ViewDragHelper源码解析
1.简介
我们了解了ViewDragHelper是可以帮助我们处理各种拖拽事件的类.使用好ViewDragHelper能帮助我们做出各种酷炫的交互,今天我们就来分析一下ViewDragHelper的使用与实现:
2.使用方法
我们这里就以翔总的这篇文章中的例子来介绍一下ViewDragHelper的使用.另外,本文中的demo可以在 这里找到
首先我们创建一个DragLayout类并继承自LinearLayout,然后我们准备在DragLayout放置三个View第一个用来被我们拖动然后停止在松手的位置,第二个可以被我们拖动,松手的时候滑动到指定位置,第三个只可以通过触摸边缘来进行拖动,
public class DragLayout extends LinearLayout { private ViewDragHelper mDragger; private View mDragView; private View mAutoBackView; private View mEdgeTrackerView; private Point mAutoBackOriginPos = new Point(); public DragLayout(Context context) { this(context, null); } public DragLayout(Context context, AttributeSet attrs) { this(context, attrs, 0); } public DragLayout(Context context, AttributeSet attrs, int defStyleAttr) { super(context, attrs, defStyleAttr); initViewDragHelper(); } private void initViewDragHelper() { mDragger = ViewDragHelper.create(this,myCallback); mDragger.setEdgeTrackingEnabled(ViewDragHelper.EDGE_ALL); } ViewDragHelper.Callback myCallback = new ViewDragHelper.Callback() { @Override //child为当前触摸区域下的View,如果返回true,就可以拖拽. public boolean tryCaptureView(View child, int pointerId) { return child == mDragView || child == mAutoBackView; } //松手时的回调 @Override public void onViewReleased(View releasedChild, float xvel, float yvel) { if (releasedChild == mAutoBackView) { mDragger.settleCapturedViewAt(mAutoBackOriginPos.x, mAutoBackOriginPos.y); invalidate(); } } //边缘触摸开始时的回调 @Override public void onEdgeDragStarted(int edgeFlags, int pointerId) { mDragger.captureChildView(mEdgeTrackerView, pointerId); } //获取水平方向允许拖拽的区域,这里是父布局的宽-子控件的宽 @Override public int getViewHorizontalDragRange(View child) { return getMeasuredWidth() - child.getMeasuredWidth(); } //获取垂直方向允许拖拽的范围 @Override public int getViewVerticalDragRange(View child) { return getMeasuredHeight() - child.getMeasuredHeight(); } //left为child即将移动到的水平位置的值,但是返回值会最终决定移动到的值 //这里直接返回了left @Override public int clampViewPositionHorizontal(View child, int left, int dx) { return left; } //同上只是这里是垂直方向 @Override public int clampViewPositionVertical(View child, int top, int dy) { return top; } }; @Override public void computeScroll() { if (mDragger.continueSettling(true)) { invalidate(); } } @Override public boolean onInterceptTouchEvent(MotionEvent ev) { return mDragger.shouldInterceptTouchEvent(ev); } @Override public boolean onTouchEvent(MotionEvent event) { mDragger.processTouchEvent(event); return true; } @Override protected void onFinishInflate() { super.onFinishInflate(); mDragView = getChildAt(0); mAutoBackView = getChildAt(1); mEdgeTrackerView = getChildAt(2); } @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(); } }
- 我们首先在构造方法里传入了当前类的对象和我们定义的
ViewDragHelper.Callback对象初始化了我们的ViewDragHelper,然后我们希望所有的边缘触摸都能触发mEdgeTrackerView的拖动,所以我们紧接着调用了mDragger.setEdgeTrackingEnabled(ViewDragHelper.EDGE_ALL);方法. - 在我们定义的
Callback中,有多个回调方法,每个回调方法都有它的作用,在代码里注释比较清楚了，我们下面也会解析每一个Callback中回调方法的作用. - 第三步我们需要在
onInterceptTouchEvent()方法和onTouchEvent()将事件委托给ViewDragHelper去处理,这样ViewDragHelper才能根据响应的事件并回调我们自己编写的Callback接口来进行响应的处理, - 由于
ViewDragHelper中的滑动是交给Srcoller类来处理的所以这里我们要重写computeScroll()方法,配合Scroller完成滚动动画. - 最后在
onFinishInflate()里获取到我们的View对象即可.
