Google最新发布的Support RecyclerView包更新到24.2.0，这次来聊聊RecyclerView的新特性和SnapHelper的关系。
一句话介绍SnapHelper: SnapHelper是RecyclerView功能的一种拓展，使RecyclerView滑动行为类似ViewPager，无论怎么滑动最终停留在某页正中间。ViewPager一次只能滑动一页，RecyclerView+SnapHelper方式可以一次滑动好几页，且最终都停留在某页正中间。非常实用和酷炫。
SnapHelper的实现原理是监听RecyclerView.OnFlingListener中的onFling接口。LinearSnapHelper是抽象类SnapHelper的具体实现。
上面的效果只需下面几行代码即可。重点在于new LinearSnapHelper().attachToRecyclerView(recyclerView);
LinearLayoutManager linearLayoutManager = new LinearLayoutManager(getContext(), LinearLayoutManager.HORIZONTAL, false);
recyclerView.setLayoutManager(linearLayoutManager);
new LinearSnapHelper().attachToRecyclerView(recyclerView);
接下来具体分析LinearSnapHelper是怎么实现类似ViewPager的功能的
attachToRecyclerView，居中处理分析
public void attachToRecyclerView(@Nullable RecyclerView recyclerView) throws IllegalStateException {
...
snapToTargetExistingView();
...
}
/**
* 1. 找到居中显示的View
* 2. 计算view离当前的位置距离, 调用mRecyclerView.smoothScrollBy使其居中
*/
private void snapToTargetExistingView() {
View snapView = findSnapView(layoutManager);
int[] snapDistance = calculateDistanceToFinalSnap(layoutManager, snapView);
if (snapDistance[0] != 0 || snapDistance[1] != 0) {
mRecyclerView.smoothScrollBy(snapDistance[0], snapDistance[1]);
}
}
/**
* 1. 找到当前RecyclerView的居中位置center
* 2. 循环遍历子节点，找出子节点居中位置最接近center的视图，及SnapView
*/
public View findSnapView(RecyclerView.LayoutManager layoutManager) {
...
}
/**
* 计算到targetView要移动的距离
*/
@Override
public int[] calculateDistanceToFinalSnap(
@NonNull RecyclerView.LayoutManager layoutManager, @NonNull View targetView) {
...
}
RecyclerView.OnFling，滑动后停止到居中位置分析
SnapHelper extends RecyclerView.OnFlingListener，重载onFling函数
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);
}
/**
* 同样的套路，先根据移动速度确定最终位置，然后startSmoothScroll
*/
private boolean snapFromFling(@NonNull 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;
}