RecyclerView的好朋友

SnapHelpter，相信很多人可能都不知道它或者没怎么关注过它，但是通过它实现的效果肯定都见过。比如短视频应用中切换视频时一划划一页的效果，这可不是ViewPager实现的啊，使用ViewPager实现的话成本太高，所以这类效果都是通过RecyclerVIew + SnapHelper来实现的，拿刚才讲的短视频切换效果来说，使用的就是RecyclerVIew和SnapHelper的子类PagerSnapHelper来实现的。

一、SnapHelper初解

说了这些，那么SnapHelper到底是什么东西呢？见名思意，Snap，翻译成中文有‘移到某位置’的意思，那么SnapHelper可以理解为‘移到某位置的帮手’，而这个被移到某位置的东西显然就是RecyclerVIew中的Item。

public abstract class SnapHelper extends RecyclerView.OnFlingListener {//....@Nullable
public abstract int[] calculateDistanceToFinalSnap(@NonNull LayoutManager layoutManager,@NonNull View targetView);@Nullable
public abstract View findSnapView(LayoutManager layoutManager);public abstract int findTargetSnapPosition(LayoutManager layoutManager, int velocityX,int velocityY);
}

可以看到SnapHelper是一个抽象类，并继承了RecyclerView.OnFlingListener这个类，其中还包括三个抽象方法，我们通过实现这三个方法，就可以帮助RecyclerView移动item到‘某位置’。

为了更好理解SnapHelper的这三个方法，先说说RecyclerView.OnFlingListener这个类。

public abstract static class OnFlingListener {/*** 可用于实现自定义投掷行为** @param velocityX X轴上的抛掷速度* @param velocityY Y轴上的抛掷速度** @return 如果处理了投掷，则为 true，否则为 false。*/public abstract boolean onFling(int velocityX, int velocityY);
}

这也是个抽象类，并且里面只有一个抽象方法，那这个类又是干啥的呢？我们都知道RecyclerView是可以滑动的，在我们手指离开屏幕后，RecyclerView还会继续顺着我们手指的方向再滑动一段距离，这个操作就是通过实现OnFlingListener接口来做到的。

SnapHelper继承了OnFlingListener实现了onFling方法，并在调用attachToRecyclerView（）方法的时候将OnFlingListener设置给了RecyclerView。

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();}}/*** 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);
}

二、三个方法

接着我们继续看SnapHelper中的三个抽象方法。

1、calculateDistanceToFinalSnap（）

/*** 计算将目标item移动到最终位置所需距离** @param layoutManager * @param targetView 需要被移动的item** @return 输出坐标将结果，out[0] 是水平轴上的距离，out[1] 是垂直轴上的距离。*/
@SuppressWarnings("WeakerAccess")
@Nullable
public abstract int[] calculateDistanceToFinalSnap(@NonNull LayoutManager layoutManager,@NonNull View targetView);public abstract int[] calculateDistanceToFinalSnap(@NonNull RecyclerView.LayoutManager layoutManager,@NonNull View targetView);

这个方法是SnapHelper中另外两个抽象方法findSnapView（）和findTargetSnapPosition（）的下游方法，其参数中的targetView就是这两个方法提供的

通过findSnapView（）提供

void snapToTargetExistingView() {/***/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]);}
}

通过findTargetSnapPosition（）提供

private boolean snapFromFling(@NonNull RecyclerView.LayoutManager layoutManager, int velocityX,int velocityY) {/**/RecyclerView.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;
}

findTargetSnapPosition（）被调用后，将找到的位置设置给smoothScroller，然后再通过layoutManager调用startSmoothScroll（）方法启动smoothScroller

public void startSmoothScroll(SmoothScroller smoothScroller) {if (mSmoothScroller != null && smoothScroller != mSmoothScroller&& mSmoothScroller.isRunning()) {mSmoothScroller.stop();}mSmoothScroller = smoothScroller;mSmoothScroller.start(mRecyclerView, this);
}

在smoothScroller的start（）方法中找到targetView

void start(RecyclerView recyclerView, LayoutManager layoutManager) {/***/mTargetView = findViewByPosition(getTargetPosition());onStart();mRecyclerView.mViewFlinger.postOnAnimation();mStarted = true;
}

最后回调到SnapHelper中创建的SmoothScroller中的onTargetFound（）方法

@Nullable
@Deprecated
protected LinearSmoothScroller createSnapScroller(RecyclerView.LayoutManager layoutManager) {/***/return new LinearSmoothScroller(mRecyclerView.getContext()) {@Overrideprotected 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);}}@Overrideprotected float calculateSpeedPerPixel(DisplayMetrics displayMetrics) {return MILLISECONDS_PER_INCH / displayMetrics.densityDpi;}};
}

