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RecyclerView绘制原理.html
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<h3 id="RecyclerView基本使用"><a href="#RecyclerView基本使用" class="headerlink" title="RecyclerView基本使用"></a>RecyclerView基本使用</h3><figure class="highlight java"><table><tr><td class="gutter"><pre><div class="line">1</div><div class="line">2</div><div class="line">3</div><div class="line">4</div><div class="line">5</div><div class="line">6</div><div class="line">7</div><div class="line">8</div><div class="line">9</div><div class="line">10</div></pre></td><td class="code"><pre><div class="line"><span class="comment">//首先设置RecyclerView的布局管理模式</span></div><div class="line">mRecyclerView.setLayoutManager(<span class="keyword">new</span> LinearLayoutManager(<span class="keyword">this</span>));</div><div class="line">mAdapter = <span class="keyword">new</span> MyAdapter(getData());</div><div class="line"><span class="comment">//设置Item项的UI装饰器</span></div><div class="line">mRecyclerView.addItemDecoration(<span class="keyword">new</span> RecyclerItemDecoration(LinearLayoutManager.VERTICAL));</div><div class="line"><span class="comment">//设置Item项的不同操作的动画</span></div><div class="line">mRecyclerView.setItemAnimator(<span class="keyword">new</span> DefaultItemAnimator());</div><div class="line"><span class="comment">//设置数据开始装配</span></div><div class="line">mRecyclerView.setAdapter(mAdapter);</div><div class="line">RecyclerView原理分析</div></pre></td></tr></table></figure>
<a id="more"></a>
<h3 id="Adapter数据适配"><a href="#Adapter数据适配" class="headerlink" title="Adapter数据适配"></a>Adapter数据适配</h3><p><code>RecyclerView.Adapter</code>类中有一个很重要的属性：<br><figure class="highlight java"><table><tr><td class="gutter"><pre><div class="line">1</div><div class="line">2</div><div class="line">3</div></pre></td><td class="code"><pre><div class="line"><span class="comment">//Adapter中被观察对象</span></div><div class="line">Observale<AdapterDataObserver></div><div class="line"><span class="keyword">private</span> <span class="keyword">final</span> AdapterDataObservable mObservable = <span class="keyword">new</span> AdapterDataObservable();</div></pre></td></tr></table></figure></p>
<p><code>RecyclerView</code>中也有一个很重要的属性<br><figure class="highlight java"><table><tr><td class="gutter"><pre><div class="line">1</div><div class="line">2</div></pre></td><td class="code"><pre><div class="line"><span class="comment">//数据观察者, AdapterDataObserver实例</span></div><div class="line"><span class="keyword">private</span> <span class="keyword">final</span> RecyclerViewDataObserver mObserver = <span class="keyword">new</span> RecyclerViewDataObserver();</div></pre></td></tr></table></figure></p>
<p>当使用<code>recyclerView.setAdapter(data)</code>设置数据时，会调用以下方法使得<code>RecyclerView</code>成为Adapter的观察者(间接)：<br><figure class="highlight java"><table><tr><td class="gutter"><pre><div class="line">1</div><div class="line">2</div><div class="line">3</div><div class="line">4</div><div class="line">5</div><div class="line">6</div><div class="line">7</div><div class="line">8</div><div class="line">9</div><div class="line">10</div><div class="line">11</div></pre></td><td class="code"><pre><div class="line"><span class="function"><span class="keyword">private</span> <span class="keyword">void</span> <span class="title">setAdapterInternal</span><span class="params">(Adapter adapter, </span></span></div><div class="line"> <span class="keyword">boolean</span> compatibleWithPrevious,</div><div class="line"> <span class="keyword">boolean</span> removeAndRecycleViews) {</div><div class="line"> ......</div><div class="line"> <span class="keyword">if</span> (adapter != <span class="keyword">null</span>) {</div><div class="line"> <span class="comment">//通过此处添加观察者，此时RecyclerView就会对Adapter中的数据进行观察监听</span></div><div class="line"> adapter.registerAdapterDataObserver(mObserver);</div><div class="line"> ......</div><div class="line"> }</div><div class="line"> ......</div><div class="line">}</div></pre></td></tr></table></figure></p>
