列表滚动过程

开始之前,我们想一下一个列表的滚动过程是怎样的?

列表的滚动一般分为两种:

  1. 手指按下 -> 手指拖拽列表移动 -> 手指停止拖拽 -> 抬起手指
  2. 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:

  1. 静止 -> 被迫拖拽移动 -> 静止
  2. 静止 -> 被迫拖拽移动 -> 自己滚动 -> 静止

监听RecyclerView的滚动

好了,我们分析完滚动的过程,再看看如何监听RecyclerView的滚动.查看源码是最好的方法.

看源码

查看RecyclerView的源码,我们可以看到以下代码:

/**
 * 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:

  1. setOnScrollListener()
  2. 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.

设置的监听器源码如下:

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

举例子

注 : 以下源码可在最后的地址中找到.

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:

  1. newState : 目前的状态
  2. dx : 水平滚动距离
  3. dy : 垂直滚动距离

onScrollStateChanged 方法

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

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

其中newState有三种值:

//静止,没有滚动
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;

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

onScrolled 方法

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

  1. recyclerView : 当前滚动的view
  2. dx : 水平滚动距离
  3. 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 时为手指向上滚动,列表滚动显示下面的内容
dy < 0 时为手指向下滚动,列表滚动显示上面的内容

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)的值表示是否能向下滚动,false表示已经滚动到底部
RecyclerView.canScrollVertically(-1)的值表示是否能向上滚动,false表示已经滚动到顶部

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 onScrollToBottom();

    void onScrollDown(ScrollRecycler recycler, int dy);

    void onScrollToTop();
}

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

addOnScrollListener(new OnScrollListener() {
    @Override
    public void onScrollStateChanged(RecyclerView recyclerView, int newState) {
        super.onScrollStateChanged(recyclerView, newState);
        callback.onStateChanged(ScrollRecycler.this, newState);
        if (newState == RecyclerView.SCROLL_STATE_IDLE) {
            if (!recyclerView.canScrollVertically(1)) {
                callback.onScrollToBottom();
            }
            if (!recyclerView.canScrollVertically(-1)) {
                callback.onScrollToTop();
            }
        }
    }

    @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);
        }
    }
});

测试以下是否可行,下面直接给出测试结果:

I/MainActivity: onStateChanged: state:1
I/MainActivity: onScrollUp: -11
I/MainActivity: onScrollUp: -10
I/MainActivity: onScrollUp: -14
I/MainActivity: onScrollUp: -22
I/MainActivity: onStateChanged: state:2
I/MainActivity: onStateChanged: state:0
I/MainActivity: onScrollToTop: 

完毕,以上就是对RecyclerView的滚动的简单研究,项目代码地址如下:Dev-Wiki/RecyclerScroll: RecyclerView滚动的Demo

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