列表的滚动一般分为两种:

手指按下 -> 手指拖拽列表移动 -> 手指停止拖拽 -> 抬起手指
手指按下 -> 手指快速拖拽后抬起手指 -> 列表继续滚动 -> 停止滚动

从上面可以看出,滚动状态分为:

|
--静止

|
--滚动

    |
--被迫拖拽移动

    |
--自己滚动

上面的过程的状态变化如下:

静止 -> 被迫拖拽移动 -> 静止
静止 -> 被迫拖拽移动 -> 自己滚动 -> 静止

源码中RecyclerView的滚动处理是：

/**
 * Set a listener that will be notified of any changes in scroll state or position.
 * @param listener Listener to set or null to clear
 * @deprecated Use {@link #addOnScrollListener(OnScrollListener)} and
 *             {@link #removeOnScrollListener(OnScrollListener)}
 */
@Deprecated
public void setOnScrollListener(OnScrollListener listener) {
    mScrollListener = listener;
}

/**
 * Add a listener that will be notified of any changes in scroll state or position.
 * <p>Components that add a listener should take care to remove it when finished.
 * Other components that take ownership of a view may call {@link #clearOnScrollListeners()}
 * to remove all attached listeners.</p>
 * @param listener listener to set or null to clear
 */
public void addOnScrollListener(OnScrollListener listener) {
    if (mScrollListeners == null) {
        mScrollListeners = new ArrayList<>();
    }
    mScrollListeners.add(listener);
}

也就是说有两种方式可以监听滚动事件:

setOnScrollListener()
addOnScrollListener()

其中 setOnScrollListener 已经过时(@deprecated),建议使用 addOnScrollListener.如果在源码中没有 addOnScrollListener 方法,可能你的版本过旧,请升级recyclerview包

设置监听器的源码如下：

public abstract static class OnScrollListener {
    /**
     * Callback method to be invoked when RecyclerView's scroll state changes.
     * @param recyclerView The RecyclerView whose scroll state has changed.
     * @param newState     The updated scroll state. One of {@link #SCROLL_STATE_IDLE},
     *                     {@link #SCROLL_STATE_DRAGGING} or {@link #SCROLL_STATE_SETTLING}.
     */
    public void onScrollStateChanged(RecyclerView recyclerView, int newState){}

    /**
     * Callback method to be invoked when the RecyclerView has been scrolled. This will be
     * called after the scroll has completed.
     * <p>
     * This callback will also be called if visible item range changes after a layout
     * calculation. In that case, dx and dy will be 0.
     *
     * @param recyclerView The RecyclerView which scrolled.
     * @param dx The amount of horizontal scroll.
     * @param dy The amount of vertical scroll.
     */
    public void onScrolled(RecyclerView recyclerView, int dx, int dy){}
}

在滚动过程中,此监听器会回调两个方法.

onScrollStateChanged : 滚动状态变化时回调
onScrolled : 滚动时回调

这两者的区别在于: 状态与过程

举个例子

demoRv = (RecyclerView) findViewById(R.id.demo_rv);
layoutManager = new LinearLayoutManager(this);
demoRv.setLayoutManager(layoutManager);
demoRv.addItemDecoration(new DividerItemDecoration(this, DividerItemDecoration.VERTICAL_LIST));

bookAdapter = new BookAdapter();
bookAdapter.fillList(MockService.getBookList());
demoRv.setAdapter(bookAdapter);

demoRv.addOnScrollListener(new RecyclerView.OnScrollListener() {
    @Override
    public void onScrollStateChanged(RecyclerView recyclerView, int newState) {
        super.onScrollStateChanged(recyclerView, newState);
        Log.i(TAG, "-----------onScrollStateChanged-----------");
        Log.i(TAG, "newState: " + newState);
    }

    @Override
    public void onScrolled(RecyclerView recyclerView, int dx, int dy) {
        super.onScrolled(recyclerView, dx, dy);
        Log.i(TAG, "-----------onScrolled-----------");
        Log.i(TAG, "dx: " + dx);
        Log.i(TAG, "dy: " + dy);
        Log.i(TAG, "CHECK_SCROLL_UP: " + recyclerView.canScrollVertically(TAG_CHECK_SCROLL_UP));
        Log.i(TAG, "CHECK_SCROLL_DOWN: " + recyclerView.canScrollVertically(TAG_CHECK_SCROLL_DOWN));
    }
});

以上代码中输出了主要个几个信息:

newState : 目前的状态
dx : 水平滚动距离
dy : 垂直滚动距离

onScrollStateChanged 方法

在void onScrollStateChanged(RecyclerView recyclerView, int newState)中回调两个变量:

recyclerView : 当前在滚动的RecyclerView
newState : 当前滚动状态.