<p>通常当我们改变Adapter中的数据源时，一般都会通过调用<code>Adapter.notifyDataSetChanged()</code>方法来刷新列表，我们来看看这个方法的实现，看看Adapter是如何通过这个方法来刷新列表的：<br><figure class="highlight java"><table><tr><td class="gutter"><pre><div class="line">1</div><div class="line">2</div><div class="line">3</div></pre></td><td class="code"><pre><div class="line"><span class="function"><span class="keyword">public</span> <span class="keyword">final</span> <span class="keyword">void</span> <span class="title">notifyDataSetChanged</span><span class="params">()</span> </span>{</div><div class="line"> mObservable.notifyChanged();</div><div class="line">}</div></pre></td></tr></table></figure></p>
<p>我们接着看<code>AdapterDataObservable.notifyChanged()</code>方法实现：<br><figure class="highlight java"><table><tr><td class="gutter"><pre><div class="line">1</div><div class="line">2</div><div class="line">3</div><div class="line">4</div><div class="line">5</div><div class="line">6</div></pre></td><td class="code"><pre><div class="line"><span class="comment">//mObservers是Observable中的属性，是一个ArrayList<T></span></div><div class="line"><span class="function"><span class="keyword">public</span> <span class="keyword">void</span> <span class="title">notifyChanged</span><span class="params">()</span> </span>{</div><div class="line"> <span class="keyword">for</span> (<span class="keyword">int</span> i = mObservers.size() - <span class="number">1</span>; i >= <span class="number">0</span>; i--) {</div><div class="line"> mObservers.get(i).onChanged();</div><div class="line"> }</div><div class="line">}</div></pre></td></tr></table></figure></p>
<p>我们再来看<code>RecyclerViewDataObserver.onChanged()</code>方法：<br><figure class="highlight java"><table><tr><td class="gutter"><pre><div class="line">1</div><div class="line">2</div><div class="line">3</div><div class="line">4</div><div class="line">5</div><div class="line">6</div><div class="line">7</div><div class="line">8</div></pre></td><td class="code"><pre><div class="line"><span class="meta">@Override</span></div><div class="line"><span class="function"><span class="keyword">public</span> <span class="keyword">void</span> <span class="title">onChanged</span><span class="params">()</span> </span>{</div><div class="line"> ......</div><div class="line"> <span class="comment">//Adapter目前没有待更新的数据</span></div><div class="line"> <span class="keyword">if</span> (!mAdapterHelper.hasPendingUpdates()) {</div><div class="line"> requestLayout();</div><div class="line"> }</div><div class="line">}</div></pre></td></tr></table></figure></p>
<p>看到<code>requestLayout()</code>这个方法，我们就明白了，调用此方法后系统会重新<code>measure, layout, draw</code>，这样列表视图就会被更新。</p>
<h3 id="RecyclerView-onMeasure"><a href="#RecyclerView-onMeasure" class="headerlink" title="RecyclerView.onMeasure()"></a>RecyclerView.onMeasure()</h3><p>我们来看看<code>RecyclerView</code>的测量方法<code>onMeasure</code>：<br><figure class="highlight java"><table><tr><td class="gutter"><pre><div class="line">1</div><div class="line">2</div><div class="line">3</div><div class="line">4</div><div class="line">5</div><div class="line">6</div><div class="line">7</div><div class="line">8</div><div class="line">9</div><div class="line">10</div><div class="line">11</div><div class="line">12</div><div class="line">13</div><div class="line">14</div><div class="line">15</div><div class="line">16</div><div class="line">17</div><div class="line">18</div><div class="line">19</div><div class="line">20</div><div class="line">21</div><div class="line">22</div><div class="line">23</div><div class="line">24</div></pre></td><td class="code"><pre><div class="line"><span class="meta">@Override</span></div><div class="line"><span class="function"><span class="keyword">protected</span> <span class="keyword">void</span> <span class="title">onMeasure</span><span class="params">(<span class="keyword">int</span> widthSpec, <span class="keyword">int</span> heightSpec)</span> </span>{</div><div class="line"> ......