3.类关系图
由于就一个类类图我们就不画了,但是作为一个强迫症患者,这个标题必须有…
4.源码分析
1.ViewDragHelper.Callback的实现
在分析ViewDragHelper之前,我们先来分析一下Callback的定义,看看Callback都定义了哪些方法:
public static abstract class Callback { //当View的拖拽状态改变时回调,state为STATE_IDLE,STATE_DRAGGING,STATE_SETTLING的一种 //STATE_IDLE: 当前未被拖拽 //STATE_DRAGGING：正在被拖拽 //STATE_SETTLING: 被拖拽后需要被安放到一个位置中的状态 public void onViewDragStateChanged(int state) {} //当View被拖拽位置发生改变时回调 //changedView ：被拖拽的View //left : 被拖拽后View的left边缘坐标 //top : 被拖拽后View的top边缘坐标 //dx : 拖动的x偏移量 //dy : 拖动的y偏移量 public void onViewPositionChanged(View changedView, int left, int top, int dx, int dy) {} //当一个View被捕获到准备开始拖动时回调, //capturedChild : 捕获的View //activePointerId : 对应的PointerId public void onViewCaptured(View capturedChild, int activePointerId) {} //当被捕获拖拽的View被释放是回调 //releasedChild : 被释放的View //xvel : 释放View的x方向上的加速度 //yvel : 释放View的y方向上的加速度 public void onViewReleased(View releasedChild, float xvel, float yvel) {} //如果parentView订阅了边缘触摸,则如果有边缘触摸就回调的接口 //edgeFlags : 当前触摸的flag 有: EDGE_LEFT,EDGE_TOP,EDGE_RIGHT,EDGE_BOTTOM //pointerId : 用来描述边缘触摸操作的id public void onEdgeTouched(int edgeFlags, int pointerId) {} //是否锁定该边缘的触摸,默认返回false,返回true表示锁定 public boolean onEdgeLock(int edgeFlags) { return false; } //边缘触摸开始时回调 //edgeFlags : 当前触摸的flag 有: EDGE_LEFT,EDGE_TOP,EDGE_RIGHT,EDGE_BOTTOM //pointerId : 用来描述边缘触摸操作的id public void onEdgeDragStarted(int edgeFlags, int pointerId) {} //在寻找当前触摸点下的子View时会调用此方法，寻找到的View会提供给tryCaptureViewForDrag()来尝试捕获。 //如果需要改变子View的遍历查询顺序可改写此方法，例如让下层的View优先于上层的View被选中。 public int getOrderedChildIndex(int index) { return index; } //获取被拖拽View child 的水平拖拽范围,返回0表示无法被水平拖拽 public int getViewHorizontalDragRange(View child) { return 0; } //获取被拖拽View child 的垂直拖拽范围,返回0表示无法被水平拖拽 public int getViewVerticalDragRange(View child) { return 0; } //尝试捕获被拖拽的View public abstract boolean tryCaptureView(View child, int pointerId); //决定拖拽View在水平方向上应该移动到的位置 //child : 被拖拽的View //left : 期望移动到位置的View的left值 //dx : 移动的水平距离 //返回值 : 直接决定View在水平方向的位置 public int clampViewPositionHorizontal(View child, int left, int dx) { return 0; } //决定拖拽View在垂直方向上应该移动到的位置 //child : 被拖拽的View //top : 期望移动到位置的View的top值 //dy : 移动的垂直距离 //返回值 : 直接决定View在垂直方向的位置 public int clampViewPositionVertical(View child, int top, int dy) { return 0; } }
想必注释已经很清楚了,正是这些回调方法,再结合ViewDragHelper中的各种方法,来帮助我们实现各种各样的拖拽的效果。
2.shouldInterceptTouchEvent()方法的实现
在这里我们假设大家都清楚了Android的事件分发机制,如果不清楚请看这里,要想处理触摸事件,我们需要在onInterceptTouchEvent(MotionEvent ev)方法里判断是否需要拦截这次触摸事件,如果此方法返回true则触摸事件将会交给onTouchEvent(MotionEvent event)处理,这样我们就能处理触摸事件了,所以我们在上面的使用方法里会这样写:
@Override public boolean onInterceptTouchEvent(MotionEvent ev) { return mDragger.shouldInterceptTouchEvent(ev); } @Override public boolean onTouchEvent(MotionEvent event) { mDragger.processTouchEvent(event); return true; }
这样就将是否拦截触摸事件,以及处理触摸事件委托给ViewDragHelper来处理了,所以我们先来看看ViewDragHelper中shouldInterceptTouchEvent();方法的实现:
public boolean shouldInterceptTouchEvent(MotionEvent ev) { //获取action final int action = MotionEventCompat.getActionMasked(ev); //获取action对应的index final int actionIndex = MotionEventCompat.getActionIndex(ev); //如果是按下的action则重置一些信息,包括各种事件点的数组 if (action == MotionEvent.ACTION_DOWN) { // Reset things for a new event stream, just in case we didn't get // the whole previous stream. cancel(); } //初始化mVelocityTracker if (mVelocityTracker == null) { mVelocityTracker = VelocityTracker.obtain(); } mVelocityTracker.addMovement(ev); //根据action来做相应的处理 switch (action) { case MotionEvent.ACTION_DOWN: { final float x = ev.getX(); final float y = ev.getY(); //获取这个事件对应的pointerId,一般情况下只有一个手指触摸时为0 //两个手指触摸时第二个手指触摸返回的pointerId为1，以此类推 final int pointerId = MotionEventCompat.getPointerId(ev, 0); //保存点的数据 //TODO (1) saveInitialMotion(x, y, pointerId); //获取当前触摸点下最顶层的子View //TODO (2) final View toCapture = findTopChildUnder((int) x, (int) y); //如果toCapture是已经捕获的View,而且正在处于被释放状态 //那么就重新捕获 if (toCapture == mCapturedView && mDragState == STATE_SETTLING) { tryCaptureViewForDrag(toCapture, pointerId); } //如果触摸了边缘,回调callback的onEdgeTouched()方法 final int edgesTouched = mInitialEdgesTouched[pointerId]; if ((edgesTouched & mTrackingEdges) != 0) { mCallback.onEdgeTouched(edgesTouched & mTrackingEdges, pointerId); } break; } //当又有一个手指触摸时 case MotionEventCompat.ACTION_POINTER_DOWN: { final int pointerId = MotionEventCompat.getPointerId(ev, actionIndex); final float x = MotionEventCompat.getX(ev, actionIndex); final float y = MotionEventCompat.getY(ev, actionIndex); //保存触摸信息 saveInitialMotion(x, y, pointerId); //因为同一时间ViewDragHelper只能操控一个View,所以当有新的手指触摸时 //只讨论当无触摸发生时,回调边缘触摸的callback //或者正在处于释放状态时重新捕获View if (mDragState == STATE_IDLE) { final int edgesTouched = mInitialEdgesTouched[pointerId]; if ((edgesTouched & mTrackingEdges) != 0) { mCallback.onEdgeTouched(edgesTouched & mTrackingEdges, pointerId); } } else if (mDragState == STATE_SETTLING) { // Catch a settling view if possible. final View toCapture = findTopChildUnder((int) x, (int) y); if (toCapture == mCapturedView) { tryCaptureViewForDrag(toCapture, pointerId); } } break; } //当手指移动时 case MotionEvent.ACTION_MOVE: { if (mInitialMotionX == null || mInitialMotionY == null) break; // First to cross a touch slop over a draggable view wins. Also report edge drags. //得到触摸点的数量,并循环处理,只处理第一个发生了拖拽的事件 final int pointerCount = MotionEventCompat.getPointerCount(ev); for (int i = 0; i < pointerCount; i++) { final int pointerId = MotionEventCompat.getPointerId(ev, i); final float x = MotionEventCompat.getX(ev, i); final float y = MotionEventCompat.getY(ev, i); //获得拖拽偏移量 final float dx = x - mInitialMotionX[pointerId]; final float dy = y - mInitialMotionY[pointerId]; //获取当前触摸点下最顶层的子View final View toCapture = findTopChildUnder((int) x, (int) y); //如果找到了最顶层View,并且产生了拖动(checkTouchSlop()返回true) //TODO (3) final boolean pastSlop = toCapture != null && checkTouchSlop(toCapture, dx, dy); if (pastSlop) { //根据callback的四个方法getView[Horizontal|Vertical]DragRange和 //clampViewPosition[Horizontal|Vertical]来检查是否可以拖动 final int oldLeft = toCapture.getLeft(); final int targetLeft = oldLeft + (int) dx; final int newLeft = mCallback.clampViewPositionHorizontal(toCapture, targetLeft, (int) dx); final int oldTop = toCapture.getTop(); final int targetTop = oldTop + (int) dy; final int newTop = mCallback.clampViewPositionVertical(toCapture, targetTop, (int) dy); final int horizontalDragRange = mCallback.getViewHorizontalDragRange( toCapture); final int verticalDragRange = mCallback.getViewVerticalDragRange(toCapture); //如果都不允许移动则跳出循环 if ((horizontalDragRange == 0 || horizontalDragRange > 0 && newLeft == oldLeft) && (verticalDragRange == 0 || verticalDragRange > 0 && newTop == oldTop)) { break; } } //记录并回调是否有边缘触摸 reportNewEdgeDrags(dx, dy, pointerId); if (mDragState == STATE_DRAGGING) { // Callback