2、findSnapView（）

/*** 找到需要被移动的item.* 如果返回 {@code null}, 则SnapHelper 不需要移动任何item.** @param layoutManager** @return 需要被移动的item*/
@SuppressWarnings("WeakerAccess")
@Nullable
public abstract View findSnapView(LayoutManager layoutManager);

这个方法会在SnapHelper绑定到RecyclerView时和RecyclerView停止滑动时被调用

void snapToTargetExistingView() {/***/View snapView = findSnapView(layoutManager);/***/
}//绑定RecyclerView时被调用
public void attachToRecyclerView(@Nullable RecyclerView recyclerView)throws IllegalStateException {/***/if (mRecyclerView != null) {/***/snapToTargetExistingView();}
}//RecyclerView停止滑到时被调用
private final RecyclerView.OnScrollListener mScrollListener =new RecyclerView.OnScrollListener() {boolean mScrolled = false;@Overridepublic void onScrollStateChanged(RecyclerView recyclerView, int newState) {super.onScrollStateChanged(recyclerView, newState);if (newState == RecyclerView.SCROLL_STATE_IDLE && mScrolled) {mScrolled = false;snapToTargetExistingView();}}@Overridepublic void onScrolled(RecyclerView recyclerView, int dx, int dy) {/***/}};

3、findTargetSnapPosition（）

/*** 找到需要被移动的目标item在adapter中的位置** @param layoutManager * @param 水平轴上的抛掷速度* @param 纵轴上的抛掷速度** @return 返回需要被移动的目标item在adapter中的位置或者无需移动时返回 {@link RecyclerView#NO_POSITION}*/
public abstract int findTargetSnapPosition(LayoutManager layoutManager, int velocityX,int velocityY);
}

这个方法会在RecyclerView触发fling操作时被调用

private boolean snapFromFling(@NonNull RecyclerView.LayoutManager layoutManager, int velocityX,int velocityY) {/***/int targetPosition = findTargetSnapPosition(layoutManager, velocityX, velocityY);if (targetPosition == RecyclerView.NO_POSITION) {return false;}smoothScroller.setTargetPosition(targetPosition);layoutManager.startSmoothScroll(smoothScroller);return true;
}@Overridepublic boolean onFling(int velocityX, int velocityY) {/***/return (Math.abs(velocityY) > minFlingVelocity || Math.abs(velocityX) > minFlingVelocity)&& snapFromFling(layoutManager, velocityX, velocityY);}

三、自定义SnapHelper实战

了解了SnapHelper三个方法的作用以及何时会调用后，我们趁热打铁，自己实现一个SnapHelper，如果想更多了解关于SnapHelper的实现，可以去看看官方实现的LinearSnapHelper和PagerSnapHelper。

这次我们继承SnapHelper，实现对RecyclerView一滑滑一页的效果，类似官方的PagerSnapHelper，但是比它更灵活，因为它的一页是一条item，我们的一页可以是多个item。