</div><div class="line"> <span class="keyword">if</span> (mLayout.mAutoMeasure) {</div><div class="line"> <span class="keyword">final</span> <span class="keyword">int</span> widthMode = MeasureSpec.getMode(widthSpec);</div><div class="line"> <span class="keyword">final</span> <span class="keyword">int</span> heightMode = MeasureSpec.getMode(heightSpec);</div><div class="line"> <span class="keyword">final</span> <span class="keyword">boolean</span> skipMeasure = widthMode == MeasureSpec.EXACTLY && heightMode == MeasureSpec.EXACTLY;</div><div class="line"> <span class="comment">//委托给LayoutManager来进行测量</span></div><div class="line"> mLayout.onMeasure(mRecycler, mState, widthSpec, heightSpec);</div><div class="line"> <span class="keyword">if</span> (skipMeasure || mAdapter == <span class="keyword">null</span>) {</div><div class="line"> <span class="keyword">return</span>;</div><div class="line"> }</div><div class="line"> <span class="keyword">if</span> (mState.mLayoutStep == State.STEP_START) {</div><div class="line"> dispatchLayoutStep1();</div><div class="line"> }</div><div class="line"> mLayout.setMeasureSpecs(widthSpec, heightSpec);</div><div class="line"> mState.mIsMeasuring = <span class="keyword">true</span>;</div><div class="line"> dispatchLayoutStep2();</div><div class="line"> mLayout.setMeasuredDimensionFromChildren(widthSpec, heightSpec);</div><div class="line"> ......</div><div class="line"> }</div><div class="line"> }</div><div class="line"> ......</div><div class="line">}</div></pre></td></tr></table></figure></p>
<p>上面<code>onMeasure</code>方法中<code>mLayout</code>变量就是我们上面设置的<code>LinearLayoutManager</code>实例，而<code>LinearLayoutManager</code>的构造函数中给变量<code>mAutoMeasure</code>值设置为true，因此测量时就会执行上面的代码，通过上面的方法我们可以看到此处测量分为两种：</p>
<p>当<code>RecyclerView</code>的宽高设置为<code>match_parent</code>或具体值的时候，<code>skipMeasure=true</code>，此时会只需要测量其自身的宽高就可以知道RecyclerView的大小，这时是<code>onMeasure</code>方法测量结束。<br>当RecyclerView的宽高设置为<code>wrap_content</code>时，<code>skipMeasure=false</code>，<code>onMeasure</code>会继续执行下面的<code>dispatchLayoutStep2()</code>，其实就是测量RecyclerView的子视图的大小最终确定<code>RecyclerView</code>的实际大小，这种情况真正的测量操作都是在方法<code>dispatchLayoutStep2()</code>里执行的:<br><figure class="highlight java"><table><tr><td class="gutter"><pre><div class="line">1</div><div class="line">2</div><div class="line">3</div><div class="line">4</div><div class="line">5</div><div class="line">6</div><div class="line">7</div><div class="line">8</div></pre></td><td class="code"><pre><div class="line"><span class="function"><span class="keyword">private</span> <span class="keyword">void</span> <span class="title">dispatchLayoutStep2</span><span class="params">()</span> </span>{</div><div class="line"> ......</div><div class="line"> mState.mItemCount = mAdapter.getItemCount();</div><div class="line"> <span class="comment">// Step 2: Run layout</span></div><div class="line"> mState.mInPreLayout = <span class="keyword">false</span>;</div><div class="line"> mLayout.onLayoutChildren(mRecycler, mState);</div><div class="line"> ......</div><div class="line">}</div></pre></td></tr></table></figure></p>
<p>从这里也可以看出<code>RecyclerView</code>真正的测量是委托给<code>LayoutManager</code>在处理，我们看看<code>LinearLayoutManager</code>的<code>onLayoutChildren</code>方法：<br><figure class="highlight java"><table><tr><td class="gutter"><pre><div class="line">1</div><div class="line">2</div><div class="line">3</div><div class="line">4</div><div class="line">5</div><div class="line">6</div><div class="line">7</div><div class="line">8</div><div class="line">9</div><div class="line">10</div><div class="line">11</div><div class="line">12</div><div class="line">13</div><div class="line">14</div><div class="line">15</div><div class="line">16</div></pre></td><td class="code"><pre><div class="line"><span class="meta">@Override</span></div><div class="line"><span class="function"><span class="keyword">public</span> <span class="keyword">void</span> <span class="title">onLayoutChildren</span><span class="params">(RecyclerView.Recycler recycler, RecyclerView.State state)</span> </span>{</div><div class="line"> ......</div><div class="line"> <span class="keyword">if</span> (mAnchorInfo.mLayoutFromEnd) {</div><div class="line"> ...</div><div class="line"> fill(recycler, mLayoutState, state, <span class="keyword">false</span>);</div><div class="line"> ......</div><div class="line"> } <span class="keyword">else</span> {</div><div class="line"> ......</div><div class="line"> fill(recycler, mLayoutState, state, <span class="keyword">false</span>);</div><div class="line"> ......</div><div class="line"> }</div><div class="line"> ......</div><div class="line"> layoutForPredictiveAnimations(recycler, state, startOffset, endOffset);</div><div class="line"> ......</div><div class="line">}</div></pre></td></tr></table></figure></p>
