RecyclerView之SnapHelper源码分析

时间 2019-12-05

标签 recyclerview snaphelper 源码分析繁體版

原文原文链接

好久没有写Android控件了，正好最近项目有个自定义控件的需求，整理了下作个总结，主要是实现相似于抖音翻页的效果，可是有有点不一样，须要在底部漏出后面的view，这样说可能很差理解，看下Demo，按页滑动，后面的View有放大缩放的动画，滑动速度太小时会有回到原位的效果，下滑也是按页滑动的效果。android

有的小伙伴可能说这个用 SnapHelper就能够了，没错，翻页是要结合这个，可是也不是纯粹靠这个，由于底部须要漏出来后面的view，因此LayoutManager就不能简单的使用LinearLayoutManager,须要去自定义LayoutManager,而后再自定义SnapHelper。缓存

若是把自定义LayoutManager和SnapHelper放在一篇里面会太长，因此咱们今天主要分析SnapHelper。bash

本文分析的源码是基于recyclerview-v7-26.1.0app

1.`Scroll`和`Fling`

这方面参考个人上篇分享：RecyclerView之Scroll和Flingide

总结一下调用栈就是：源码分析

SnapHelper
onFling ---> snapFromFling 
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上面获得最终位置targetPosition,把位置给RecyclerView.SmoothScroller, 而后就开始滑动了：post

RecyclerView.SmoothScroller
start --> onAnimation
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在滑动过程当中若是targetPosition对应的targetView已经layout出来了，就会回调SnapHelper，而后计算获得到当前位置到targetView的距离dx,dy动画

SnapHelper
onTargetFound ---> calculateDistanceToFinalSnap
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而后把距离dx,dy更新给RecyclerView.Action:ui

RecyclerView.Action
update --> runIfNecessary --> recyclerView.mViewFlinger.smoothScrollBy
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最后调用RecyclerView.ViewFlinger, 而后又回到onAnimationthis

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();
        }
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2.`SnapHelper`源码分析

上面其实已经接触到部分的SnapHelper源码, SnapHelper实际上是一个抽象类，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.
     * <p>
     * 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.
     * <p>
     * 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.
     * <p>
     * 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);
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上面三个方法就是咱们重写SnapHelper须要实现的，很重要，简单介绍下它们的做用和调用时机：

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

咱们看下SnapHelper怎么用的，其实就一行代码：

this.snapHelper.attachToRecyclerView(view);
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SnapHelper正是经过该方法附着到RecyclerView上，从而实现辅助RecyclerView滚动对齐操做,那咱们就从上面的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();
        }
    }
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在attachToRecyclerView()方法中会清掉SnapHelper以前保存的RecyclerView对象的回调(若是有的话)，对新设置进来的RecyclerView对象设置回调,而后初始化一个Scroller对象,最后调用snapToTargetExistingView()方法对SnapView进行对齐调整。

snapToTargetExistingView()

该方法的做用是对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]);
        }
    }
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snapToTargetExistingView()方法就是先找到SnapView，而后计算SnapView当前坐标到目的坐标之间的距离，而后调用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，通常findSnapView会返回null，不须要对齐。

回调

SnapHelper要有对齐功能，确定须要知道RecyclerView的滚动scroll和fling过程的，这个就是经过回调接口实现。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();
        }
    }
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一开始会先清空以前的回调接口而后再注册接口，先看下destroyCallbacks:

/**
     * Called when the instance of a {@link RecyclerView} is detached.
     */
    private void destroyCallbacks() {
        mRecyclerView.removeOnScrollListener(mScrollListener);
        mRecyclerView.setOnFlingListener(null);
    }
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能够看出SnapHelper对RecyclerView设置了两个回调，一个是OnScrollListener对象mScrollListener，另一个就是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);
    }
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SnapHelper实现了RecyclerView.OnFlingListener接口，因此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;
                    }
                }
            };
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mScrolled = true表示以前滚动过，RecyclerView.SCROLL_STATE_IDLE表示滚动中止，这个不清楚的能够看考以前的博客RecyclerView之Scroll和Fling。这个监听器的实现其实很简单，就是在滚动中止的时候调用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操做触发的时候会回调，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.
     * <p>
     * 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);
    }
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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);
    }
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首先会获取mRecyclerView.getMinFlingVelocity()须要进行fling操做的最小速率，只有超过该速率，Item才能在手指离开的时候进行Fling操做。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;
    }
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首先判断是否是实现了ScrollVectorProvider接口，系统提供的Layoutmanager默认都实现了该接口

建立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进行减速中止。

经过findTargetSnapPosition()方法，以layoutManager和速率做为参数，找到targetSnapPosition,这个方法就是自定义SnapHelper须要实现的。

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

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;
            }
        };
    }
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在经过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);
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传入的第一个参数targetView就是咱们但愿滚动到的位置对应的View，最后一个参数就是咱们能够用来通知滚动器要减速滚动的距离。

其实就是咱们要在这个方法里面告诉滚动器在目标子View layout出来后还须要滚动多少距离，而后经过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); } /** * <p>Calculates the time for deceleration so that transition from LinearInterpolator to * DecelerateInterpolator looks smooth.</p> * * @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

能够看到，第二个方法返回值越大，须要滚动的时间越长，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实现滚动对齐就是经过给RecyclerView设置OnScrollerListener和OnFlingListener这两个监听器实现的。整个过程以下：

在onFling操做触发的时候首先经过findTargetSnapPosition找到最终须要滚动到的位置，而后启动平滑滚动器滚动到指定位置，

在指定位置须要渲染的View -targetView layout出来后，系统会回调onTargetFound,而后调用calculateDistanceToFinalSnap方法计算targetView须要减速滚动的距离，而后经过Action更新给滚动器。

在滚动中止的时候，也就是state变成SCROLL_STATE_IDLE时会调用snapToTargetExistingView，经过findSnapView找到SnapView，而后经过calculateDistanceToFinalSnap计算获得滚动的距离，作最后的对齐调整。

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。