ItemTouchHelper的基本使用上次已经介绍了，今天来分析下ItemTouchHelper的源码，我们从attachToRecyclerView方法入手
/**
* Attaches the ItemTouchHelper to the provided RecyclerView. If TouchHelper is already
* attached to a RecyclerView, it will first detach from the previous one. You can call this
* method with {@code null} to detach it from the current RecyclerView.
*
* @param recyclerView The RecyclerView instance to which you want to add this helper or
* {@code null} if you want to remove ItemTouchHelper from the current
* RecyclerView.
*/
public void attachToRecyclerView(@Nullable RecyclerView recyclerView) {
if (mRecyclerView == recyclerView) {
return; // nothing to do
}
if (mRecyclerView != null) {
destroyCallbacks();
}
mRecyclerView = recyclerView;
if (mRecyclerView != null) {
final Resources resources = recyclerView.getResources();
mSwipeEscapeVelocity = resources
.getDimension(R.dimen.item_touch_helper_swipe_escape_velocity);
mMaxSwipeVelocity = resources
.getDimension(R.dimen.item_touch_helper_swipe_escape_max_velocity);
setupCallbacks();
}
}
可以看到就是将recyclerView赋值给mRecyclerView ，并调用了setupCallbacks方法，来到setupCallbacks方法：
private void setupCallbacks() {
ViewConfiguration vc = ViewConfiguration.get(mRecyclerView.getContext());
mSlop = vc.getScaledTouchSlop();
//说明ItemTouchHelper本身继承了ItemDecoration
//ItemDecoration我们一般用于画分割线
mRecyclerView.addItemDecoration(this);
mRecyclerView.addOnItemTouchListener(mOnItemTouchListener);
mRecyclerView.addOnChildAttachStateChangeListener(this);
initGestureDetector();
}
其中初始化了判断移动的阀值mSlop ，然后调用mRecyclerView.addItemDecoration(this)，我们看ItemTouchHelper继承ItemDecoration干了什么，发现ItemTouchHelper改写了onDraw和onDrawOver方法
/**
* 该方法在recyclerView的draw方法中调用，draw方法会调用onDraw方法
*
* @param c
* @param parent
* @param state
*/
@Override
public void onDrawOver(Canvas c, RecyclerView parent, RecyclerView.State state) {
float dx = 0, dy = 0;
if (mSelected != null) {
getSelectedDxDy(mTmpPosition);
dx = mTmpPosition[0];
dy = mTmpPosition[1];
}
mCallback.onDrawOver(c, parent, mSelected,
mRecoverAnimations, mActionState, dx, dy);
}
/**
* 该方法在recyclerView的onDraw方法中调用
*
* @param c
* @param parent
* @param state
*/
@Override
public void onDraw(Canvas c, RecyclerView parent, RecyclerView.State state) {
// we don't know if RV changed something so we should invalidate this index.
mOverdrawChildPosition = -1;
float dx = 0, dy = 0;
if (mSelected != null) {
getSelectedDxDy(mTmpPosition);
dx = mTmpPosition[0];
dy = mTmpPosition[1];
}
mCallback.onDraw(c, parent, mSelected,
mRecoverAnimations, mActionState, dx, dy);
}
两个方法都差不多，先判断mSelected 是不是null，获取dx，dy，即横向的偏移量和纵向的偏移量；接下来调用mCallback的方法，mCallback就是我们使用的ItemTouchHelper.Callback，下面是Callback的onDraw方法
void onDraw(Canvas c, RecyclerView parent, ViewHolder selected,
List<ItemTouchHelper.RecoverAnimation> recoverAnimationList,
int actionState, float dX, float dY) {
final int recoverAnimSize = recoverAnimationList.size();
for (int i = 0; i < recoverAnimSize; i++) {
final ItemTouchHelper.RecoverAnimation anim = recoverAnimationList.get(i);
anim.update();
final int count = c.save();
onChildDraw(c, parent, anim.mViewHolder, anim.mX, anim.mY, anim.mActionState,
false);
c.restoreToCount(count);
}
if (selected != null) {
final int count = c.save();
onChildDraw(c, parent, selected, dX, dY, actionState, true);
c.restoreToCount(count);
}
}
我们关注onChildDraw方法，我们追踪代码发现，最后调用的是ItemTouchUIUtilImpl中的onDraw和onDrawOver方法，ItemTouchUIUtilImpl是ItemTouchUIUtil的实现类。mSelected 其实就是我们选中的ViewHodler，在接下来我们的分析中会知道，这边先提一下。下面贴一下ItemTouchUIUtilImpl的代码
/**
* Package private class to keep implementations. Putting them inside ItemTouchUIUtil makes them
* public API, which is not desired in this case.