<p>很明显可以看到，最终执行了fill()方法：<br><figure class="highlight java"><table><tr><td class="gutter"><pre><div class="line">1</div><div class="line">2</div><div class="line">3</div><div class="line">4</div><div class="line">5</div><div class="line">6</div><div class="line">7</div><div class="line">8</div><div class="line">9</div><div class="line">10</div></pre></td><td class="code"><pre><div class="line"><span class="function"><span class="keyword">int</span> <span class="title">fill</span><span class="params">(RecyclerView.Recycler recycler, LayoutState layoutState, RecyclerView.State state, <span class="keyword">boolean</span> stopOnFocusable)</span> </span>{</div><div class="line"> ......</div><div class="line"> LayoutChunkResult layoutChunkResult = <span class="keyword">new</span> LayoutChunkResult();</div><div class="line"> <span class="keyword">while</span> ((layoutState.mInfinite || remainingSpace > <span class="number">0</span>) && layoutState.hasMore(state)) {</div><div class="line"> layoutChunkResult.resetInternal();</div><div class="line"> layoutChunk(recycler, state, layoutState, layoutChunkResult);</div><div class="line"> ......</div><div class="line"> }</div><div class="line"> ......</div><div class="line">}</div></pre></td></tr></table></figure></p>
<p>上面的while判断条件中remainingSpace可以理解为当前列表中是否还有多余的位置可用于添加绘制child，而<code>layoutState.hasMore(state)</code>则是判断当前绘制的child索引位置是否在Adapter数据范围内<br><figure class="highlight java"><table><tr><td class="gutter"><pre><div class="line">1</div><div class="line">2</div><div class="line">3</div></pre></td><td class="code"><pre><div class="line"><span class="function"><span class="keyword">boolean</span> <span class="title">hasMore</span><span class="params">(RecyclerView.State state)</span> </span>{</div><div class="line"> <span class="keyword">return</span> mCurrentPosition >= <span class="number">0</span> && mCurrentPosition < state.getItemCount();</div><div class="line">}</div></pre></td></tr></table></figure></p>
<p>再来看上面的<code>layoutChunk()</code>方法：<br><figure class="highlight java"><table><tr><td class="gutter"><pre><div class="line">1</div><div class="line">2</div><div class="line">3</div><div class="line">4</div><div class="line">5</div><div class="line">6</div><div class="line">7</div><div class="line">8</div><div class="line">9</div><div class="line">10</div><div class="line">11</div><div class="line">12</div><div class="line">13</div><div class="line">14</div><div class="line">15</div><div class="line">16</div><div class="line">17</div><div class="line">18</div><div class="line">19</div><div class="line">20</div><div class="line">21</div><div class="line">22</div><div class="line">23</div><div class="line">24</div><div class="line">25</div><div class="line">26</div></pre></td><td class="code"><pre><div class="line"><span class="function"><span class="keyword">void</span> <span class="title">layoutChunk</span><span class="params">(RecyclerView.Recycler recycler, RecyclerView.State state,</span></span></div><div class="line"> LayoutState layoutState, LayoutChunkResult result) {</div><div class="line"> View view = layoutState.next(recycler);</div><div class="line"> ......</div><div class="line"> LayoutParams params = (LayoutParams) view.getLayoutParams();</div><div class="line"> <span class="keyword">if</span> (layoutState.mScrapList == <span class="keyword">null</span>) {</div><div class="line"> <span class="keyword">if</span> (mShouldReverseLayout == (layoutState.mLayoutDirection</div><div class="line"> == LayoutState.LAYOUT_START)) {</div><div class="line"> addView(view);</div><div class="line"> } <span class="keyword">else</span> {</div><div class="line"> addView(view, <span class="number">0</span>);</div><div class="line"> }</div><div class="line"> } <span class="keyword">else</span> {</div><div class="line"> <span class="keyword">if</span> (mShouldReverseLayout == (layoutState.mLayoutDirection</div><div class="line"> == LayoutState.LAYOUT_START)) {</div><div class="line"> addDisappearingView(view);</div><div class="line"> } <span class="keyword">else</span> {</div><div class="line"> addDisappearingView(view, <span class="number">0</span>);</div><div class="line"> }</div><div class="line"> }</div><div class="line"> measureChildWithMargins(view, <span class="number">0</span>, <span class="number">0</span>);</div><div class="line"> ......</div><div class="line"> <span class="comment">// To calculate correct layout position, we subtract margins.</span></div><div class="line"> layoutDecorated(view, left + params.leftMargin, top + params.topMargin, right - params.rightMargin, bottom - params.bottomMargin);</div><div class="line"> ......</div><div class="line">}</div></pre></td></tr></table></figure></p>
