很久没有写Android控件了，正好最近项目有个自定义控件的需求，是基于RecylerView整理了下，本文先看下Scroll和Fling的联系和区别
本文分析的源码是基于recyclerview-v7-26.1.0
1.Scroll和状态
Scroll大家都知道，我们可以给RecyclerView添加监听，在RecyclerView滚动的时候会回调。
RecyclerView.addOnScrollListener(mScrollListener);
就是在RecyclerView滚动的时候会回调
/**
* An OnScrollListener can be added to a RecyclerView to receive messages when a scrolling event
* has occurred on that RecyclerView.
*
* @see RecyclerView#addOnScrollListener(OnScrollListener)
* @see RecyclerView#clearOnChildAttachStateChangeListeners()
*
*/
public abstract static class OnScrollListener {
/**
* Callback method to be invoked when RecyclerView's scroll state changes.
*
* @param recyclerView The RecyclerView whose scroll state has changed.
* @param newState The updated scroll state. One of {@link #SCROLL_STATE_IDLE},
* {@link #SCROLL_STATE_DRAGGING} or {@link #SCROLL_STATE_SETTLING}.
*/
public void onScrollStateChanged(RecyclerView recyclerView, int newState){}
/**
* Callback method to be invoked when the RecyclerView has been scrolled. This will be
* called after the scroll has completed.
*
* This callback will also be called if visible item range changes after a layout
* calculation. In that case, dx and dy will be 0.
*
* @param recyclerView The RecyclerView which scrolled.
* @param dx The amount of horizontal scroll.
* @param dy The amount of vertical scroll.
*/
public void onScrolled(RecyclerView recyclerView, int dx, int dy){}
}
滚动有三种状态：
/**
* The RecyclerView is not currently scrolling.
* @see #getScrollState()
*/
public static final int SCROLL_STATE_IDLE = 0;
/**
* The RecyclerView is currently being dragged by outside input such as user touch input.
* @see #getScrollState()
*/
public static final int SCROLL_STATE_DRAGGING = 1;
/**
* The RecyclerView is currently animating to a final position while not under
* outside control.
* @see #getScrollState()
*/
public static final int SCROLL_STATE_SETTLING = 2;
这三种回调顺序:
- SCROLL_STATE_DRAGGING, 先是手指拖拽的状态
- SCROLL_STATE_SETTLING，再是手指松开但是
RecyclerView还在滑动 - SCROLL_STATE_IDLE， 最后是
RecyclerView滚动停止状态。
2.Fling和Scroll的关系
Fling是怎样一种状态，手指在屏幕上滑动RecyclerView然后松手，RecyclerView中的内容会顺着惯性继续往手指滑动的方向继续滚动直到停止，这个过程叫做Fling。Fling操作从手指离开屏幕瞬间被触发，在滚动停止时结束。其实RecyclerView在Fling过程中会把state设置为 SCROLL_STATE_SETTLING。看下源码：
在Fling触发的时候会回调SnapHelper中的onFling
@Override
public boolean onFling(int velocityX, int velocityY) {
LayoutManager layoutManager = mRecyclerView.getLayoutManager();
if (layoutManager == null) {
return false;
}
RecyclerView.Adapter adapter = mRecyclerView.getAdapter();
if (adapter == null) {
return false;
}
int minFlingVelocity = mRecyclerView.getMinFlingVelocity();
return (Math.abs(velocityY) > minFlingVelocity || Math.abs(velocityX) > minFlingVelocity)
&& snapFromFling(layoutManager, velocityX, velocityY);
}
private boolean snapFromFling(@NonNull LayoutManager layoutManager, int velocityX,
int velocityY) {
...
