RecyclerView的滚动事件OnScrollListener研究
bnrj7351
7年前
<h2><strong>（1）滚动事件分类</strong></h2> <p>列表的滚动一般分为两种:</p> <pre> <code class="language-java">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</code></pre> <p>上面的过程的状态变化如下:</p> <pre> <code class="language-java">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</code></pre> <h2><strong>（2）监听RecyclerView的滚动</strong></h2> <p>有两种方式可以监听滚动事件:</p> <pre> <code class="language-java">1.setOnScrollListener(OnScrollListener listener) 2.addOnScrollListener(OnScrollListener listener)</code></pre> <p>其中 setOnScrollListener 由于可能出现空指针的风险，已经过时。建议用addOnScrollListener。</p> <h2><strong>（3）OnScrollListener</strong></h2> <pre> <code class="language-java">/** * An OnScrollListener can be added to a RecyclerView to receive messages when a scrolling event * has occurred on that RecyclerView. * <p> * @see RecyclerView#addOnScrollListener(OnScrollListener) * @see RecyclerView#clearOnChildAttachStateChangeListeners() * */ 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){} }</code></pre> <p>OnScrollListener类是个抽象类，有两个方法：</p> <pre> <code class="language-java">void onScrollStateChanged(RecyclerView recyclerView, int newState): 滚动状态变化时回调 void onScrolled(RecyclerView recyclerView, int dx, int dy): 滚动时回调</code></pre> <h3><strong>3.1 onScrollStateChanged(RecyclerView recyclerView, int newState)方法</strong></h3> <p>回调的两个变量的含义：</p> <p>recyclerView: 当前在滚动的RecyclerView</p> <p>newState: 当前滚动状态.</p> <p>其中newState有三种值:</p> <pre> <code class="language-java">/** * The RecyclerView is not currently scrolling.（静止没有滚动） */ public static final int SCROLL_STATE_IDLE = 0; /** * The RecyclerView is currently being dragged by outside input such as user touch input. *（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; /** * The RecyclerView is currently animating to a final position while not under outside control. *（自动滚动） */ public static final int SCROLL_STATE_SETTLING = 2;</code></pre> <h3><strong>3.2 onScrolled(RecyclerView recyclerView, int dx, int dy)方法</strong></h3> <p>回调的三个变量含义:</p> <p>recyclerView : 当前滚动的view</p> <p>dx : 水平滚动距离</p> <p>dy : 垂直滚动距离</p> <p>dx > 0时为手指向左滚动, <strong>列表滚动显示右面的内容</strong></p> <p>dx < 0时为手指向右滚动, <strong>列表滚动显示左面的内容</strong></p> <p>dy > 0时为手指向上滚动, <strong>列表滚动显示下面的内容</strong></p> <p>dy < 0时为手指向下滚动, <strong>列表滚动显示上面的内容</strong></p> <h2><strong>（4）canScrollVertically和canScrollHorizontally方法</strong></h2> <pre> <code class="language-java">public boolean canScrollVertically (int direction) 这个方法是判断View在竖直方向是否还能向上，向下滑动。 其中，direction为 -1 表示手指向下滑动（屏幕向上滑动）， 1 表示手指向上滑动（屏幕向下滑动）。 public boolean canScrollHorizontally (int direction) 这个方法用来判断 水平方向的滑动</code></pre> <p>例如：</p> <p>RecyclerView.canScrollVertically(1)的值表示是否能向下滚动， <strong> <em>false表示已经滚动到底部</em> </strong></p> <p>RecyclerView.canScrollVertically(-1)的值表示是否能向上滚动， <strong> <em>false表示已经滚动到顶部</em> </strong></p> <h2><strong>（5）两种判断是否到底部的方法：</strong></h2> <h3><strong>5.1方法一：</strong></h3> <p>如果 当前</p> <p><em>第一个可见item的位置 + 当前可见的item个数 >= item的总个数</em></p> <p>这样就可以判断出来，是在底部了。</p> <pre> <code class="language-java">loadingMoreListener = new RecyclerView.OnScrollListener() { @Override public void onScrollStateChanged(RecyclerView recyclerView, int newState) { super.onScrollStateChanged(recyclerView, newState); } @Override public void onScrolled(RecyclerView recyclerView, int dx, int dy) { super.onScrolled(recyclerView, dx, dy); if (dy > 0) //向下滚动 { int visibleItemCount = mLinearLayoutManager.getChildCount(); int totalItemCount = mLinearLayoutManager.getItemCount(); int pastVisiblesItems = mLinearLayoutManager.findFirstVisibleItemPosition(); if (!loading && (visibleItemCount + pastVisiblesItems) >= totalItemCount) { loading = true; loadMoreDate(); } } } };</code></pre> <p>通过</p> <p>visibleItemCount + pastVisiblesItems) >= totalItemCount</p> <p>来判断是否是底部。</p> <h3><strong>5.2方法二：</strong></h3> <p>通过canScrollVertically 来判断</p> <pre> <code class="language-java">loadingMoreListener = new RecyclerView.OnScrollListener() { @Override public void onScrollStateChanged(RecyclerView recyclerView, int newState) { super.onScrollStateChanged(recyclerView, newState); if(!loading && !recyclerView.canScrollVertically(1)){ loading = true; loadMoreDate(); } } @Override public void onScrolled(RecyclerView recyclerView, int dx, int dy) { super.onScrolled(recyclerView, dx, dy); // if (dy > 0) //向下滚动 // { // int visibleItemCount = mLinearLayoutManager.getChildCount(); // int totalItemCount = mLinearLayoutManager.getItemCount(); // int pastVisiblesItems = mLinearLayoutManager.findFirstVisibleItemPosition(); // // if (!loading && (visibleItemCount + pastVisiblesItems) >= totalItemCount) { // loading = true; // loadMoreDate(); // } // } } };</code></pre> <p> </p> <p> </p> <p>来自：http://www.jianshu.com/p/ce347cf991db</p> <p> </p>