*/
class ItemTouchUIUtilImpl {
static class Lollipop extends Honeycomb {
@Override
public void onDraw(Canvas c, RecyclerView recyclerView, View view,
float dX, float dY, int actionState, boolean isCurrentlyActive) {
if (isCurrentlyActive) {
Object originalElevation = view.getTag(R.id.item_touch_helper_previous_elevation);
if (originalElevation == null) {
originalElevation = ViewCompat.getElevation(view);
float newElevation = 1f + findMaxElevation(recyclerView, view);
ViewCompat.setElevation(view, newElevation);
view.setTag(R.id.item_touch_helper_previous_elevation, originalElevation);
}
}
super.onDraw(c, recyclerView, view, dX, dY, actionState, isCurrentlyActive);
}
private float findMaxElevation(RecyclerView recyclerView, View itemView) {
final int childCount = recyclerView.getChildCount();
float max = 0;
for (int i = 0; i < childCount; i++) {
final View child = recyclerView.getChildAt(i);
if (child == itemView) {
continue;
}
final float elevation = ViewCompat.getElevation(child);
if (elevation > max) {
max = elevation;
}
}
return max;
}
@Override
public void clearView(View view) {
final Object tag = view.getTag(R.id.item_touch_helper_previous_elevation);
if (tag != null && tag instanceof Float) {
ViewCompat.setElevation(view, (Float) tag);
}
view.setTag(R.id.item_touch_helper_previous_elevation, null);
super.clearView(view);
}
}
static class Honeycomb implements ItemTouchUIUtil {
@Override
public void clearView(View view) {
ViewCompat.setTranslationX(view, 0f);
ViewCompat.setTranslationY(view, 0f);
}
@Override
public void onSelected(View view) {
}
@Override
public void onDraw(Canvas c, RecyclerView recyclerView, View view,
float dX, float dY, int actionState, boolean isCurrentlyActive) {
ViewCompat.setTranslationX(view, dX);
ViewCompat.setTranslationY(view, dY);
}
@Override
public void onDrawOver(Canvas c, RecyclerView recyclerView,
View view, float dX, float dY, int actionState, boolean isCurrentlyActive) {
}
}
static class Gingerbread implements ItemTouchUIUtil {
private void draw(Canvas c, RecyclerView parent, View view,
float dX, float dY) {
c.save();
c.translate(dX, dY);
parent.drawChild(c, view, 0);
c.restore();
}
@Override
public void clearView(View view) {
view.setVisibility(View.VISIBLE);
}
@Override
public void onSelected(View view) {
view.setVisibility(View.INVISIBLE);
}
@Override
public void onDraw(Canvas c, RecyclerView recyclerView, View view,
float dX, float dY, int actionState, boolean isCurrentlyActive) {
if (actionState != ItemTouchHelper.ACTION_STATE_DRAG) {
draw(c, recyclerView, view, dX, dY);
}
}
@Override
public void onDrawOver(Canvas c, RecyclerView recyclerView,
View view, float dX, float dY,
int actionState, boolean isCurrentlyActive) {
if (actionState == ItemTouchHelper.ACTION_STATE_DRAG) {
draw(c, recyclerView, view, dX, dY);
}
}
}
}
到了这里，我们发现，都是canvas的矩阵变换效果(api28中是设置View的属性)，也就是我们拖拽和侧滑，最终的动画效果就是利用的canvas，那么具体我们要执行到哪个ViewHodler上，是在哪里判断的呢？我们继续来到setupCallbacks方法
private void setupCallbacks() {
ViewConfiguration vc = ViewConfiguration.get(mRecyclerView.getContext());
mSlop = vc.getScaledTouchSlop();
//说明ItemTouchHelper本事实现了ItemDecoration
//ItemDecoration我们一般用于画分割线
mRecyclerView.addItemDecoration(this);
//addOnItemTouchListener方法做了什么？
mRecyclerView.addOnItemTouchListener(mOnItemTouchListener);
mRecyclerView.addOnChildAttachStateChangeListener(this);
initGestureDetector();
}
之前分析了mRecyclerView.addItemDecoration方法，知道了RecyclerView每次onDraw的时候，都会调用ItemTouchHelper.Callback的onDraw方法，我们再分析mRecyclerView.addOnItemTouchListener做了什么？来到RecyclerView的addOnItemTouchListener方法
/**
* Add an {@link OnItemTouchListener} to intercept touch events before they are dispatched
* to child views or this view's standard scrolling behavior.