might have started an edge drag break; } //如果产生了拖动则调用tryCaptureViewForDrag() //TODO (4) if (pastSlop && tryCaptureViewForDrag(toCapture, pointerId)) { break; } } //保存触摸点的信息 saveLastMotion(ev); break; } //当有一个手指抬起时,清除这个手指的触摸数据 case MotionEventCompat.ACTION_POINTER_UP: { final int pointerId = MotionEventCompat.getPointerId(ev, actionIndex); clearMotionHistory(pointerId); break; } //清除所有触摸数据 case MotionEvent.ACTION_UP: case MotionEvent.ACTION_CANCEL: { cancel(); break; } } //如果mDragState等于正在拖拽则返回true return mDragState == STATE_DRAGGING; }
上面就是整个shouldInterceptTouchEvent()的实现,上面的注释也足够清楚了,我们这里就先不分析某一种触摸事件,大家可以看到我上面留了几个TODO,下文会一起分析,这里我假设大家都已经对触摸事件分发处理都有充分的理解了,我们下面就直接看ViewDragHelper里processTouchEvent()方法的实现.
3.processTouchEvent()方法的实现
public void processTouchEvent(MotionEvent ev) { final int action = MotionEventCompat.getActionMasked(ev); final int actionIndex = MotionEventCompat.getActionIndex(ev); ...（省去部分代码） switch (action) { case MotionEvent.ACTION_DOWN: { ...（省去部分代码） break; } case MotionEventCompat.ACTION_POINTER_DOWN: { ...（省去部分代码） break; } case MotionEvent.ACTION_MOVE: { //如果现在已经是拖拽状态 if (mDragState == STATE_DRAGGING) { final int index = MotionEventCompat.findPointerIndex(ev, mActivePointerId); final float x = MotionEventCompat.getX(ev, index); final float y = MotionEventCompat.getY(ev, index); final int idx = (int) (x - mLastMotionX[mActivePointerId]); final int idy = (int) (y - mLastMotionY[mActivePointerId]); //拖拽至指定位置 //TODO (5) dragTo(mCapturedView.getLeft() + idx, mCapturedView.getTop() + idy, idx, idy); saveLastMotion(ev); } else { // Check to see if any pointer is now over a draggable view. //如果还不是拖拽状态,就检测是否经过了一个View final int pointerCount = MotionEventCompat.getPointerCount(ev); for (int i = 0; i < pointerCount; i++) { final int pointerId = MotionEventCompat.getPointerId(ev, i); final float x = MotionEventCompat.getX(ev, i); final float y = MotionEventCompat.getY(ev, i); final float dx = x - mInitialMotionX[pointerId]; final float dy = y - mInitialMotionY[pointerId]; reportNewEdgeDrags(dx, dy, pointerId); if (mDragState == STATE_DRAGGING) { // Callback might have started an edge drag. break; } final View toCapture = findTopChildUnder((int) x, (int) y); if (checkTouchSlop(toCapture, dx, dy) && tryCaptureViewForDrag(toCapture, pointerId)) { break; } } saveLastMotion(ev); } break; } //当多个手指中的一个手机松开时 case MotionEventCompat.ACTION_POINTER_UP: { final int pointerId = MotionEventCompat.getPointerId(ev, actionIndex); //如果当前点正在被拖拽,则再剩余还在触摸的点钟寻找是否正在View上 if (mDragState == STATE_DRAGGING && pointerId == mActivePointerId) { // Try to find another pointer that's still holding on to the captured view. int newActivePointer = INVALID_POINTER; final int pointerCount = MotionEventCompat.getPointerCount(ev); for (int i = 0; i < pointerCount; i++) { final int id = MotionEventCompat.getPointerId(ev, i); if (id == mActivePointerId) { // This one's going away, skip. continue; } final float x = MotionEventCompat.getX(ev, i); final float y = MotionEventCompat.getY(ev, i); if (findTopChildUnder((int) x, (int) y) == mCapturedView && tryCaptureViewForDrag(mCapturedView, id)) { newActivePointer = mActivePointerId; break; } } if (newActivePointer == INVALID_POINTER) { // We didn't find another pointer still touching the view, release it. //如果没找到则释放View //TODO (6) releaseViewForPointerUp(); } } clearMotionHistory(pointerId); break; } case MotionEvent.ACTION_UP: { //如果是拖拽状态的释放则调用 //releaseViewForPointerUp() if (mDragState == STATE_DRAGGING) { releaseViewForPointerUp(); } cancel(); break; } case MotionEvent.ACTION_CANCEL: { if (mDragState == STATE_DRAGGING) { dispatchViewReleased(0, 0); } cancel(); break; } } }