其中newState有三种值:

//滚动停止
public static final int SCROLL_STATE_IDLE = 0;

//正在被外部拖拽,一般为用户正在用手指滚动
public static final int SCROLL_STATE_DRAGGING = 1;

//自动滚动
public static final int SCROLL_STATE_SETTLING = 2;

onScrolled 方法

在void onScrolled(RecyclerView recyclerView, int dx, int dy)方法中回调了三个变量:

recyclerView : 当前滚动的view
dx : 水平滚动距离
dy : 垂直滚动距离

真机实践

运行代码

运行以上代码,然后按照上面的滚动过程分别进行两种滚动.

第一种方式缓慢滚动结果如下:

I/MainActivity: 
-----------onScrollStateChanged-----------

I/MainActivity: newState: 1
I/MainActivity: 
-----------onScrolled-----------

I/MainActivity: dx: 0
I/MainActivity: dy: -6
I/MainActivity: CHECK_SCROLL_UP: true
I/MainActivity: CHECK_SCROLL_DOWN: true

------------------------n个onScrolled--------------------

I/MainActivity: 
-----------onScrolled-----------

I/MainActivity: dx: 0
I/MainActivity: dy: -2
I/MainActivity: CHECK_SCROLL_UP: true
I/MainActivity: CHECK_SCROLL_DOWN: false
I/MainActivity: 
-----------onScrollStateChanged-----------

I/MainActivity: newState: 0

第二种快速滚动结果如下:

I/MainActivity: -----------onScrollStateChanged-----------
I/MainActivity: newState: 1
I/MainActivity: -----------onScrolled-----------
I/MainActivity: dx: 0
I/MainActivity: dy: 59
I/MainActivity: CHECK_SCROLL_UP: true
I/MainActivity: CHECK_SCROLL_DOWN: true
--------------------------n个onScrolled-------------------
I/MainActivity: -----------onScrolled-----------
I/MainActivity: dx: 0
I/MainActivity: dy: 54
I/MainActivity: CHECK_SCROLL_UP: true
I/MainActivity: CHECK_SCROLL_DOWN: true
I/MainActivity: -----------onScrollStateChanged-----------
I/MainActivity: newState: 2
I/MainActivity: -----------onScrolled-----------
I/MainActivity: dx: 0
I/MainActivity: dy: 56
I/MainActivity: CHECK_SCROLL_UP: true
I/MainActivity: CHECK_SCROLL_DOWN: true
--------------------------n个onScrolled-------------------
I/MainActivity: -----------onScrolled-----------
I/MainActivity: dx: 0
I/MainActivity: dy: 14
I/MainActivity: CHECK_SCROLL_UP: true
I/MainActivity: CHECK_SCROLL_DOWN: true
I/MainActivity: -----------onScrolled-----------
I/MainActivity: dx: 0
I/MainActivity: dy: 1
I/MainActivity: CHECK_SCROLL_UP: true
I/MainActivity: CHECK_SCROLL_DOWN: true
I/MainActivity: -----------onScrollStateChanged-----------
I/MainActivity: newState: 0

分析结果

且在滚动过程中发现:

1.滚动方向

dy > 0 时为向上滚动
dy < 0 时为向下滚动

2.回调过程

缓慢拖拽回调过程:

1. newState = RecyclerView.SCROLL_STATE_DRAGGING;
2. dy 多次改变
3. newState = RecyclerView.SCROLL_STATE_IDLE

快速滚动回调过程:

1. newState = RecyclerView.SCROLL_STATE_DRAGGING;
2. dy 多次改变
3. newState = RecyclerView.SCROLL_STATE_SETTLING;
4. dy 多次改变
5. newState = RecyclerView.SCROLL_STATE_IDLE;

3.顶端与底部

以上信息中还打印了canScrollVertically的信息,其中:

RecyclerView.canScrollVertically(1)的值表示是否滚动到底部
RecyclerView.canScrollVertically(-1)的值表示是否滚动到顶部

封装

基于以上,我们可以封装一个可以回调滚动状态和方向的RecyclerView.

先建立事件监听的接口OnScrollCallback,代码如下

public interface OnScrollCallback {

    void onStateChanged(ScrollRecycler recycler, int state);

    void onScrollUp(ScrollRecycler recycler, int dy);

    void onScrollDown(ScrollRecycler recycler, int dy);
}

再写一个类ScrollRecycler继承RecyclerView,在类中添加以下方法:

public void setOnScrollCallback(final OnScrollCallback callback) {
    if (callback == null) {
        return;
    }
    addOnScrollListener(new OnScrollListener() {
        @Override
        public void onScrollStateChanged(RecyclerView recyclerView, int newState) {
            super.onScrollStateChanged(recyclerView, newState);
            callback.onStateChanged(ScrollRecycler.this, newState);
        }

        @Override
        public void onScrolled(RecyclerView recyclerView, int dx, int dy) {
            super.onScrolled(recyclerView, dx, dy);
            if (dy > 0) {
                callback.onScrollDown(ScrollRecycler.this, dy);
            } else {
                callback.onScrollUp(ScrollRecycler.this, dy);
            }
        }
    });
}

转自RecyclerView的滚动事件分析

滚动的监听及相关处理

onScrollStateChanged 方法

onScrolled 方法

真机实践

运行代码

分析结果

封装

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