SmoothScroller smoothScroller = createScroller(layoutManager);
if (smoothScroller == null) {
return false;
}
int targetPosition = findTargetSnapPosition(layoutManager, velocityX, velocityY);
if (targetPosition == RecyclerView.NO_POSITION) {
return false;
}
smoothScroller.setTargetPosition(targetPosition);
layoutManager.startSmoothScroll(smoothScroller);
return true;
}
在onFling中调用snapFromFling， findTargetSnapPosition会根据滑动速率计算出要滑动到的位置，然后设置给smoothScroller:
public void setTargetPosition(int targetPosition) {
mTargetPosition = targetPosition;
}
/**
* Returns the adapter position of the target item
*
* @return Adapter position of the target item or
* {@link RecyclerView#NO_POSITION} if no target view is set.
*/
public int getTargetPosition() {
return mTargetPosition;
}
在snapHelper中smoothScroller默认是LinearSmoothScroller,
public class LinearSmoothScroller extends RecyclerView.SmoothScroller
然后会调用smoothScroller滚动到指定位置, 开始滚动之前会调用start方法, 会设置mTargetView就是我们上面设置的要滚到的位置：
/**
* Starts a smooth scroll for the given target position.
* In each animation step, {@link RecyclerView} will check
* for the target view and call either
* {@link #onTargetFound(android.view.View, RecyclerView.State, SmoothScroller.Action)} or
* {@link #onSeekTargetStep(int, int, RecyclerView.State, SmoothScroller.Action)} until
* SmoothScroller is stopped.
*
* Note that if RecyclerView finds the target view, it will automatically stop the
* SmoothScroller. This does not mean that scroll will stop, it only means it will
* stop calling SmoothScroller in each animation step.
*/
void start(RecyclerView recyclerView, LayoutManager layoutManager) {
mRecyclerView = recyclerView;
mLayoutManager = layoutManager;
if (mTargetPosition == RecyclerView.NO_POSITION) {
throw new IllegalArgumentException("Invalid target position");
}
mRecyclerView.mState.mTargetPosition = mTargetPosition;
mRunning = true;
mPendingInitialRun = true;
mTargetView = findViewByPosition(getTargetPosition());
onStart();
mRecyclerView.mViewFlinger.postOnAnimation();
}
在RecyclerView.SmoothScroller滑动是会调用onAnimation，在滚动过程中如果发现getChildPosition(mTargetView) == mTargetPosition,也就是目标位置已经layout出来了，那么就会回调 onTargetFound.
public abstract static class SmoothScroller {
private void onAnimation(int dx, int dy) {
final RecyclerView recyclerView = mRecyclerView;
if (!mRunning || mTargetPosition == RecyclerView.NO_POSITION || recyclerView == null) {
stop();
}
mPendingInitialRun = false;
if (mTargetView != null) {
// verify target position
if (getChildPosition(mTargetView) == mTargetPosition) {
onTargetFound(mTargetView, recyclerView.mState, mRecyclingAction);
mRecyclingAction.runIfNecessary(recyclerView);
stop();
} else {
Log.e(TAG, "Passed over target position while smooth scrolling.");
mTargetView = null;
}
}
...
}
}
public int getChildPosition(View view) {
return mRecyclerView.getChildLayoutPosition(view);
}
没有悬念，绕了一大圈终于回到SnapHelper中onTargetFound,在onTargetFound中会调用calculateDistanceToFinalSnap:
return new LinearSmoothScroller(mRecyclerView.getContext()) {
@Override
protected void onTargetFound(View targetView, RecyclerView.State state, Action action) {
int[] snapDistances = calculateDistanceToFinalSnap(mRecyclerView.getLayoutManager(),
targetView);
final int dx = snapDistances[0];
final int dy = snapDistances[1];
final int time = calculateTimeForDeceleration(Math.max(Math.abs(dx), Math.abs(dy)));
if (time > 0) {
action.update(dx, dy, time, mDecelerateInterpolator);
}
}
@Override
protected float calculateSpeedPerPixel(DisplayMetrics displayMetrics) {
return MILLISECONDS_PER_INCH / displayMetrics.densityDpi;
}
};
计算到目标位置的坐标然后更新给RecyclerView.Action,然后在上面的onAnimation会调用mRecyclingAction.runIfNecessary(recyclerView);，然后会调用smoothScrollBy:
void runIfNecessary(RecyclerView recyclerView) {
if (mJumpToPosition >= 0) {
final int position = mJumpToPosition;
mJumpToPosition = NO_POSITION;
recyclerView.jumpToPositionForSmoothScroller(position);
mChanged = false;
return;
}
if (mChanged) {
validate();
if (mInterpolator == null) {
if (mDuration == UNDEFINED_DURATION) {
recyclerView.mViewFlinger.smoothScrollBy(mDx, mDy);
} else {
recyclerView.mViewFlinger.smoothScrollBy(mDx, mDy, mDuration);
}
} else {
recyclerView.mViewFlinger.smoothScrollBy(
mDx, mDy, mDuration, mInterpolator);
}
mConsecutiveUpdates++;
if (mConsecutiveUpdates > 10) {
// A new action is being set in every animation step. This looks like a bad
// implementation. Inform developer.