上面就是processTouchEvent()方法的实现,我们省去了部分大致与shouldInterceptTouchEvent()相同的逻辑代码,通过事件传递机制我们知道,如果程序已经进入到processTouchEvent()中,也就意味着触摸事件就不会再向下传递,都会交给此方法处理,所以在这里我们就需要处理拖拽事件了,通过上面的注释,我们也看到了在MotionEvent.ACTION_MOVE,MotionEventCompat.ACTION_POINTER_UP,MotionEvent.ACTION_UP和MotionEvent.ACTION_CANCEL都分别进行了处理 ,我们知道触摸事件大致的流程是:
ACTION_DOWN -> ACTION_MOVE -> ... -> ACTION_MOVE -> ACTION_UP
再配合事件的分发机制,我们就能很清晰的分析出一次完整的事件调用过程,所以整个ViewDragHelper的拖拽过程也能很清晰的分为三个步骤:
捕获拖拽目标View -> 拖拽目标View -> 处理目标View释放操作
最后我们再分析上面两段代码的6个TODO:
4.saveInitialMotion()方法
private void saveInitialMotion(float x, float y, int pointerId) { //确保各个数组的大小足够存放数据 ensureMotionHistorySizeForId(pointerId); //保存x坐标 mInitialMotionX[pointerId] = mLastMotionX[pointerId] = x; //保存y坐标 mInitialMotionY[pointerId] = mLastMotionY[pointerId] = y; //保存是否触摸到边缘 mInitialEdgesTouched[pointerId] = getEdgesTouched((int) x, (int) y); //保存当前id是否在触摸,用于后续验证 mPointersDown |= 1 << pointerId; }
5.findTopChildUnder()方法
public View findTopChildUnder(int x, int y) { final int childCount = mParentView.getChildCount(); for (int i = childCount - 1; i >= 0; i--) { final View child = mParentView.getChildAt(mCallback.getOrderedChildIndex(i)); if (x >= child.getLeft() && x < child.getRight() && y >= child.getTop() && y < child.getBottom()) { return child; } } return null; }
代码很简单就是根据x和y坐标和来找到指定View,注意这里回调了callback中的getOrderedChildIndex()方法,所以我们可以在这里返回指定的View的index.
6.checkTouchSlop()方法
private boolean checkTouchSlop(View child, float dx, float dy) { if (child == null) { return false; } final boolean checkHorizontal = mCallback.getViewHorizontalDragRange(child) > 0; final boolean checkVertical = mCallback.getViewVerticalDragRange(child) > 0; if (checkHorizontal && checkVertical) { return dx * dx + dy * dy > mTouchSlop * mTouchSlop; } else if (checkHorizontal) { return Math.abs(dx) > mTouchSlop; } else if (checkVertical) { return Math.abs(dy) > mTouchSlop; } return false; }
用来根据mTouchSlop最小拖动的距离来判断是否属于拖动,mTouchSlop根据我们设定的灵敏度决定.
7.tryCaptureViewForDrag()方法
boolean tryCaptureViewForDrag(View toCapture, int pointerId) { //如果已经捕获该View 直接返回true if (toCapture == mCapturedView && mActivePointerId == pointerId) { // Already done! return true; } //根据mCallback.tryCaptureView()方法来最终决定是否可以捕获View if (toCapture != null && mCallback.tryCaptureView(toCapture, pointerId)) { mActivePointerId = pointerId; //如果可以则调用captureChildView(),并返回true captureChildView(toCapture, pointerId); return true; } return false; }
可以看到如果可以捕获View则调用了captureChildView()方法:
public void captureChildView(View childView, int activePointerId) { if (childView.getParent() != mParentView) { throw new IllegalArgumentException("captureChildView: parameter must be a descendant " + "of the ViewDragHelper's tracked parent view (" + mParentView + ")"); } //赋值mCapturedView mCapturedView = childView; mActivePointerId = activePointerId; //回调callback mCallback.onViewCaptured(childView, activePointerId); //设定mDragState的状态为STATE_DRAGGING setDragState(STATE_DRAGGING); }
如果程序执行到这里,就证明View已经处于拖拽状态了,后续的触摸操作,将直接根据mDragState为STATE_DRAGGING的状态处理.