*
* <p>Client code may use listeners to implement item manipulation behavior. Once a listener
* returns true from
* {@link OnItemTouchListener#onInterceptTouchEvent(RecyclerView, MotionEvent)} its
* {@link OnItemTouchListener#onTouchEvent(RecyclerView, MotionEvent)} method will be called
* for each incoming MotionEvent until the end of the gesture.</p>
*
* @param listener Listener to add
* @see SimpleOnItemTouchListener
*/
public void addOnItemTouchListener(OnItemTouchListener listener) {
mOnItemTouchListeners.add(listener);
}
添加到了mOnItemTouchListeners集合中，那么mOnItemTouchListeners用来做什么呢？我们发现在RecyclerView的dispatchOnItemTouchIntercept方法中，用到了这个mOnItemTouchListeners集合
而dispatchOnItemTouchIntercept方法是在RecyclerView的onInterceptTouchEvent事件中调用的
@Override
public boolean onInterceptTouchEvent(MotionEvent e) {
if (mLayoutFrozen) {
// When layout is frozen, RV does not intercept the motion event.
// A child view e.g. a button may still get the click.
return false;
}
if (dispatchOnItemTouchIntercept(e)) {
cancelTouch();
//消费了事件，直接返回
return true;
}
...
}
private boolean dispatchOnItemTouchIntercept(MotionEvent e) {
final int action = e.getAction();
if (action == MotionEvent.ACTION_CANCEL || action == MotionEvent.ACTION_DOWN) {
mActiveOnItemTouchListener = null;
}
final int listenerCount = mOnItemTouchListeners.size();
for (int i = 0; i < listenerCount; i++) {
final OnItemTouchListener listener = mOnItemTouchListeners.get(i);
if (listener.onInterceptTouchEvent(this, e) && action != MotionEvent.ACTION_CANCEL) {
//赋值
mActiveOnItemTouchListener = listener;
return true;
}
}
return false;
}
也就是说，事件分发时，在onInterceptTouchEvent方法中又传递给了ItemTouchHelper的OnItemTouchListener的onInterceptTouchEvent方法，并且如果该方法返回了true，则消费事件，并且又把这个OnItemTouchListener 赋值给了mActiveOnItemTouchListener，我们再看跟踪这个mActiveOnItemTouchListener，最终发现了调用的地方
@Override
public boolean onTouchEvent(MotionEvent e) {
if (mLayoutFrozen || mIgnoreMotionEventTillDown) {
return false;
}
if (dispatchOnItemTouch(e)) {
cancelTouch();
return true;
}
...
}
private boolean dispatchOnItemTouch(MotionEvent e) {
final int action = e.getAction();
if (mActiveOnItemTouchListener != null) {
if (action == MotionEvent.ACTION_DOWN) {
// Stale state from a previous gesture, we're starting a new one. Clear it.
mActiveOnItemTouchListener = null;
} else {
mActiveOnItemTouchListener.onTouchEvent(this, e);
if (action == MotionEvent.ACTION_CANCEL || action == MotionEvent.ACTION_UP) {
// Clean up for the next gesture.
mActiveOnItemTouchListener = null;
}
return true;
}
}
// Listeners will have already received the ACTION_DOWN via dispatchOnItemTouchIntercept
// as called from onInterceptTouchEvent; skip it.