<p>上面的方法中<code>addView</code>与<code>addDisappearingView</code>最终都是调用的<code>RecyclerView</code>的<code>addView</code>方法，也就是将子<code>child</code>添加到<code>RecyclerView</code>中。</p>
<p>我们再来看看<code>View view = layoutState.next(recycler);</code>这行代码的实现：<br><figure class="highlight java"><table><tr><td class="gutter"><pre><div class="line">1</div><div class="line">2</div><div class="line">3</div><div class="line">4</div><div class="line">5</div><div class="line">6</div><div class="line">7</div><div class="line">8</div><div class="line">9</div><div class="line">10</div><div class="line">11</div></pre></td><td class="code"><pre><div class="line"><span class="function">View <span class="title">next</span><span class="params">(RecyclerView.Recycler recycler)</span> </span>{</div><div class="line"> <span class="keyword">if</span> (mScrapList != <span class="keyword">null</span>) {</div><div class="line"> <span class="keyword">return</span> nextViewFromScrapList();</div><div class="line"> }</div><div class="line"> <span class="keyword">final</span> View view = recycler.getViewForPosition(mCurrentPosition);</div><div class="line"> <span class="comment">//获取某个位置需要展示的View</span></div><div class="line"></div><div class="line"> mCurrentPosition += mItemDirection;</div><div class="line"> <span class="comment">//将当前绘制的child的索引下移一位，配合while循环</span></div><div class="line"> <span class="keyword">return</span> view;</div><div class="line">}</div></pre></td></tr></table></figure></p>
<p>我们看看上面的获取position位置的view是如何获取的：<br><figure class="highlight java"><table><tr><td class="gutter"><pre><div class="line">1</div><div class="line">2</div><div class="line">3</div><div class="line">4</div><div class="line">5</div><div class="line">6</div><div class="line">7</div><div class="line">8</div><div class="line">9</div><div class="line">10</div><div class="line">11</div><div class="line">12</div><div class="line">13</div><div class="line">14</div><div class="line">15</div><div class="line">16</div><div class="line">17</div><div class="line">18</div><div class="line">19</div><div class="line">20</div><div class="line">21</div><div class="line">22</div><div class="line">23</div><div class="line">24</div><div class="line">25</div><div class="line">26</div><div class="line">27</div><div class="line">28</div><div class="line">29</div><div class="line">30</div><div class="line">31</div><div class="line">32</div><div class="line">33</div><div class="line">34</div><div class="line">35</div><div class="line">36</div><div class="line">37</div><div class="line">38</div><div class="line">39</div><div class="line">40</div><div class="line">41</div><div class="line">42</div><div class="line">43</div><div class="line">44</div><div class="line">45</div><div class="line">46</div><div class="line">47</div><div class="line">48</div><div class="line">49</div><div class="line">50</div><div class="line">51</div><div class="line">52</div><div class="line">53</div><div class="line">54</div><div class="line">55</div><div class="line">56</div><div class="line">57</div><div class="line">58</div></pre></td><td class="code"><pre><div class="line"><span class="function">View <span class="title">getViewForPosition</span><span class="params">(<span class="keyword">int</span> position, <span class="keyword">boolean</span> dryRun)</span> </span>{</div><div class="line"> ......</div><div class="line"> ViewHolder holder = <span class="keyword">null</span>;</div><div class="line"> <span class="comment">// 0) If there is a changed scrap, try to find from there</span></div><div class="line"> <span class="keyword">if</span> (mState.isPreLayout()) {</div><div class="line"> holder = getChangedScrapViewForPosition(position);</div><div class="line"> fromScrap = holder != <span class="keyword">null</span>;</div><div class="line"> }</div><div class="line"> <span class="comment">// 1) Find from scrap by position</span></div><div class="line"> <span class="keyword">if</span> (holder == <span class="keyword">null</span>) {</div><div class="line"> holder = getScrapViewForPosition(position, INVALID_TYPE, dryRun);</div><div class="line"> ......