今天，簡單講講Android 如何監聽滑動。

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不廢話了，主要是需要做一個功能，實現RecyclerView滑動時，讓一個標題欄固定顯示在頂部。

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基本知識：

列表的滾動一般分為兩種:

手指按下 -> 手指拖拽列表移動 -> 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 -> 列表繼續滾動 -> 停止滾動

從上面可以看出,滾動狀態分為:

|--靜止 |--滾動|--被迫拖拽移動|--自己滾動

上面的過程的狀態變化如下:

靜止 -> 被迫拖拽移動 -> 靜止

靜止 -> 被迫拖拽移動 -> 自己滾動 -> 靜止

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監聽RecyclerView的滾動

好了,我們分析完滾動的過程,再看看如何監聽RecyclerView的滾動.查看源碼是最好的方法.

看源碼

查看RecyclerView的源碼,我們可以看到以下代碼:

/*** 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); }

也就是說有兩種方式可以監聽滾動事件:

其中 setOnScrollListener 已經過時(?設置的監聽器源碼如下:

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){} }

在滾動過程中,此監聽器會回調兩個方法.

onScrollStateChanged : 滾動狀態變化時回調
onScrolled : 滾動時回調

這兩者的區別在于:?狀態與過程

舉例子

注 : 以下源碼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() {@Overridepublic void onScrollStateChanged(RecyclerView recyclerView, int newState) {super.onScrollStateChanged(recyclerView, newState);Log.i(TAG, "-----------onScrollStateChanged-----------");Log.i(TAG, "newState: " + newState);}@Overridepublic 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));} });

以上代碼中輸出了主要個幾個信息:

newState : 目前的狀態

dx : 水平滾動距離

dy : 垂直滾動距離

onScrollStateChanged 方法

在

recyclerView : 當前在滾動的RecyclerView

newState : 當前滾動狀態.

其中newState有三種值:

//停止滾動 public static final int SCROLL_STATE_IDLE = 0;//正在被外部拖拽,一般為用戶正在用手指滾動 public static final int SCROLL_STATE_DRAGGING = 1;//自動滾動開始 public static final int SCROLL_STATE_SETTLING = 2;

onScrolled 方法

在

• recyclerView : 當前滾動的view

• dx : 水平滾動距離

• dy : 垂直滾動距離

真機實踐

運行代碼

運行以上代碼,然后按照上面的滾動過程分別進行兩種滾動.

第一種方式緩慢滾動結果如下:

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個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

第二種快速滾動結果如下:

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個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個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

分析結果

且在滾動過程中發現:

1.滾動方向

dy > 0 時為向上滾動
dy < 0 時為向下滾動

2.回調過程

緩慢拖拽回調過程:

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

快速滾動回調過程:

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

3.頂端與底部

以上信息中還打印了

RecyclerView.canScrollVertically(-1)的值表示是否滾動到頂部

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這些是基本知識，參考文章：https://www.cnblogs.com/ldq2016/p/6645952.html

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具體實現：

在界面上設置一個標題欄，在RecyclerView 中的一個item和標題欄一致，當RecyclerView滑動到標題欄時，顯示界面上的標題欄，小于位置時，隱藏界面上的標題欄。我這個RecyclerView 的標題欄是第2個item。

mRecyclerView.addOnScrollListener(new RecyclerView.OnScrollListener() {@Overridepublic void onScrollStateChanged(RecyclerView recyclerView, int newState) {super.onScrollStateChanged(recyclerView, newState);}@Overridepublic void onScrolled(RecyclerView recyclerView, int dx, int dy) {super.onScrolled(recyclerView, dx, dy);int firstItemPosition = linearLayoutManager.findFirstVisibleItemPosition();Log.e("TAG", " PersonalHomepageActivity " + firstItemPosition);if (firstItemPosition >= 1 && llBookTitle.getVisibility() == View.GONE) {llBookTitle.setVisibility(View.VISIBLE);} else if (firstItemPosition == 0 && llBookTitle.getVisibility() == View.VISIBLE) {llBookTitle.setVisibility(View.GONE);}}});

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Android RecyclerView 監聽滑動就講完了。

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就這么簡單。

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