(), attrs);

}

@Override

protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {

int measureWidth = MeasureSpec.getSize(widthMeasureSpec);

int measureHeight = MeasureSpec.getSize(heightMeasureSpec);

setMeasuredDimension(measureWidth, measureHeight);

mContentView = getChildAt(0);

mDrawerView = getChildAt(1);

MarginLayoutParams params = (MarginLayoutParams) mContentView.getLayoutParams();

int childWidthMeasureSpec = MeasureSpec.makeMeasureSpec(

measureWidth- (params.leftMargin + params.rightMargin), MeasureSpec.EXACTLY);

int childHeightMeasureSpec = MeasureSpec.makeMeasureSpec(

measureHeight - (params.topMargin + params.bottomMargin), MeasureSpec.EXACTLY);

mContentView.measure(childWidthMeasureSpec, childHeightMeasureSpec);

mDrawerView.measure(widthMeasureSpec, heightMeasureSpec);

}

@Override

protected void onLayout(boolean changed, int l, int t, int r, int b) {

if (changed) {

MarginLayoutParams params = (MarginLayoutParams) mContentView.getLayoutParams();

mContentView.layout(params.leftMargin, params.topMargin,

mContentView.getMeasuredWidth() + params.leftMargin,

mContentView.getMeasuredHeight() + params.topMargin);

params = (MarginLayoutParams) mDrawerView.getLayoutParams();

mDrawerView.layout(params.leftMargin, mCurTop + params.topMargin,

mDrawerView.getMeasuredWidth() + params.leftMargin,

mCurTop + mDrawerView.getMeasuredHeight() + params.topMargin);

}

}

}

怎么样，简单吧。效果也有了，代码也有了，ViewDragHelper也体验了，接下来就该苦逼的看源码了。

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3 ViewDragHelper局部源码浅析

==========================

上面的例子中我们可以知道，使用ViewDragHelper的第一步就是通过他提供的静态工厂方法create获取实例，因为ViewDragHelper的构造方法是私有的。既然这样那我们先看下这些静态工厂方法，如下：

public class ViewDragHelper {

public static ViewDragHelper create(ViewGroup forParent, float sensitivity, Callback cb) {

final ViewDragHelper helper = create(forParent, cb);

helper.mTouchSlop = (int) (helper.mTouchSlop * (1 / sensitivity));

return helper;

}

public static ViewDragHelper create(ViewGroup forParent, Callback cb) {

return new ViewDragHelper(forParent.getContext(), forParent, cb);

}

}

可以看见，三个参数的create方法实质调运的还是两个参数的create。其中forParent一般是我们自定义的ViewGroup，cb是控制子View相关状态的回调抽象类实现对象，sensitivity是用来调节mTouchSlop的，至于mTouchSlop是啥以及sensitivity的作用下面会有解释。接着可以发现两个参数的create实质是调运了ViewDragHelper的构造函数，那我们就来分析一下这个构造函数，如下源码：

private ViewDragHelper(Context context, ViewGroup forParent, Callback cb) {

…

//对参数进行赋值

mParentView = forParent;

mCallback = cb;

//通过ViewConfiguration等将dp转px得到mEdgeSize

final ViewConfiguration vc = ViewConfiguration.get(context);

final float density = context.getResources().getDisplayMetrics().density;

mEdgeSize = (int) (EDGE_SIZE * density + 0.5f);

//通过ViewConfiguration获取TouchSlop，默认为8

mTouchSlop = vc.getScaledTouchSlop();

//获得允许执行一个fling手势动作的最大速度值

mMaxVelocity = vc.getScaledMaximumFlingVelocity();

//获得允许执行一个fling手势动作的最小速度值

mMinVelocity = vc.getScaledMinimumFlingVelocity();

//通过兼容包的ScrollerCompat实例化Scroller，动画插值器为sInterpolator

mScroller = ScrollerCompat.create(context, sInterpolator);

}

可以看见，构造函数其实没有做啥特别的事情，主要就是一些参数的实例化，最主要的就是实例化了一个Scroller的内部成员，而且还在ViewDragHelper中重写了插值器，如下：

private static final Interpolator sInterpolator = new Interpolator() {

public float getInterpolation(float t) {

t -= 1.0f;

return t * t * t * t * t + 1.0f;

}

};

关于动画插值器这里不再说了，之前博文有讲过。我们还是把视线回到上面实例部分，可以看见，在获取ViewDragHelper实例之后我们接着重写了ViewGroup的onInterceptTouchEvent和onTouchEvent方法，在其中触发了ViewDragHelper的shouldInterceptTouchEvent和processTouchEvent方法。所以下面我们就来分析这两个方法，首先我们看下shouldInterceptTouchEvent方法，如下：

//这玩意返回值的作用在前面博客中有分析，我们先来看下ACTION_DOWN事件

public boolean shouldInterceptTouchEvent(MotionEvent ev) {

final int action = MotionEventCompat.getActionMasked(ev);

final int actionIndex = MotionEventCompat.getActionIndex(ev);

if (action == MotionEvent.ACTION_DOWN) {

//每次ACTION_DOWN都会调用cancel()，该方法中mVelocityTracker被清空&#x