Log.e(TAG, "Smooth Scroll action is being updated too frequently. Make sure"
+ " you are not changing it unless necessary");
}
mChanged = false;
} else {
mConsecutiveUpdates = 0;
}
}
在smoothScrollBy中就会设置状态SCROLL_STATE_SETTLING:
public void smoothScrollBy(int dx, int dy, int duration, Interpolator interpolator) {
if (mInterpolator != interpolator) {
mInterpolator = interpolator;
mScroller = new OverScroller(getContext(), interpolator);
}
setScrollState(SCROLL_STATE_SETTLING);
mLastFlingX = mLastFlingY = 0;
mScroller.startScroll(0, 0, dx, dy, duration);
if (Build.VERSION.SDK_INT < 23) {
// b/64931938 before API 23, startScroll() does not reset getCurX()/getCurY()
// to start values, which causes fillRemainingScrollValues() put in obsolete values
// for LayoutManager.onLayoutChildren().
mScroller.computeScrollOffset();
}
postOnAnimation();
}
上面绕了一圈，总结一下调用栈就是：
SnapHelper
onFling ---> snapFromFling
上面得到最终位置targetPosition,把位置给RecyclerView.SmoothScroller, 然后就开始滑动了：
RecyclerView.SmoothScroller
start --> onAnimation
在滑动过程中如果targetPosition对应的targetView已经layout出来了，就会回调SnapHelper，然后计算得到到当前位置到targetView的距离dx,dy
SnapHelper
onTargetFound ---> calculateDistanceToFinalSnap
然后把距离dx,dy更新给RecyclerView.Action:
RecyclerView.Action
update --> runIfNecessary --> recyclerView.mViewFlinger.smoothScrollBy
最后调用RecyclerView.ViewFlinger, 然后又回到onAnimation
class ViewFlinger implements Runnable
public void smoothScrollBy(int dx, int dy, int duration, Interpolator interpolator) {
if (mInterpolator != interpolator) {
mInterpolator = interpolator;
mScroller = new OverScroller(getContext(), interpolator);
}
setScrollState(SCROLL_STATE_SETTLING);
mLastFlingX = mLastFlingY = 0;
mScroller.startScroll(0, 0, dx, dy, duration);
postOnAnimation();
}
3.触发fling操作
接着我们再看看Fling操作在RecyclerView中是什么时候触发的。
先看下RecyclerView的onTouch方法：
@Override
public boolean onTouchEvent(MotionEvent e) {
case MotionEvent.ACTION_UP: {
mVelocityTracker.addMovement(vtev);
eventAddedToVelocityTracker = true;
mVelocityTracker.computeCurrentVelocity(1000, mMaxFlingVelocity);
final float xvel = canScrollHorizontally
? -mVelocityTracker.getXVelocity(mScrollPointerId) : 0;
final float yvel = canScrollVertically
? -mVelocityTracker.getYVelocity(mScrollPointerId) : 0;
if (!((xvel != 0 || yvel != 0) && fling((int) xvel, (int) yvel))) {
setScrollState(SCROLL_STATE_IDLE);
}
resetTouch();
} break;
}
在up触发的时候会调用native方法去获得X和Y方向的速率：
/**
* Compute the current velocity based on the points that have been
* collected. Only call this when you actually want to retrieve velocity
* information, as it is relatively expensive. You can then retrieve
* the velocity with {@link #getXVelocity()} and
* {@link #getYVelocity()}.
