demo实现效果图见下，可自由拖拽的view，还在自己造轮子吗？使用系统androidx包（原v4）下的ViewDragHelper 几行代码即可搞定..
实现
ViewDragHelper是用于编写自定义ViewGroup的工具类。它提供了许多有用的操作和状态跟踪，以允许用户在其父级ViewGroup中拖动和重新放置视图，具体可见 官网API。好，那我们就开始自定义一个简单的ViewGroup，并创建ViewDragHelper，代码见下：
public class DragViewGroup extends RelativeLayout {
ViewDragHelper mDragHelper;
public DragViewGroup(Context context) {
this(context, null);
}
public DragViewGroup(Context context, AttributeSet attrs) {
this(context, attrs, 0);
}
public DragViewGroup(Context context, AttributeSet attrs, int defStyleAttr) {
super(context, attrs, defStyleAttr);
mDragHelper = ViewDragHelper.create(this, 1.0f, new ViewDragCallback());
}
...
}
复制代码其中ViewDragCallback是我自己创建的内部类，继承自ViewDragHelper.Callback实现类。
private static class ViewDragCallback extends ViewDragHelper.Callback {
@Override
public boolean tryCaptureView(@NonNull View child, int pointerId) {
// 决定child是否可以被拖拽，具体见下文源码分析
return true;
}
@Override
public int clampViewPositionHorizontal(@NonNull View child, int left, int dx) {
// 可决定child横向的偏移计算，见下文
return left;
}
@Override
public int clampViewPositionVertical(@NonNull View child, int top, int dy) {
// 可决定child竖向的偏移计算，见下文
return top;
}
}
复制代码重写DragViewGroup的方法onInterceptHoverEvent和onTouchEvent方法：
public class DragViewGroup extends RelativeLayout {
...
@Override
public boolean onInterceptHoverEvent(MotionEvent event) {
return mDragHelper.shouldInterceptTouchEvent(event);
}
@SuppressLint("ClickableViewAccessibility")
@Override
public boolean onTouchEvent(MotionEvent event) {
mDragHelper.processTouchEvent(event);
return true;
}
...
}
复制代码这是我们的layout文件，其中DragViewGroup是我们上面定义的ViewGroup，TextView就是待拖拽的child view。
<com.blog.a.drag.DragViewGroup
android:layout_width="match_parent"
android:layout_height="match_parent"
>
<TextView
android:layout_width="70dp"
android:layout_height="70dp"
android:text="可拖拽"
android:gravity="center"
android:textColor="#fff"
android:background="#6495ED"
/>
</com.blog.a.drag.DragViewGroup>
复制代码是不是非常省事，博客的栗子我都上传到了gitHub上，感兴趣的可以下载看下。
源码
本篇文章主要分析下，当触摸事件开始到结束，processTouchEvent的处理过程：
public boolean onTouchEvent(MotionEvent event) {
mDragHelper.processTouchEvent(event);
return true;
}
复制代码MotionEvent.ACTION_DOWN
当手指刚接触屏幕时，会触发ACTION_DOWN 事件，通过MotionEvent我们能获取到点击事件发生的 x, y 坐标，注意这里的getX/getY的坐标是相对于当前view而言的。Pointer是触摸点的概念，一个MotionEvent可能会包含多个Pointer触摸点的信息，而每个Pointer触摸点都会有一个自己的id和index。具体往下看。
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);
tryCaptureViewForDrag(toCapture, pointerId);
// mTrackingEdges默认是0，可通过ViewDragHelper#setEdgeTrackingEnabled(int)
// 来设置，用来控制触碰边缘回调onEdgeTouched
final int edgesTouched = mInitialEdgesTouched[pointerId];
if ((edgesTouched & mTrackingEdges) != 0) {
mCallback.onEdgeTouched(edgesTouched & mTrackingEdges, pointerId);
}
break;
}
复制代码这里的findTopChildUnder方法是用来获取当前x, y坐标点所在的view，默认是最上层的，当然我们也可以通过callback#getOrderedChildIndex(int) 接口来自定义view遍历顺序，代码见下：
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;
}
复制代码这里的saveInitialMotion方法是用来保存当前触摸位置信息，其中getEdgesTouched方法用来判断x, y是否位于此viewGroup边缘之外，并返回保存相应result结果。todo:下篇准备写一下关于位运算符的文章，很有意思。
private void saveInitialMotion(float x, float y, int pointerId) {
ensureMotionHistorySizeForId(pointerId);
mInitialMotionX[pointerId] = mLastMotionX[pointerId] = x;
mInitialMotionY[pointerId] = mLastMotionY[pointerId] = y;
mInitialEdgesTouched[pointerId] = getEdgesTouched((int) x, (int) y);
mPointersDown |= 1 << pointerId;
}
复制代码其中tryCaptureViewForDrag方法内，mCapturedView是当前触摸的视图view，如果相同则直接返回，否则会进行mCallback#tryCaptureView(View, int)判断，这个是不是很眼熟，我们可以重写这个回调来控制toCapture这个view能否被捕获，即能否被拖拽操作。
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;
}
复制代码这里的captureChildView方法用来保存信息，并设置拖拽状态。能注意到，这里还有个捕获view是否是child view的判断。
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);
}
复制代码MotionEvent.ACTION_POINTER_DOWN
当用户又使用一个手指接触屏幕时，会触发ACTION_POINTER_DOWN 事件，与上面的ACTION_DOWN 相似，就不细展开了。由于ViewDragHelper一次只能操作一个视图，所以这里会先进行状态判断，如果视图还未被捕获拖动，则逻辑与上面的ACTION_POINTER_DOWN一致，反之，会判断触摸点是否在当前视图内，如果符合条件，则更新Pointer，这里很重要，体现在ui效果上就是，一个手指按住view，另一个手指仍然可以拖拽此view。
case MotionEvent.ACTION_POINTER_DOWN: {
final int pointerId = ev.getPointerId(actionIndex);
final float x = ev.getX(actionIndex);
final float y = ev.getY(actionIndex);
saveInitialMotion(x, y, pointerId);
// A ViewDragHelper can only manipulate one view at a time.
