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:
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.
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...
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。
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.
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:
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;
} 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.
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.
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.
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
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.
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:
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.