if (action != MotionEvent.ACTION_DOWN) {
final int listenerCount = mOnItemTouchListeners.size();
for (int i = 0; i < listenerCount; i++) {
final OnItemTouchListener listener = mOnItemTouchListeners.get(i);
if (listener.onInterceptTouchEvent(this, e)) {
mActiveOnItemTouchListener = listener;
return true;
}
}
}
return false;
}
如果事件分发到onTouchEvent(onInterceptTouchEvent为true或者子控件没有消费事件)，那么dispatchOnItemTouch方法会被调用，在不是ACTION_DOWN事件的情况下，如果mActiveOnItemTouchListener不为null，mActiveOnItemTouchListener的onTouchEvent方法会被调用，并且返回true，如果mActiveOnItemTouchListener为null，则又会调用OnItemTouchListener的onInterceptTouchEvent方法
值得注意的是onInterceptTouchEvent方法中，OnItemTouchListener是能接受到ACTION_DOWN事件的，但是onTouchEvent事件中，OnItemTouchListener不能接受到ACTION_DOWN事件
上述事件分发的方法，调用有点乱，没什么头绪，那么我们先来看OnItemTouchListener这个对象，该对象在ItemTouchHelper中
private final OnItemTouchListener mOnItemTouchListener
= new OnItemTouchListener() {
@Override
public boolean onInterceptTouchEvent(RecyclerView recyclerView, MotionEvent event) {
mGestureDetector.onTouchEvent(event);
if (DEBUG) {
Log.d(TAG, "intercept: x:" + event.getX() + ",y:" + event.getY() + ", " + event);
}
final int action = MotionEventCompat.getActionMasked(event);
if (action == MotionEvent.ACTION_DOWN) {
mActivePointerId = event.getPointerId(0);
mInitialTouchX = event.getX();
mInitialTouchY = event.getY();
obtainVelocityTracker();
if (mSelected == null) {
final RecoverAnimation animation = findAnimation(event);
if (animation != null) {
mInitialTouchX -= animation.mX;
mInitialTouchY -= animation.mY;
endRecoverAnimation(animation.mViewHolder, true);
if (mPendingCleanup.remove(animation.mViewHolder.itemView)) {
mCallback.clearView(mRecyclerView, animation.mViewHolder);
}
select(animation.mViewHolder, animation.mActionState);
updateDxDy(event, mSelectedFlags, 0);
}
}
} else if (action == MotionEvent.ACTION_CANCEL || action == MotionEvent.ACTION_UP) {
mActivePointerId = ACTIVE_POINTER_ID_NONE;
select(null, ACTION_STATE_IDLE);
} else if (mActivePointerId != ACTIVE_POINTER_ID_NONE) {
// in a non scroll orientation, if distance change is above threshold, we
// can select the item
final int index = event.findPointerIndex(mActivePointerId);
if (DEBUG) {
Log.d(TAG, "pointer index " + index);
}
if (index >= 0) {
checkSelectForSwipe(action, event, index);
}
}
if (mVelocityTracker != null) {
mVelocityTracker.addMovement(event);
}
return mSelected != null;
}
@Override
public void onTouchEvent(RecyclerView recyclerView, MotionEvent event) {
mGestureDetector.onTouchEvent(event);
if (DEBUG) {
Log.d(TAG,
"on touch: x:" + mInitialTouchX + ",y:" + mInitialTouchY + ", :" + event);
}
if (mVelocityTracker != null) {
mVelocityTracker.addMovement(event);
}
if (mActivePointerId == ACTIVE_POINTER_ID_NONE) {
return;
}
final int action = MotionEventCompat.getActionMasked(event);
final int activePointerIndex = event.findPointerIndex(mActivePointerId);
if (activePointerIndex >= 0) {
checkSelectForSwipe(action, event, activePointerIndex);
}
ViewHolder viewHolder = mSelected;
if (viewHolder == null) {
return;
}
switch (action) {
case MotionEvent.ACTION_MOVE: {
// Find the index of the active pointer and fetch its position
if (activePointerIndex >= 0) {
updateDxDy(event, mSelectedFlags, activePointerIndex);
moveIfNecessary(viewHolder);
mRecyclerView.removeCallbacks(mScrollRunnable);
mScrollRunnable.run();
mRecyclerView.invalidate();
}
break;
}
case MotionEvent.ACTION_CANCEL:
if (mVelocityTracker != null) {
mVelocityTracker.clear();
}
// fall through
case MotionEvent.ACTION_UP:
select(null, ACTION_STATE_IDLE);
mActivePointerId = ACTIVE_POINTER_ID_NONE;
break;
case MotionEvent.ACTION_POINTER_UP: {
final int pointerIndex = MotionEventCompat.getActionIndex(event);
final int pointerId = event.getPointerId(pointerIndex);
if (pointerId == mActivePointerId) {
// This was our active pointer going up. Choose a new
// active pointer and adjust accordingly.
