SnapHelper 源码解析

 

1.attachToRecyclerView

SnapHelper使用入口：attachToRecyclerView(),代码如下：

 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();
         //初始化Scroll对象，用来计算fling的距离
        mGravityScroller = new Scroller(mRecyclerView.getContext(),
                new DecelerateInterpolator());
        //滑动到目标位置
        snapToTargetExistingView();
    }
}


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


private void setupCallbacks() throws IllegalStateException {
    if (mRecyclerView.getOnFlingListener() != null) {
        throw new IllegalStateException("An instance of OnFlingListener already set.");                                  
    }
    mRecyclerView.addOnScrollListener(mScrollListener);
    mRecyclerView.setOnFlingListener(this);
}

2、snapToTargetExistingView

寻找目标位置及将目标View划定到屏幕中间是由snapToTargetExistingView()函数来实现的，snapToTargetExistingView的代码如下：

void snapToTargetExistingView() {
    if (mRecyclerView == null) {
        return;
    }
    
    RecyclerView.LayoutManager layoutManager = mRecyclerView.getLayoutManager();
    if (layoutManager == null) {
        return;
    }
    //fling前的当前View
    View snapView = findSnapView(layoutManager);
    if (snapView == null) {
       return;
    }
    //fling滑动的距离
    int[] snapDistance = calculateDistanceToFinalSnap(layoutManager, snapView);
    if (snapDistance[0] != 0 || snapDistance[1] != 0) {
        mRecyclerView.smoothScrollBy(snapDistance[0], snapDistance[1]);
    }
}

2.1、findSnapView

findSnapView()函数是查找离当前RecyclerView容器中心最近的View，具体实现代码参考LinearSnaoHelper类，代码如下：

@Override
public View findSnapView(RecyclerView.LayoutManager layoutManager) {
    if (layoutManager.canScrollVertically()) {
        return findCenterView(layoutManager, getVerticalHelper(layoutManager));
    } else if (layoutManager.canScrollHorizontally()) {
        return findCenterView(layoutManager, getHorizontalHelper(layoutManager));
    }
        return null;
}


@Nullable
private View findCenterView(RecyclerView.LayoutManager layoutManager,
            OrientationHelper helper) {
    int childCount = layoutManager.getChildCount();
    if (childCount == 0) {
        return null;
    }

    View closestChild = null;
    final int center;//容器的中间位置
    if (layoutManager.getClipToPadding()) {
        center = helper.getStartAfterPadding() + helper.getTotalSpace() / 2;
    } else {
        center = helper.getEnd() / 2;
    }
    int absClosest = Integer.MAX_VALUE;

    for (int i = 0; i < childCount; i++) {
        final View child = layoutManager.getChildAt(i);
        int childCenter = helper.getDecoratedStart(child)
                    + (helper.getDecoratedMeasurement(child) / 2);//条目的中间位置
        int absDistance = Math.abs(childCenter - center);

            /** if child center is closer than previous closest, set it as closest  **/
        if (absDistance < absClosest) {//找出与容器中间位置距离最近的条目
            absClosest = absDistance;
            closestChild = child;
        }
    }
    return closestChild;
}

2.2、calculateDistanceToFinalSnap

calculateDistanceToFinalSnap()函数

 @Override
public int[] calculateDistanceToFinalSnap(
    @NonNull RecyclerView.LayoutManager layoutManager, @NonNull View targetView) {
    if (layoutManager.canScrollHorizontally()) {
    out[0] = distanceToCenter(layoutManager, targetView,
         getHorizontalHelper(layoutManager));
    } else {
        out[0] = 0;
    }

    if (layoutManager.canScrollVertically()) {
        out[1] = distanceToCenter(layoutManager, targetView,
                getVerticalHelper(layoutManager));
    } else {
        out[1] = 0;
    }
    return out;
}

private int distanceToCenter(@NonNull RecyclerView.LayoutManager layoutManager,
        @NonNull View targetView, OrientationHelper helper) {
    final int childCenter = helper.getDecoratedStart(targetView)
            + (helper.getDecoratedMeasurement(targetView) / 2);
    final int containerCenter;
    if (layoutManager.getClipToPadding()) {
        containerCenter = helper.getStartAfterPadding() + helper.getTotalSpace() / 2;
    } else {
        containerCenter = helper.getEnd() / 2;
    }
    return childCenter - containerCenter;
}

在进行fling过程中需要对fling操作进行处理：

@Override
public boolean onFling(int velocityX, int velocityY) {
    RecyclerView.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 RecyclerView.LayoutManager layoutManager, 
int velocityX,int velocityY) {
    if (!(layoutManager instanceof RecyclerView.SmoothScroller.ScrollVectorProvider)) {
        return false;
    }

    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;
}


 @Override
    public int findTargetSnapPosition(RecyclerView.LayoutManager layoutManager, int velocityX, int velocityY) {
    if (!(layoutManager instanceof RecyclerView.SmoothScroller.ScrollVectorProvider)) {
        return RecyclerView.NO_POSITION;
    }
    final int itemCount = layoutManager.getItemCount();
    if (itemCount == 0) {
        return RecyclerView.NO_POSITION;
    }

    final View currentView = findSnapView(layoutManager);
    if (currentView == null) {
        return RecyclerView.NO_POSITION;
    }

    final int currentPosition = layoutManager.getPosition(currentView);
    if (currentPosition == RecyclerView.NO_POSITION) {
        return RecyclerView.NO_POSITION;
    }

    RecyclerView.SmoothScroller.ScrollVectorProvider vectorProvider =
                (RecyclerView.SmoothScroller.ScrollVectorProvider) layoutManager;
     
    PointF vectorForEnd = vectorProvider.computeScrollVectorForPosition(itemCount - 1);
    if (vectorForEnd == null) {
            // cannot get a vector for the given position.
        return RecyclerView.NO_POSITION;
    }

    int vDeltaJump, hDeltaJump;
    if (layoutManager.canScrollHorizontally()) {
        hDeltaJump = estimateNextPositionDiffForFling(layoutManager,
                    getHorizontalHelper(layoutManager), velocityX, 0);
        if (vectorForEnd.x < 0) {
            hDeltaJump = -hDeltaJump;
        }
    } else {
        hDeltaJump = 0;
    }
    if (layoutManager.canScrollVertically()) {
        vDeltaJump = estimateNextPositionDiffForFling(layoutManager,
                    getVerticalHelper(layoutManager), 0, velocityY);
        if (vectorForEnd.y < 0) {
            vDeltaJump = -vDeltaJump;
        }
    } else {
        vDeltaJump = 0;
    }

    int deltaJump = layoutManager.canScrollVertically() ? vDeltaJump : hDeltaJump;
    if (deltaJump == 0) {
        return RecyclerView.NO_POSITION;
    }

    int targetPos = currentPosition + deltaJump;
    if (targetPos < 0) {
        targetPos = 0;
    }
    if (targetPos >= itemCount) {
        targetPos = itemCount - 1;
    }
    return targetPos;
}