《Android 基础（四十三）》 SnapHelper

/**
 * Class intended to support snapping for a {@link RecyclerView}.
 * <p>
 * SnapHelper tries to handle fling as well but for this to work properly, the
 * {@link RecyclerView.LayoutManager} must implement the {@link ScrollVectorProvider} interface or
 * you should override {@link #onFling(int, int)} and handle fling manually.
 */

LinearLayoutManager llm = new LinearLayoutManager(this, LinearLayoutManager.HORIZONTAL, false);
CardAdapter cardAdapter = new CardAdapter(this);
LinearSnapHelper linearSnapHelper = new LinearSnapHelper();

mRvContent.setLayoutManager(llm);
mRvContent.setAdapter(cardAdapter);
linearSnapHelper.attachToRecyclerView(mRvContent);

LinearLayoutManager llm = new LinearLayoutManager(this, LinearLayoutManager.VERTICAL, false);
CardAdapter cardAdapter = new CardAdapter(this);
LinearSnapHelper linearSnapHelper = new LinearSnapHelper();

mRvContent.setLayoutManager(llm);
mRvContent.setAdapter(cardAdapter);
linearSnapHelper.attachToRecyclerView(mRvContent);

LinearLayoutManager llm = new LinearLayoutManager(this, LinearLayoutManager.HORIZONTAL, false);
CardAdapter cardAdapter = new CardAdapter(this);
PagerSnapHelper pagerSnapHelper = new PagerSnapHelper();

mRvContent.setLayoutManager(llm);
mRvContent.setAdapter(cardAdapter);
pagerSnapHelper.attachToRecyclerView(mRvContent);

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();
    }
}

private void destroyCallbacks() {
       mRecyclerView.removeOnScrollListener(mScrollListener);
       mRecyclerView.setOnFlingListener(null);
   }

private void setupCallbacks() throws IllegalStateException {
     //如果onFlingListener已经设置过，则抛出异常
     if (mRecyclerView.getOnFlingListener() != null) {
         throw new IllegalStateException("An instance of OnFlingListener already set.");
     }
     mRecyclerView.addOnScrollListener(mScrollListener);
     mRecyclerView.setOnFlingListener(this);
 }

// Handles the snap on scroll case.
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;
                    //当滑动状态为idle状态时并且刚结束滚动
                    snapToTargetExistingView();
                }
            }

            @Override
            public void onScrolled(RecyclerView recyclerView, int dx, int dy) {
                if (dx != 0 || dy != 0) {
                    mScrolled = true;
                }
            }
        };

void snapToTargetExistingView() {
    if (mRecyclerView == null) {
        return;
    }
    LayoutManager layoutManager = mRecyclerView.getLayoutManager();
    if (layoutManager == null) {
        return;
    }
    //找到SnapView
    View snapView = findSnapView(layoutManager);
    if (snapView == null) {
        return;
    }
    //计算偏移量然后平滑的滑动过去
    int[] snapDistance = calculateDistanceToFinalSnap(layoutManager, snapView);
    if (snapDistance[0] != 0 || snapDistance[1] != 0) {
        mRecyclerView.smoothScrollBy(snapDistance[0], snapDistance[1]);
    }
}

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;
    }
    //获取触发fling的最小速度
    int minFlingVelocity = mRecyclerView.getMinFlingVelocity();
    //横向或者纵向必须有一个滑动速度可以触发fling，snapFromFling和上面滚动监听器中的实现思路类似
    return (Math.abs(velocityY) > minFlingVelocity || Math.abs(velocityX) > minFlingVelocity)
            && snapFromFling(layoutManager, velocityX, velocityY);
}

private boolean snapFromFling(@NonNull LayoutManager layoutManager, int velocityX,
        int velocityY) {
     //这就是为何前文中提到需要实现ScrollVectorProvider否则旧得自己实现onFling接口
    if (!(layoutManager instanceof ScrollVectorProvider)) {
        return false;
    }

    //创建scroller
    RecyclerView.SmoothScroller smoothScroller = createSnapScroller(layoutManager);
    if (smoothScroller == null) {
        return false;
    }

    //找到目标snap位置
    int targetPosition = findTargetSnapPosition(layoutManager, velocityX, velocityY);
    if (targetPosition == RecyclerView.NO_POSITION) {
        return false;
    }

    //平滑的滚动过去
    smoothScroller.setTargetPosition(targetPosition);
    layoutManager.startSmoothScroll(smoothScroller);
    return true;
}