8.dragTo()方法的实现
private void dragTo(int left, int top, int dx, int dy) { int clampedX = left; int clampedY = top; final int oldLeft = mCapturedView.getLeft(); final int oldTop = mCapturedView.getTop(); if (dx != 0) { //回调callback来决定View最终被拖拽的x方向上的偏移量 clampedX = mCallback.clampViewPositionHorizontal(mCapturedView, left, dx); //移动View ViewCompat.offsetLeftAndRight(mCapturedView, clampedX - oldLeft); } if (dy != 0) { //回调callback来决定View最终被拖拽的y方向上的偏移量 clampedY = mCallback.clampViewPositionVertical(mCapturedView, top, dy); //移动View ViewCompat.offsetTopAndBottom(mCapturedView, clampedY - oldTop); } if (dx != 0 || dy != 0) { final int clampedDx = clampedX - oldLeft; final int clampedDy = clampedY - oldTop; //回调callback mCallback.onViewPositionChanged(mCapturedView, clampedX, clampedY, clampedDx, clampedDy); } }
因为dragTo()方法是在processTouchEvent()中的MotionEvent.ACTION_MOVE case被调用所以当程序运行到这里时View就会不断的被拖动了。如果一旦手指释放则最终会调用releaseViewForPointerUp()方法
8.releaseViewForPointerUp()方法的实现
private void releaseViewForPointerUp() { //计算出当前x和y方向上的加速度 mVelocityTracker.computeCurrentVelocity(1000, mMaxVelocity); final float xvel = clampMag( VelocityTrackerCompat.getXVelocity(mVelocityTracker, mActivePointerId), mMinVelocity, mMaxVelocity); final float yvel = clampMag( VelocityTrackerCompat.getYVelocity(mVelocityTracker, mActivePointerId), mMinVelocity, mMaxVelocity); dispatchViewReleased(xvel, yvel); }
计算完加速度后就调用了dispatchViewReleased():
private void dispatchViewReleased(float xvel, float yvel) { //设定当前正处于释放阶段 mReleaseInProgress = true; //回调callback的onViewReleased()方法 mCallback.onViewReleased(mCapturedView, xvel, yvel); mReleaseInProgress = false; //设定状态 if (mDragState == STATE_DRAGGING) { // onViewReleased didn't call a method that would have changed this. Go idle. //如果onViewReleased()中没有调用任何方法,则状态设定为STATE_IDLE setDragState(STATE_IDLE); } }
所以最后释放后的处理交给了callback中的onViewReleased()方法,如果我们什么都不做,那么这个被拖拽的View就是停止在当前位置,或者我们可以调用ViewDragHelper提供给我们的这几个方法:
- settleCapturedViewAt(int finalLeft, int finalTop) 以松手前的滑动速度为初速动，让捕获到的View自动滚动到指定位置。只能在Callback的onViewReleased()中调用。
- flingCapturedView(int minLeft, int minTop, int maxLeft, int maxTop) 以松手前的滑动速度为初速动，让捕获到的View在指定范围内fling。只能在Callback的onViewReleased()中调用。
- smoothSlideViewTo(View child, int finalLeft, int finalTop) 指定某个View自动滚动到指定的位置，初速度为0，可在任何地方调用。
引用自这篇文章,具体释放后的原理我们就不分析了,其实就是配合Scroller这个类来实现,具体也可以参照上面这篇文章。好,我们关于ViewDragHelper的源码分析就到这里.
5.开源项目中的使用
ViewDragHelper在各种关于拖拽和各种手势动画的开源库中使用广泛,我这里就简要列出一些,大家可以多去看看是如何使用ViewDragHelper的:
6.个人评价
ViewDragHelper的出现,大大简化了我们开发相关触摸和拖拽功能的复杂度和代码量,帮助我们比较容易的实现各种效果,让我们开发酷炫的交互更加容易了。但是从一些开源项目中发现,ViewDragHelper中还是有一些不足之处,比如给Scroller提供了一个固定的Interpolator,导致如果我们想实现例如反弹效果的话,还要把ViewDragHelper的代码拷贝一份并修改Interpolator,这样做肯定是不太好的.当然建议我们自己修改一个ViewDragHelper后如果项目里有多处使用,可以包装成一个提供给我们自己项目的模块使用,防止出现更多的多余代码.