if (mDragState == STATE_IDLE) {
// If we're idle we can do anything! Treat it like a normal down event.
final View toCapture = findTopChildUnder((int) x, (int) y);
tryCaptureViewForDrag(toCapture, pointerId);
final int edgesTouched = mInitialEdgesTouched[pointerId];
if ((edgesTouched & mTrackingEdges) != 0) {
mCallback.onEdgeTouched(edgesTouched & mTrackingEdges, pointerId);
}
} else if (isCapturedViewUnder((int) x, (int) y)) {
tryCaptureViewForDrag(mCapturedView, pointerId);
}
break;
}
复制代码MotionEvent.ACTION_MOVE
当手指在屏幕移动时，如果视图正在被拖动，则会先判断当前mActivePointerId是否有效，无效则跳过当前move事件。随后获取当前x, y并计算与上次x, y移动距离。之后触发dragTo拖动逻辑，最后保存保存这次的位置。核心方法dragTo分析见下文：
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);
} else {
// Check to see if any pointer is now over a draggable view.
...
}
break;
}
复制代码在move过程中，通过dragTo方法来传入目标x, y 和横向和竖向的偏移量，并通过callback回调来通知开发者，开发者可重写clampViewPositionHorizontal与clampViewPositionVertical这两个回调方法，来自定义clampedX，clampedY目标位置。随后使用offsetLeftAndRight和offsetTopAndBottom 方法分别在相应的方向偏移（clampedX - oldLeft）和（clampedY - oldTo）的像素。最后触发onViewPositionChanged位置修改的回调。
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);
}
}
复制代码如果当手指在屏幕移动时，发现视图未处于拖动状态呢？首先会去检查是否有其他Pointer是否有效。随后触发边缘拖动回调，随后再进行状态检查，应该是为了避免此时状态由未拖动->拖动状态了，如：smoothSlideViewTo方法就有这个能力。如果此时mDragState处于未拖动状态，则会重新获取x,y 所在视图view并重新设置拖拽状态，这个逻辑与down逻辑一样。
case MotionEvent.ACTION_MOVE: {
if (mDragState == STATE_DRAGGING) {
// If pointer is invalid then skip the ACTION_MOVE.
...
} else {
// Check to see if any pointer is now over a draggable view.
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];
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;
}
复制代码MotionEvent.ACTION_POINTER_UP
当处于多触摸点时，当一手指从屏幕上松开时，首先判断正在拖动视图的触摸点是否是当前触摸点，如果是，则再去检查视图上是否还有其他有效的触摸点，如果没有则释放，此时view就惯性停住了。如果还有，则清理当前up掉的触摸点数据。
case MotionEvent.ACTION_POINTER_UP: {
final int pointerId = ev.getPointerId(actionIndex);
// 判断当前触摸点是否是正在拖动视图的触摸点
if (mDragState == STATE_DRAGGING && pointerId == mActivePointerId) {
// 检查是否有其他有效触摸点
int newActivePointer = INVALID_POINTER;
final int pointerCount = ev.getPointerCount();
// 遍历ev内触摸点
for (int i = 0; i < pointerCount; i++) {
final int id = ev.getPointerId(i);
if (id == mActivePointerId) {
// This one's going away, skip.
continue;
}
final float x = ev.getX(i);
final float y = ev.getY(i);
// 如果在视图上，并且可拖动，则标记找到了
if (findTopChildUnder((int) x, (int) y) == mCapturedView
&& tryCaptureViewForDrag(mCapturedView, id)) {
newActivePointer = mActivePointerId;
break;
}
}
if (newActivePointer == INVALID_POINTER) {
// 如果没有发现其他触摸点在拖拽视图view，则释放掉就可以了
releaseViewForPointerUp();
}
}
// 清理当前up掉的触摸点数据
clearMotionHistory(pointerId);
break;
}
复制代码MotionEvent.ACTION_UP
当手指从屏幕上离开时，会先判断当前状态，如果此时mDragState处于拖动状态，则释放，view惯性停住。通过cancel方法改变状态，清空当前触摸点数据并接触速度检测mVelocityTracker。
case MotionEvent.ACTION_UP: {
if (mDragState == STATE_DRAGGING) {
releaseViewForPointerUp();
}
cancel();
break;
}
复制代码好了，本文到这里，关于ViewDrafHHelper的介绍就结束了。如果本文对你有用，来点个赞吧，大家的肯定也是阿呆i坚持写作的动力。
