RecyclerView的滚动事件分析

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

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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)} */@Deprecatedpublic 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. 其中 setOnScrollListener 已经过时( 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

   在

2. recyclerView : 当前在滚动的RecyclerView

3. newState : 当前滚动状态.

其中newState有三种值:

//停止滚动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

在

• 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: 1I/MainActivity: -----------onScrolled-----------I/MainActivity: dx: 0I/MainActivity: dy: -6I/MainActivity: CHECK_SCROLL_UP: trueI/MainActivity: CHECK_SCROLL_DOWN: true------------------------n个onScrolled--------------------I/MainActivity: -----------onScrolled-----------I/MainActivity: dx: 0I/MainActivity: dy: -2I/MainActivity: CHECK_SCROLL_UP: trueI/MainActivity: CHECK_SCROLL_DOWN: falseI/MainActivity: -----------onScrollStateChanged-----------I/MainActivity: newState: 0

第二种快速滚动结果如下:

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

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publicinterfaceOnScrollCallback { void onStateChanged(ScrollRecycler recycler, int state); voidonScrollUp(ScrollRecycler recycler,int dy); void onScrollDown(ScrollRecycler recycler, int dy); }

再写一个类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);            }        }    });}