ViewDragHelper是个工具类，用来辅助ViewGroup内控件的操作和拖拽。

1. ViewDragHelper创建

ViewDragHelper通过静态方法create()创建，

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

Callback是一个回调，用来处理事件，
主要方法

// 是否需要捕获这个child
abstract boolean tryCaptureView(View child, int pointerId)// 修改水平位置
public int clampViewPositionHorizontal(View child, int left, int dx)// 修改垂直位置
public int clampViewPositionVertical(View child, int top, int dy)

2. 简单的拖拽实现

自定义ViewDragHelperView实现FrameLayout，在onInterceptTouchEvent()和onTouchEvent()方法中，添加ViewDragHelperView进行拦截处理。

public class ViewDragHelperView extends FrameLayout {private ViewDragHelper mViewDragHelper;public ViewDragHelperView(@NonNull Context context) {this(context, null);}public ViewDragHelperView(@NonNull Context context, @Nullable AttributeSet attrs) {this(context, attrs, 0);}public ViewDragHelperView(@NonNull Context context, @Nullable AttributeSet attrs, int defStyleAttr) {super(context, attrs, defStyleAttr);mViewDragHelper = ViewDragHelper.create(this, new CustomCallback());}@Overridepublic boolean onInterceptTouchEvent(MotionEvent ev) {return mViewDragHelper.shouldInterceptTouchEvent(ev);}@Overridepublic boolean onTouchEvent(MotionEvent event) {mViewDragHelper.processTouchEvent(event);return true;}private class CustomCallback extends ViewDragHelper.Callback {@Overridepublic boolean tryCaptureView(@NonNull View view, int pointerId) {return true;}@Overridepublic int clampViewPositionHorizontal(@NonNull View child, int left, int dx) {return left;}@Overridepublic int clampViewPositionVertical(@NonNull View child, int top, int dy) {return top;}}}

布局文件，ViewDragHelperView中有两个子控件

<?xml version="1.0" encoding="utf-8"?>
<com.blog.demo.custom.widget.ViewDragHelperViewxmlns:android="http://schemas.android.com/apk/res/android"android:layout_width="match_parent"android:layout_height="match_parent"><ImageViewandroid:layout_width="@dimen/margin_dpi_50"android:layout_height="@dimen/margin_dpi_50"android:layout_margin="@dimen/margin_dpi_50"android:src="@drawable/icon_pyq"/><ImageViewandroid:layout_width="@dimen/margin_dpi_50"android:layout_height="@dimen/margin_dpi_50"android:layout_margin="100dp"android:src="@drawable/icon_link"/></com.blog.demo.custom.widget.ViewDragHelperView>

显示如下

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3. 拖拽后的回弹

在CallBack里，首先要记录控件的初始位置，其次要捕捉释放的时机
在onViewReleased()方法里调用ViewDragHelper的settleCapturedViewAt()方法

private class CustomCallback extends ViewDragHelper.Callback {private int mLeft;private int mTop;...  ...// 获取捕捉的子控件@Overridepublic void onViewCaptured(@NonNull View capturedChild, int activePointerId) {mLeft = capturedChild.getLeft();mTop = capturedChild.getTop();}// 拖拽结束@Overridepublic void onViewReleased(@NonNull View releasedChild, float xvel, float yvel) {mViewDragHelper.settleCapturedViewAt(mLeft, mTop);invalidate();}
}

在settleCapturedViewAt()方法里面，实际是使用Scroll实现的回弹

public boolean settleCapturedViewAt(int finalLeft, int finalTop) {if (!mReleaseInProgress) {throw new IllegalStateException("Cannot settleCapturedViewAt outside of a call to "+ "Callback#onViewReleased");}return forceSettleCapturedViewAt(finalLeft, finalTop,(int) mVelocityTracker.getXVelocity(mActivePointerId),(int) mVelocityTracker.getYVelocity(mActivePointerId));
}private boolean forceSettleCapturedViewAt(int finalLeft, int finalTop, int xvel, int yvel) {final int startLeft = mCapturedView.getLeft();final int startTop = mCapturedView.getTop();final int dx = finalLeft - startLeft;final int dy = finalTop - startTop;if (dx == 0 && dy == 0) {// Nothing to do. Send callbacks, be done.mScroller.abortAnimation();setDragState(STATE_IDLE);return false;}final int duration = computeSettleDuration(mCapturedView, dx, dy, xvel, yvel);mScroller.startScroll(startLeft, startTop, dx, dy, duration);setDragState(STATE_SETTLING);return true;
}

所以在View的computeScroll()方法里面需要添加ViewDragHelper的continueSettling()方法进行计算。

public class ViewDragHelperView extends FrameLayout {private ViewDragHelper mViewDragHelper;... ...@Overridepublic void computeScroll() {if (mViewDragHelper.continueSettling(true)) {invalidate();}}
}