其实这次要实现的效果在很多App中都能看到，尤其是应用商城类的App。

光说还是有点懵，先看看实现的最终效果吧~

public class MyGallerySnapHelper extends SnapHelper {protected RecyclerView mRecyclerView;@Nullableprivate OrientationHelper mHorizontalHelper;private int pageSize;@Overridepublic void attachToRecyclerView(@Nullable RecyclerView recyclerView) throws IllegalStateException {mRecyclerView = recyclerView;super.attachToRecyclerView(recyclerView);}@Nullable@Overridepublic int[] calculateDistanceToFinalSnap(@NonNull RecyclerView.LayoutManager layoutManager, @NonNull View view) {int[] out = new int[2];//RecyclerView为横向方向时if (layoutManager.canScrollHorizontally()) {out[0] = distance2Start(layoutManager, view,getHorizontalHelper(layoutManager));}return out;}private int distance2Start(@NonNull RecyclerView.LayoutManager layoutManager, @NonNull View targetView, OrientationHelper helper) {//获取item的宽度int columnWidth = helper.getDecoratedMeasurement(targetView);//获取item的下标int position = layoutManager.getPosition(targetView);//计算RecyclerView一屏可以展示多少itempageSize = (mRecyclerView.getWidth() - mRecyclerView.getPaddingStart() - mRecyclerView.getPaddingEnd()) / getHorizontalHelper(layoutManager).getDecoratedMeasurement(targetView);//计算item处于第几屏int pageIndex = position / pageSize;//计算上一步所得屏数中第一个item的下标int currentPageStart = pageIndex * pageSize;//计算传入item和它所属屏数第一个item的距离int distance = ((position - currentPageStart)) * columnWidth;//获取传入item的顶部在RecyclerView中的位置（像素）final int childStart = helper.getDecoratedStart(targetView);return childStart - distance;}@Nullable@Overridepublic View findSnapView(RecyclerView.LayoutManager layoutManager) {return findStartView(layoutManager, getHorizontalHelper(layoutManager));}private View findStartView(RecyclerView.LayoutManager layoutManager, OrientationHelper helper) {int childCount = layoutManager.getChildCount();if (childCount == 0) return null;int lastPosition = 0;//获取最后一个完整可见item的下标if (layoutManager instanceof LinearLayoutManager) {lastPosition = ((LinearLayoutManager) layoutManager).findLastCompletelyVisibleItemPosition();}int absClosest = Integer.MAX_VALUE;View snapView = null;//如最后一个完整可见item的下标等于列表最后一个item的下标if (lastPosition == layoutManager.getItemCount() - 1) {snapView = layoutManager.getChildAt(lastPosition);} else {//找到距离RecyclerView顶部最近的itemfor (int i = 0; i < childCount; i++) {View child = layoutManager.getChildAt(i);int absDistance = helper.getDecoratedStart(child);if (absDistance < absClosest) {absClosest = absDistance;snapView = child;}}}return snapView;}@Overridepublic int findTargetSnapPosition(RecyclerView.LayoutManager layoutManager, int velocityX, int velocityY) {//找到距离RecyclerView顶部最近的itemView snapView = findSnapView(layoutManager);if (snapView == null) return RecyclerView.NO_POSITION;//得到距离RecyclerView顶部最近的item的下标int startMostPosition = layoutManager.getPosition(snapView);if (startMostPosition == RecyclerView.NO_POSITION) return RecyclerView.NO_POSITION;//滑动方向，ture为正方向滑动 false为反方向滑动final boolean forwardDirection;if (layoutManager.canScrollHorizontally()) {forwardDirection = velocityX > 0;} else {forwardDirection = velocityY > 0;}View childAt = layoutManager.getChildAt(0);//计算RecyclerView一屏可以展示多少itemif (childAt != null) {pageSize = (mRecyclerView.getWidth() - mRecyclerView.getPaddingStart() - mRecyclerView.getPaddingEnd()) / getHorizontalHelper(layoutManager).getDecoratedMeasurement(childAt);}//计算item处于第几屏int pageIndex = startMostPosition / pageSize;//计算上一步所得屏数中第一个item的下标int currentPageStart = pageIndex * pageSize;//根据滑动方向，在当前屏首的下标上加减数量return forwardDirection ? Math.min(currentPageStart + pageSize, layoutManager.getItemCount() - 1) : Math.max(0, currentPageStart + pageSize - 1);}@Nullable@Overrideprotected RecyclerView.SmoothScroller createScroller(RecyclerView.LayoutManager layoutManager) {return !(layoutManager instanceof RecyclerView.SmoothScroller.ScrollVectorProvider) ? null : new LinearSmoothScroller(this.mRecyclerView.getContext()) {protected void onTargetFound(View targetView, RecyclerView.State state, RecyclerView.SmoothScroller.Action action) {if (mRecyclerView != null) {int[] snapDistances = calculateDistanceToFinalSnap(mRecyclerView.getLayoutManager(), targetView);int dx = snapDistances[0];int dy = snapDistances[1];int time = this.calculateTimeForDeceleration(Math.max(Math.abs(dx), Math.abs(dy)));if (time > 0) {action.update(dx, dy, time, this.mDecelerateInterpolator);}}}protected float calculateSpeedPerPixel(DisplayMetrics displayMetrics) {return 50.0F / (float) displayMetrics.densityDpi;}};}@NonNullprivate OrientationHelper getHorizontalHelper(@NonNull RecyclerView.LayoutManager layoutManager) {if (this.mHorizontalHelper == null || this.mHorizontalHelper.getLayoutManager() != layoutManager) {this.mHorizontalHelper = OrientationHelper.createHorizontalHelper(layoutManager);}return this.mHorizontalHelper;}
}

写的有点粗糙，只支持了LayoutManager为LinearLayoutManager时的水平方向，没有竖直方向的逻辑，也没有RecyclerView倒序时的逻辑，也没多少代码，懒得写了，想用的自己加吧。。