RecyclerView之Scroll和Fling

RecyclerView · 發表 2018-11-11 17:28:00

摘要：很久沒有寫Android控制元件了，正好最近專案有個自定義控制元件的需求，是基於RecylerView 整理了下，本文先看下Scroll 和Fling 的聯絡和區別本文分析的原始碼是基於recyclerview-v7-2...

很久沒有寫Android控制元件了，正好最近專案有個自定義控制元件的需求，是基於RecylerView 整理了下，本文先看下Scroll 和Fling 的聯絡和區別

本文分析的原始碼是基於recyclerview-v7-26.1.0

1.`Scroll` 和狀態

Scroll 大家都知道，我們可以給RecyclerView 新增監聽，在RecyclerView 滾動的時候會回撥。

RecyclerView.addOnScrollListener(mScrollListener);

就是在RecyclerView 滾動的時候會回撥

/**
* An OnScrollListener can be added to a RecyclerView to receive messages when a scrolling event
* has occurred on that RecyclerView.
* <p>
* @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 newStateThe 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.
* <p>
* 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){}
}

滾動有三種狀態：

/**
* 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;

這三種回撥順序:

RecyclerView
RecyclerView

2.`Fling` 和`Scroll` 的關係

Fling 是怎樣一種狀態，手指在螢幕上滑動RecyclerView然後鬆手，RecyclerView中的內容會順著慣性繼續往手指滑動的方向繼續滾動直到停止，這個過程叫做Fling 。Fling 操作從手指離開螢幕瞬間被觸發，在滾動停止時結束。其實RecyclerView 在Fling 過程中會把state設定為SCROLL_STATE_SETTLING 。看下原始碼：

在Fling觸發的時候會回撥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 中呼叫snapFromFling ，findTargetSnapPosition 會根據滑動速率計算出要滑動到的位置，然後設定給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 預設是LinearSmoothScroller ,

public class LinearSmoothScroller extends RecyclerView.SmoothScroller

然後會呼叫smoothScroller 滾動到指定位置, 開始滾動之前會呼叫start 方法, 會設定mTargetView 就是我們上面設定的要滾到的位置：

/**
* Starts a smooth scroll for the given target position.
* <p>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.</p>
*
* <p>Note that if RecyclerView finds the target view, it will automatically stop the
* SmoothScroller. This <b>does not</b> mean that scroll will stop, it only means it will
* stop calling SmoothScroller in each animation step.</p>
*/
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 滑動是會呼叫onAnimation ，在滾動過程中如果發現getChildPosition(mTargetView) == mTargetPosition ,也就是目標位置已經layout出來了，那麼就會回撥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);
}

沒有懸念，繞了一大圈終於回到SnapHelper 中onTargetFound ,在onTargetFound 中會呼叫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;
}
};

計算到目標位置的座標然後更新給RecyclerView.Action ,然後在上面的onAnimation 會呼叫mRecyclingAction.runIfNecessary(recyclerView); ，然後會呼叫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 中就會設定狀態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();
}

上面繞了一圈，總結一下呼叫棧就是：

SnapHelper
onFling ---> snapFromFling

上面得到最終位置targetPosition ,把位置給RecyclerView.SmoothScroller , 然後就開始滑動了：

RecyclerView.SmoothScroller
start --> onAnimation

在滑動過程中如果targetPosition 對應的targetView 已經layout出來了，就會回撥SnapHelper ，然後計算得到到當前位置到targetView 的距離dx,dy

SnapHelper
onTargetFound ---> calculateDistanceToFinalSnap

然後把距離dx,dy 更新給RecyclerView.Action :

RecyclerView.Action
update --> runIfNecessary --> recyclerView.mViewFlinger.smoothScrollBy

最後呼叫RecyclerView.ViewFlinger , 然後又回到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.觸發`fling` 操作

接著我們再看看Fling操作在RecyclerView 中是什麼時候觸發的。

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觸發的時候會呼叫native方法去獲得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);
}

如果有其中一個不為0就會呼叫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;
}

這裡有個mMinFlingVelocity 欄位，如果x和y方向的速率小於這個欄位，就不會觸發fling操作，直接return false，然後就會把RecyclerView 的狀態設定為SCROLL_STATE_IDLE 。mMinFlingVelocity 在RecyclerView 構造的時候會從ViewConfiguration 中獲取

final ViewConfiguration vc = ViewConfiguration.get(context);
mMinFlingVelocity = vc.getScaledMinimumFlingVelocity();
mMaxFlingVelocity = vc.getScaledMaximumFlingVelocity();

在ViewConfiguration 建構函式中：

/**
* 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;
}

所以如果滑動速率小於50的話就不會觸發fling操作。

從上面分析知道，fling操作是在手指離開的時候觸發然後直到滑動停止這中間的一段操作。在滑動過程中其實state狀態是SCROLL_STATE_SETTLING ,然後通過RecyclerView.ViewFlinger 進行滑動。

RecyclerView之Scroll和Fling

1.Scroll 和狀態

2.Fling 和Scroll 的關係

3.觸發fling 操作

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