*
* @param units The units you would like the velocity in. A value of 1
* provides pixels per millisecond, 1000 provides pixels per second, etc.
* @param maxVelocity The maximum velocity that can be computed by this method.
* This value must be declared in the same unit as the units parameter. This value
* must be positive.
*/
public void computeCurrentVelocity(int units, float maxVelocity) {
nativeComputeCurrentVelocity(mPtr, units, maxVelocity);
}
如果有其中一个不为0就会调用fling操作：
public boolean fling(int velocityX, int velocityY) {
if (mLayout == null) {
Log.e(TAG, "Cannot fling without a LayoutManager set. "
+ "Call setLayoutManager with a non-null argument.");
return false;
}
if (mLayoutFrozen) {
return false;
}
final boolean canScrollHorizontal = mLayout.canScrollHorizontally();
final boolean canScrollVertical = mLayout.canScrollVertically();
if (!canScrollHorizontal || Math.abs(velocityX) < mMinFlingVelocity) {
velocityX = 0;
}
if (!canScrollVertical || Math.abs(velocityY) < mMinFlingVelocity) {
velocityY = 0;
}
if (velocityX == 0 && velocityY == 0) {
// If we don't have any velocity, return false
return false;
}
if (!dispatchNestedPreFling(velocityX, velocityY)) {
final boolean canScroll = canScrollHorizontal || canScrollVertical;
dispatchNestedFling(velocityX, velocityY, canScroll);
if (mOnFlingListener != null && mOnFlingListener.onFling(velocityX, velocityY)) {
return true;
}
if (canScroll) {
int nestedScrollAxis = ViewCompat.SCROLL_AXIS_NONE;
if (canScrollHorizontal) {
nestedScrollAxis |= ViewCompat.SCROLL_AXIS_HORIZONTAL;
}
if (canScrollVertical) {
nestedScrollAxis |= ViewCompat.SCROLL_AXIS_VERTICAL;
}
startNestedScroll(nestedScrollAxis, TYPE_NON_TOUCH);
velocityX = Math.max(-mMaxFlingVelocity, Math.min(velocityX, mMaxFlingVelocity));
velocityY = Math.max(-mMaxFlingVelocity, Math.min(velocityY, mMaxFlingVelocity));
mViewFlinger.fling(velocityX, velocityY);
return true;
}
}
return false;
}
这里有个mMinFlingVelocity字段，如果x和y方向的速率小于这个字段，就不会触发fling操作，直接return false，然后就会把RecyclerView的状态设置为SCROLL_STATE_IDLE。 mMinFlingVelocity在RecyclerView构造的时候会从ViewConfiguration中获取
final ViewConfiguration vc = ViewConfiguration.get(context);
mMinFlingVelocity = vc.getScaledMinimumFlingVelocity();
mMaxFlingVelocity = vc.getScaledMaximumFlingVelocity();
在ViewConfiguration构造函数中：
/**
* Minimum velocity to initiate a fling, as measured in dips per second
*/
private static final int MINIMUM_FLING_VELOCITY = 50;
/**
* Maximum velocity to initiate a fling, as measured in dips per second
*/
private static final int MAXIMUM_FLING_VELOCITY = 8000;
public ViewConfiguration() {
mMinimumFlingVelocity = MINIMUM_FLING_VELOCITY;
mMaximumFlingVelocity = MAXIMUM_FLING_VELOCITY;
}
所以如果滑动速率小于50的话就不会触发fling操作。
从上面分析知道，fling操作是在手指离开的时候触发然后直到滑动停止这中间的一段操作。在滑动过程中其实state状态是SCROLL_STATE_SETTLING,然后通过RecyclerView.ViewFlinger进行滑动。