RecyclerView之SnapHelper源码分析-白红宇

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，整理了下做个总结，主要是实现类似于抖音翻页的效果，但是有有点不同，需要在底部漏出后面的view，这样说可能不好理解，看下Demo，按页滑动，后面的View有放大缩放的动画，滑动速度过小时会有回到原位的效果，下滑也是按页滑动的效果。

有的小伙伴可能说这个用 SnapHelper就可以了，没错，翻页是要结合这个，但是也不是纯粹靠这个，因为底部需要漏出来后面的view，所以LayoutManager就不能简单的使用LinearLayoutManager,需要去自定义LayoutManager,然后再自定义SnapHelper。

如果把自定义LayoutManager和SnapHelper放在一篇里面会太长，所以我们今天主要分析SnapHelper。

本文分析的源码是基于recyclerview-v7-26.1.0

1.`Scroll`和`Fling`

这方面参考我的上篇分享：

总结一下调用栈就是：

SnapHelperonFling ---> snapFromFling 复制代码

上面得到最终位置targetPosition,把位置给RecyclerView.SmoothScroller, 然后就开始滑动了：

RecyclerView.SmoothScrollerstart --> onAnimation复制代码

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

SnapHelperonTargetFound ---> calculateDistanceToFinalSnap复制代码

然后把距离dx,dy更新给RecyclerView.Action:

RecyclerView.Actionupdate --> runIfNecessary --> recyclerView.mViewFlinger.smoothScrollBy复制代码

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();        }复制代码

2.`SnapHelper`源码分析

上面其实已经接触到部分的SnapHelper源码, SnapHelper其实是一个抽象类，有三个抽象方法：

/**     * Override to provide a particular adapter target position for snapping.     *     * @param layoutManager the {@link RecyclerView.LayoutManager} associated with the attached     *                      {@link RecyclerView}     * @param velocityX fling velocity on the horizontal axis     * @param velocityY fling velocity on the vertical axis     *     * @return the target adapter position to you want to snap or {@link RecyclerView#NO_POSITION}     *         if no snapping should happen     */    public abstract int findTargetSnapPosition(LayoutManager layoutManager, int velocityX,            int velocityY);    /**     * Override this method to snap to a particular point within the target view or the container     * view on any axis.     *      * This method is called when the {@link SnapHelper} has intercepted a fling and it needs     * to know the exact distance required to scroll by in order to snap to the target view.     *     * @param layoutManager the {@link RecyclerView.LayoutManager} associated with the attached     *                      {@link RecyclerView}     * @param targetView the target view that is chosen as the view to snap     *     * @return the output coordinates the put the result into. out[0] is the distance     * on horizontal axis and out[1] is the distance on vertical axis.     */    @SuppressWarnings("WeakerAccess")    @Nullable    public abstract int[] calculateDistanceToFinalSnap(@NonNull LayoutManager layoutManager,            @NonNull View targetView);    /**     * Override this method to provide a particular target view for snapping.     * 
     * This method is called when the {@link SnapHelper} is ready to start snapping and requires     * a target view to snap to. It will be explicitly called when the scroll state becomes idle     * after a scroll. It will also be called when the {@link SnapHelper} is preparing to snap     * after a fling and requires a reference view from the current set of child views.     * 
     * If this method returns {@code null}, SnapHelper will not snap to any view.     *     * @param layoutManager the {@link RecyclerView.LayoutManager} associated with the attached     *                      {@link RecyclerView}     *     * @return the target view to which to snap on fling or end of scroll     */    @SuppressWarnings("WeakerAccess")    @Nullable    public abstract View findSnapView(LayoutManager layoutManager);复制代码

上面三个方法就是我们重写SnapHelper需要实现的，很重要，简单介绍下它们的作用和调用时机：

findTargetSnapPosition用来找到最终的目标位置，在fling操作刚触发的时候会根据速度计算一个最终目标位置，然后开始fling操作 calculateDistanceToFinalSnap 这个用来计算滑动到最终位置还需要滑动的距离，在一开始attachToRecyclerView或者targetView layout的时候会调用 findSnapView用来找到上面的targetView，就是需要对其的view，在calculateDistanceToFinalSnap调用之前会调用该方法。

我们看下SnapHelper怎么用的，其实就一行代码：

this.snapHelper.attachToRecyclerView(view);复制代码

SnapHelper正是通过该方法附着到RecyclerView上，从而实现辅助RecyclerView滚动对齐操作,那我们就从上面的attachToRecyclerView开始入手：

public void attachToRecyclerView(@Nullable RecyclerView recyclerView)            throws IllegalStateException {        if (mRecyclerView == recyclerView) {            return; // nothing to do        }        if (mRecyclerView != null) {            destroyCallbacks();        }        mRecyclerView = recyclerView;        if (mRecyclerView != null) {            setupCallbacks();            mGravityScroller = new Scroller(mRecyclerView.getContext(),                    new DecelerateInterpolator());            snapToTargetExistingView();        }    }复制代码