final int newPointerIndex = pointerIndex == 0 ? 1 : 0;
mActivePointerId = event.getPointerId(newPointerIndex);
updateDxDy(event, mSelectedFlags, pointerIndex);
}
break;
}
}
}
@Override
public void onRequestDisallowInterceptTouchEvent(boolean disallowIntercept) {
if (!disallowIntercept) {
return;
}
select(null, ACTION_STATE_IDLE);
}
};
首先，我们先看onInterceptTouchEvent的ACTION_DOWN，其中findAnimation方法（源码有点多，就不贴出来了）会根据手指按下的位置，找到选中的子控件，然后通过select方法赋值给mSelected，最后return mSelected != null;而ACTION_UP，则会通过select方法将mSelected置空，并且判断是否需要执行侧滑动画，并最终根据判断是否要调用Callback的onSwiped方法，所以ItemTouchHelper的OnItemTouchListener的onInterceptTouchEvent方法只是找到mSelected和释放mSelected
之前了解到onInterceptTouchEvent的onTouchEvent方法中是不会有ACTION_DOWN的，我们先来看ACTION_MOVE，首先调用了updateDxDy方法
void updateDxDy(MotionEvent ev, int directionFlags, int pointerIndex) {
final float x = ev.getX(pointerIndex);
final float y = ev.getY(pointerIndex);
// Calculate the distance moved
mDx = x - mInitialTouchX;
mDy = y - mInitialTouchY;
if ((directionFlags & LEFT) == 0) {
mDx = Math.max(0, mDx);
}
if ((directionFlags & RIGHT) == 0) {
mDx = Math.min(0, mDx);
}
if ((directionFlags & UP) == 0) {
mDy = Math.max(0, mDy);
}
if ((directionFlags & DOWN) == 0) {
mDy = Math.min(0, mDy);
}
}
就是根据标志位得出相应的偏移，接下来调用moveIfNecessary方法
/**
* Checks if we should swap w/ another view holder.
*/
void moveIfNecessary(ViewHolder viewHolder) {
if (mRecyclerView.isLayoutRequested()) {
return;
}
if (mActionState != ACTION_STATE_DRAG) {
return;
}
final float threshold = mCallback.getMoveThreshold(viewHolder);
final int x = (int) (mSelectedStartX + mDx);
final int y = (int) (mSelectedStartY + mDy);
if (Math.abs(y - viewHolder.itemView.getTop()) < viewHolder.itemView.getHeight() * threshold
&& Math.abs(x - viewHolder.itemView.getLeft())
< viewHolder.itemView.getWidth() * threshold) {
return;
}
List<ViewHolder> swapTargets = findSwapTargets(viewHolder);
if (swapTargets.size() == 0) {
return;
}
// may swap.
ViewHolder target = mCallback.chooseDropTarget(viewHolder, swapTargets, x, y);
if (target == null) {
mSwapTargets.clear();
mDistances.clear();
return;
}
final int toPosition = target.getAdapterPosition();
final int fromPosition = viewHolder.getAdapterPosition();
if (mCallback.onMove(mRecyclerView, viewHolder, target)) {
// keep target visible
mCallback.onMoved(mRecyclerView, viewHolder, fromPosition,
target, toPosition, x, y);
}
}
发现最后调用了Callback的onMove方法，这也是我们需要改写的拖拽方法，而ACTION_UP和onInterceptTouchEvent中的差不多
最后总结一下，ItemTouchHelper是通过OnItemTouchListener获取到选中的ViewHolder，并通过它的内部类Callback，调用ItemTouchUIUtilImpl中的方法进行绘制工作