显示如下

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4. 代码解析

processTouchEvent(MotionEvent ev)方法处理手势事件

public void processTouchEvent(@NonNull MotionEvent ev) {final int action = ev.getActionMasked();final int actionIndex = ev.getActionIndex();... ...if (mVelocityTracker == null) {mVelocityTracker = VelocityTracker.obtain();}mVelocityTracker.addMovement(ev);switch (action) {case MotionEvent.ACTION_DOWN: {final float x = ev.getX();final float y = ev.getY();final int pointerId = ev.getPointerId(0);final View toCapture = findTopChildUnder((int) x, (int) y);saveInitialMotion(x, y, pointerId);// Since the parent is already directly processing this touch event,// there is no reason to delay for a slop before dragging.// Start immediately if possible.tryCaptureViewForDrag(toCapture, pointerId);final int edgesTouched = mInitialEdgesTouched[pointerId];if ((edgesTouched & mTrackingEdges) != 0) {mCallback.onEdgeTouched(edgesTouched & mTrackingEdges, pointerId);}break;}... ...case MotionEvent.ACTION_MOVE: {if (mDragState == STATE_DRAGGING) {// If pointer is invalid then skip the ACTION_MOVE.if (!isValidPointerForActionMove(mActivePointerId)) break;final int index = ev.findPointerIndex(mActivePointerId);final float x = ev.getX(index);final float y = ev.getY(index);final int idx = (int) (x - mLastMotionX[mActivePointerId]);final int idy = (int) (y - mLastMotionY[mActivePointerId]);dragTo(mCapturedView.getLeft() + idx, mCapturedView.getTop() + idy, idx, idy);saveLastMotion(ev);}}... ...case MotionEvent.ACTION_UP: {if (mDragState == STATE_DRAGGING) {releaseViewForPointerUp();}cancel();break;}case MotionEvent.ACTION_CANCEL: {if (mDragState == STATE_DRAGGING) {dispatchViewReleased(0, 0);}cancel();break;}}
}

ACTION_DOWN事件

findTopChildUnder()查找拦截子控件

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;
}

tryCaptureViewForDrag()拦截事件，并调用Callback的tryCaptureView()方法回调，查询是否拦截该事件。

boolean tryCaptureViewForDrag(View toCapture, int pointerId) {if (toCapture == mCapturedView && mActivePointerId == pointerId) {// Already done!return true;}if (toCapture != null && mCallback.tryCaptureView(toCapture, pointerId)) {mActivePointerId = pointerId;captureChildView(toCapture, pointerId);return true;}return false;
}

captureChildView()方法设置拦截对象，调用Callback的onViewCaptured()方法回调。

public void captureChildView(@NonNull 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 + ")");}mCapturedView = childView;mActivePointerId = activePointerId;mCallback.onViewCaptured(childView, activePointerId);setDragState(STATE_DRAGGING);
}

setDragState()方法修改状态，调用Callback的onViewDragStateChanged()方法回调。

void setDragState(int state) {mParentView.removeCallbacks(mSetIdleRunnable);if (mDragState != state) {mDragState = state;mCallback.onViewDragStateChanged(state);if (mDragState == STATE_IDLE) {mCapturedView = null;}}
}

ACTION_MOVE事件

dragTo()方法移动拦截控件，Callback的clampViewPositionHorizontal()和clampViewPositionVertical()分别返回水平和垂直的移动量，最后调用Callback的onViewPositionChanged()方法回调

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) {clampedX = mCallback.clampViewPositionHorizontal(mCapturedView, left, dx);ViewCompat.offsetLeftAndRight(mCapturedView, clampedX - oldLeft);}if (dy != 0) {clampedY = mCallback.clampViewPositionVertical(mCapturedView, top, dy);ViewCompat.offsetTopAndBottom(mCapturedView, clampedY - oldTop);}if (dx != 0 || dy != 0) {final int clampedDx = clampedX - oldLeft;final int clampedDy = clampedY - oldTop;mCallback.onViewPositionChanged(mCapturedView, clampedX, clampedY,clampedDx, clampedDy);}
}

saveLastMotion()方法保留当前手势位置。

private void saveLastMotion(MotionEvent ev) {final int pointerCount = ev.getPointerCount();for (int i = 0; i < pointerCount; i++) {final int pointerId = ev.getPointerId(i);// If pointer is invalid then skip saving on ACTION_MOVE.if (!isValidPointerForActionMove(pointerId)) {continue;}final float x = ev.getX(i);final float y = ev.getY(i);mLastMotionX[pointerId] = x;mLastMotionY[pointerId] = y;}
}

ACTION_UP和ACTION_CANCEL事件

releaseViewForPointerUp()方法计算移动时的速率，回调Callback的onViewReleased()方法

private void releaseViewForPointerUp() {mVelocityTracker.computeCurrentVelocity(1000, mMaxVelocity);final float xvel = clampMag(mVelocityTracker.getXVelocity(mActivePointerId),mMinVelocity, mMaxVelocity);final float yvel = clampMag(mVelocityTracker.getYVelocity(mActivePointerId),mMinVelocity, mMaxVelocity);dispatchViewReleased(xvel, yvel);
}private void dispatchViewReleased(float xvel, float yvel) {mReleaseInProgress = true;mCallback.onViewReleased(mCapturedView, xvel, yvel);mReleaseInProgress = false;if (mDragState == STATE_DRAGGING) {// onViewReleased didn't call a method that would have changed this. Go idle.setDragState(STATE_IDLE);}
}public void cancel() {mActivePointerId = INVALID_POINTER;clearMotionHistory();if (mVelocityTracker != null) {mVelocityTracker.recycle();mVelocityTracker = null;}
}