在attachToRecyclerView()方法中会清掉SnapHelper之前保存的RecyclerView对象的回调(如果有的话)，对新设置进来的RecyclerView对象设置回调,然后初始化一个Scroller对象,最后调用snapToTargetExistingView()方法对SnapView进行对齐调整。

snapToTargetExistingView()

该方法的作用是对SnapView进行滚动调整，以使得SnapView达到对齐效果。

看下源码：

void snapToTargetExistingView() {        if (mRecyclerView == null) {            return;        }        LayoutManager layoutManager = mRecyclerView.getLayoutManager();        if (layoutManager == null) {            return;        }        View snapView = findSnapView(layoutManager);        if (snapView == null) {            return;        }        int[] snapDistance = calculateDistanceToFinalSnap(layoutManager, snapView);        if (snapDistance[0] != 0 || snapDistance[1] != 0) {            mRecyclerView.smoothScrollBy(snapDistance[0], snapDistance[1]);        }    }复制代码

snapToTargetExistingView()方法就是先找到SnapView，然后计算SnapView当前坐标到目的坐标之间的距离，然后调用RecyclerView.smoothScrollBy()方法实现对RecyclerView内容的平滑滚动，从而将SnapView移到目标位置，达到对齐效果。

其实这个时候RecyclerView还没进行layout，一般findSnapView会返回null，不需要对齐。

回调

SnapHelper要有对齐功能，肯定需要知道RecyclerView的滚动scroll和fling过程的，这个就是通过回调接口实现。再看下attachToRecyclerView的源码：

public void attachToRecyclerView(@Nullable RecyclerView recyclerView)            throws IllegalStateException {        if (mRecyclerView == recyclerView) {            return; // nothing to do        }        if (mRecyclerView != null) {            destroyCallbacks();        }        mRecyclerView = recyclerView;        if (mRecyclerView != null) {            setupCallbacks();            mGravityScroller = new Scroller(mRecyclerView.getContext(),                    new DecelerateInterpolator());            snapToTargetExistingView();        }    }复制代码

一开始会先清空之前的回调接口然后再注册接口，先看下destroyCallbacks:

/**     * Called when the instance of a {@link RecyclerView} is detached.     */    private void destroyCallbacks() {        mRecyclerView.removeOnScrollListener(mScrollListener);        mRecyclerView.setOnFlingListener(null);    }复制代码

可以看出SnapHelper对RecyclerView设置了两个回调，一个是OnScrollListener对象mScrollListener，另外一个就是OnFlingListener对象。

再看下setupCallbacks:

/**     * Called when an instance of a {@link RecyclerView} is attached.     */    private void setupCallbacks() throws IllegalStateException {        if (mRecyclerView.getOnFlingListener() != null) {            throw new IllegalStateException("An instance of OnFlingListener already set.");        }        mRecyclerView.addOnScrollListener(mScrollListener);        mRecyclerView.setOnFlingListener(this);    }复制代码

SnapHelper实现了RecyclerView.OnFlingListener接口，所以OnFlingListener就是SnapHelper自身。

先来看下RecyclerView.OnScrollListener对象mScrollListener

RecyclerView.OnScrollListener

先看下mScrollListener是怎么实现的：

private final RecyclerView.OnScrollListener mScrollListener =            new RecyclerView.OnScrollListener() {                boolean mScrolled = false;                @Override                public void onScrollStateChanged(RecyclerView recyclerView, int newState) {                    super.onScrollStateChanged(recyclerView, newState);                    if (newState == RecyclerView.SCROLL_STATE_IDLE && mScrolled) {                        mScrolled = false;                        snapToTargetExistingView();                    }                }                @Override                public void onScrolled(RecyclerView recyclerView, int dx, int dy) {                    if (dx != 0 || dy != 0) {                        mScrolled = true;                    }                }            };复制代码

mScrolled = true表示之前滚动过，RecyclerView.SCROLL_STATE_IDLE表示滚动停止，这个不清楚的可以看考之前的博客。这个监听器的实现其实很简单，就是在滚动停止的时候调用snapToTargetExistingView对目标View进行滚动调整对齐。

RecyclerView.OnFlingListener

RecyclerView.OnFlingListener接口只有一个方法，这个就是在Fling操作触发的时候会回调，返回true就是已处理，返回false就会交给系统处理。

/**     * This class defines the behavior of fling if the developer wishes to handle it.     *      * Subclasses of {@link OnFlingListener} can be used to implement custom fling behavior.     *     * @see #setOnFlingListener(OnFlingListener)     */    public abstract static class OnFlingListener {        /**         * Override this to handle a fling given the velocities in both x and y directions.         * Note that this method will only be called if the associated {@link LayoutManager}         * supports scrolling and the fling is not handled by nested scrolls first.         *         * @param velocityX the fling velocity on the X axis         * @param velocityY the fling velocity on the Y axis         *         * @return true if the fling was handled, false otherwise.         */        public abstract boolean onFling(int velocityX, int velocityY);    }复制代码

看下SnapHelper怎么实现onFling()方法:

@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);    }复制代码

首先会获取mRecyclerView.getMinFlingVelocity()需要进行fling操作的最小速率，只有超过该速率，Item才能在手指离开的时候进行Fling操作。关键就是调用snapFromFling方法实现平滑滚动。

snapFromFling

看下怎么实现的：

private boolean snapFromFling(@NonNull LayoutManager layoutManager, int velocityX,            int velocityY) {        if (!(layoutManager instanceof ScrollVectorProvider)) {            return false;        }        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;    }复制代码

首先判断是不是实现了ScrollVectorProvider接口，系统提供的Layoutmanager默认都实现了该接口

创建SmoothScroller对象,默认是LinearSmoothScroller对象，会用LinearInterpolator进行平滑滚动，在目标位置成为Recyclerview的子View时会用DecelerateInterpolator进行减速停止。

通过findTargetSnapPosition()方法，以layoutManager和速率作为参数，找到targetSnapPosition,这个方法就是自定义SnapHelper需要实现的。

把targetSnapPosition设置给平滑滚动器，然后开始进行滚动操作。

很明显重点就是要看下平滑滚动器了。

LinearSmoothScroller

看下系统怎么实现：

@Nullable    protected LinearSmoothScroller createSnapScroller(LayoutManager layoutManager) {        if (!(layoutManager instanceof ScrollVectorProvider)) {            return null;        }        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;            }        };    }复制代码

在通过findTargetSnapPosition()方法找到的targetSnapPosition成为Recyclerview的子View时(根据Recyclerview的缓存机制，这个时候可能该View在屏幕上还看不到),会回调onTargetFound，看下系统定义：

/**         * Called when the target position is laid out. This is the last callback SmoothScroller         * will receive and it should update the provided {@link Action} to define the scroll         * details towards the target view.         * @param targetView    The view element which render the target position.         * @param state         Transient state of RecyclerView         * @param action        Action instance that you should update to define final scroll action         *                      towards the targetView         */        protected abstract void onTargetFound(View targetView, State state, Action action);复制代码

传入的第一个参数targetView就是我们希望滚动到的位置对应的View，最后一个参数就是我们可以用来通知滚动器要减速滚动的距离。

其实就是我们要在这个方法里面告诉滚动器在目标子View layout出来后还需要滚动多少距离，然后通过Action通知滚动器。

第二个方法是计算滚动速率，返回值会影响onTargetFound中的calculateTimeForDeceleration方法，看下源码：

private final float MILLISECONDS_PER_PX;    public LinearSmoothScroller(Context context) {        MILLISECONDS_PER_PX = calculateSpeedPerPixel(context.getResources().getDisplayMetrics());    }    /**     * Calculates the time it should take to scroll the given distance (in pixels)     *     * @param dx Distance in pixels that we want to scroll     * @return Time in milliseconds     * @see #calculateSpeedPerPixel(android.util.DisplayMetrics)     */    protected int calculateTimeForScrolling(int dx) {        // In a case where dx is very small, rounding may return 0 although dx > 0.        // To avoid that issue, ceil the result so that if dx > 0, we'll always return positive        // time.        return (int) Math.ceil(Math.abs(dx) * MILLISECONDS_PER_PX);    }    /**     * Calculates the time for deceleration so that transition from LinearInterpolator to     * DecelerateInterpolator looks smooth.
     *     * @param dx Distance to scroll     * @return Time for DecelerateInterpolator to smoothly traverse the distance when transitioning     * from LinearInterpolation     */    protected int calculateTimeForDeceleration(int dx) {        // we want to cover same area with the linear interpolator for the first 10% of the        // interpolation. After that, deceleration will take control.        // area under curve (1-(1-x)^2) can be calculated as (1 - x/3) * x * x        // which gives 0.100028 when x = .3356        // this is why we divide linear scrolling time with .3356        return  (int) Math.ceil(calculateTimeForScrolling(dx) / .3356);    }复制代码

可以看到，第二个方法返回值越大，需要滚动的时间越长，也就是滚动越慢。

3.总结

到这里，SnapHelper的源码就分析完了，整理下思路，SnapHelper辅助RecyclerView实现滚动对齐就是通过给RecyclerView设置OnScrollerListener和OnFlingListener这两个监听器实现的。整个过程如下：

在onFling操作触发的时候首先通过findTargetSnapPosition找到最终需要滚动到的位置，然后启动平滑滚动器滚动到指定位置，

在指定位置需要渲染的View -targetView layout出来后，系统会回调onTargetFound,然后调用calculateDistanceToFinalSnap方法计算targetView需要减速滚动的距离，然后通过Action更新给滚动器。

在滚动停止的时候，也就是state变成SCROLL_STATE_IDLE时会调用snapToTargetExistingView，通过findSnapView找到SnapView，然后通过calculateDistanceToFinalSnap计算得到滚动的距离，做最后的对齐调整。

前面分享的Demo就留到下一篇博客再说了，其实只要理解了SnapHelper的源码，自定义就很简单了。

对Demo感兴趣的欢迎关注下一篇博客了。

完。

1.Scroll和Fling

2.SnapHelper源码分析