5. 边缘触发

利用ViewDragHelper可以轻松实现边缘触发

// 设置识别哪些方向的触发
public void setEdgeTrackingEnabled(int edgeFlags)

Callback回调里面也有相应的回调方法。

// 边缘被触摸
public void onEdgeTouched(int edgeFlags, int pointerId)// 边缘开始拉拽
public void onEdgeDragStarted(int edgeFlags, int pointerId)

在ViewDragHelperView设置边缘触发

public ViewDragHelperView(@NonNull Context context, @Nullable AttributeSet attrs, int defStyleAttr) {super(context, attrs, defStyleAttr);mViewDragHelper = ViewDragHelper.create(this, new CustomCallback());mViewDragHelper.setEdgeTrackingEnabled(ViewDragHelper.EDGE_LEFT);
}

在Callback中的onEdgeDragStarted()方法里，加入CaptureView

private class CustomCallback extends ViewDragHelper.Callback {... ...@Overridepublic void onEdgeDragStarted(int edgeFlags, int pointerId) {Log.d("ViewDragHelperView", "onEdgeDragStarted");if (edgeFlags == ViewDragHelper.EDGE_LEFT) {mViewDragHelper.captureChildView(getChildAt(0), pointerId);}}
}

显示如下

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6. ViewDragHelper中的Button

Button在ViewDragHelperView中无法被拖动

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我们来看一下shouldInterceptTouchEvent()方法，当发生ACTION_DOWN事件时，只有mDragState == STATE_SETTLING时才会调用tryCaptureViewForDrag()方法，并修改状态，所有返回值为false。而Button会拦截手势，因此不会调用processTouchEvent()方法了

public boolean shouldInterceptTouchEvent(@NonNull MotionEvent ev) {final int action = ev.getActionMasked();final int actionIndex = ev.getActionIndex();if (action == MotionEvent.ACTION_DOWN) {// Reset things for a new event stream, just in case we didn't get// the whole previous stream.cancel();}if (mVelocityTracker == null) {mVelocityTracker = VelocityTracker.obtain();}mVelocityTracker.addMovement(ev);switch (action) {case MotionEvent.ACTION_DOWN: {final float x = ev.getX();final float y = ev.getY();final int pointerId = ev.getPointerId(0);saveInitialMotion(x, y, pointerId);final View toCapture = findTopChildUnder((int) x, (int) y);// Catch a settling view if possible.if (toCapture == mCapturedView && mDragState == STATE_SETTLING) {tryCaptureViewForDrag(toCapture, pointerId);}final int edgesTouched = mInitialEdgesTouched[pointerId];if ((edgesTouched & mTrackingEdges) != 0) {mCallback.onEdgeTouched(edgesTouched & mTrackingEdges, pointerId);}break;}... ...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.final int pointerCount = ev.getPointerCount();for (int i = 0; i < pointerCount; i++) {final int pointerId = ev.getPointerId(i);// If pointer is invalid then skip the ACTION_MOVE.if (!isValidPointerForActionMove(pointerId)) continue;final float x = ev.getX(i);final float y = ev.getY(i);final float dx = x - mInitialMotionX[pointerId];final float dy = y - mInitialMotionY[pointerId];final View toCapture = findTopChildUnder((int) x, (int) y);final boolean pastSlop = toCapture != null && checkTouchSlop(toCapture, dx, dy);if (pastSlop) {// check the callback's// getView[Horizontal|Vertical]DragRange methods to know// if you can move at all along an axis, then see if it// would clamp to the same value. If you can't move at// all in every dimension with a nonzero range, bail.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 hDragRange = mCallback.getViewHorizontalDragRange(toCapture);final int vDragRange = mCallback.getViewVerticalDragRange(toCapture);if ((hDragRange == 0 || (hDragRange > 0 && newLeft == oldLeft))&& (vDragRange == 0 || (vDragRange > 0 && newTop == oldTop))) {break;}}reportNewEdgeDrags(dx, dy, pointerId);if (mDragState == STATE_DRAGGING) {// Callback might have started an edge dragbreak;}if (pastSlop && tryCaptureViewForDrag(toCapture, pointerId)) {break;}}saveLastMotion(ev);break;}... ...}return mDragState == STATE_DRAGGING;
}

当发生ACTION_MOVE事件时，会调用checkTouchSlop()来查看是否拦截手势，默认是不拦截。

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;
}

所以如果需要拦截的话，修改Callback的getViewHorizontalDragRange()和getViewVerticalDragRange()方法

@Override
public int getViewHorizontalDragRange(@NonNull View child) {if (child == mClickableView) {return 1;}return super.getViewHorizontalDragRange(child);
}@Override
public int getViewVerticalDragRange(@NonNull View child) {if (child == mClickableView) {return 1;}return super.getViewVerticalDragRange(child);
}