</div><div class="line"> }</div><div class="line"> <span class="keyword">if</span> (holder == <span class="keyword">null</span>) {</div><div class="line"> <span class="keyword">final</span> <span class="keyword">int</span> offsetPosition = mAdapterHelper.findPositionOffset(position);</div><div class="line"> ......</div><div class="line"> <span class="keyword">final</span> <span class="keyword">int</span> type = mAdapter.getItemViewType(offsetPosition);</div><div class="line"> <span class="comment">// 2) Find from scrap via stable ids, if exists</span></div><div class="line"> <span class="keyword">if</span> (mAdapter.hasStableIds()) {</div><div class="line"> holder = getScrapViewForId(mAdapter.getItemId(offsetPosition), type, dryRun);</div><div class="line"> ......</div><div class="line"> }</div><div class="line"> <span class="comment">//mViewCacheExtension的缓存是由开发者自己实现来控制ViewHolder的缓存策略</span></div><div class="line"> <span class="keyword">if</span> (holder == <span class="keyword">null</span> && mViewCacheExtension != <span class="keyword">null</span>) {</div><div class="line"> <span class="comment">// We are NOT sending the offsetPosition because LayoutManager does not</span></div><div class="line"> <span class="comment">// know it.</span></div><div class="line"> <span class="keyword">final</span> View view = mViewCacheExtension.getViewForPositionAndType(<span class="keyword">this</span>, position, type);</div><div class="line"> <span class="keyword">if</span> (view != <span class="keyword">null</span>) {</div><div class="line"> holder = getChildViewHolder(view);</div><div class="line"> ......</div><div class="line"> }</div><div class="line"> }</div><div class="line"> <span class="keyword">if</span> (holder == <span class="keyword">null</span>) { <span class="comment">// fallback to recycler</span></div><div class="line"> ......</div><div class="line"> holder = getRecycledViewPool().getRecycledView(type);</div><div class="line"> ......</div><div class="line"> }</div><div class="line"> <span class="keyword">if</span> (holder == <span class="keyword">null</span>) {</div><div class="line"> holder = mAdapter.createViewHolder(RecyclerView.<span class="keyword">this</span>, type);</div><div class="line"> ......</div><div class="line"> }</div><div class="line"> }</div><div class="line"> ......</div><div class="line"></div><div class="line"> <span class="keyword">boolean</span> bound = <span class="keyword">false</span>;</div><div class="line"> <span class="keyword">if</span> (mState.isPreLayout() && holder.isBound()) {</div><div class="line"> <span class="comment">// do not update unless we absolutely have to.</span></div><div class="line"> holder.mPreLayoutPosition = position;</div><div class="line"> } <span class="keyword">else</span> <span class="keyword">if</span> (!holder.isBound() || holder.needsUpdate() || holder.isInvalid()) {</div><div class="line"> ......</div><div class="line"> holder.mOwnerRecyclerView = RecyclerView.<span class="keyword">this</span>;</div><div class="line"> <span class="comment">//此处就是调用Adapter中bindViewHolder方法</span></div><div class="line"> mAdapter.bindViewHolder(holder, offsetPosition);</div><div class="line"> ......</div><div class="line"> }</div><div class="line"> ......</div><div class="line"> <span class="keyword">return</span> holder.itemView;</div><div class="line">}</div></pre></td></tr></table></figure></p>
<p>将指定位置的View获取得到之后添加到RecyclerView中，紧接着再来看后面执行的<code>measureChildWithMargins</code>方法：<br><figure class="highlight java"><table><tr><td class="gutter"><pre><div class="line">1</div><div class="line">2</div><div class="line">3</div><div class="line">4</div><div class="line">5</div><div class="line">6</div><div class="line">7</div><div class="line">8</div><div class="line">9</div><div class="line">10</div><div class="line">11</div><div class="line">12</div><div class="line">13</div></pre></td><td class="code"><pre><div class="line"><span class="function"><span class="keyword">public</span> <span class="keyword">void</span> <span class="title">measureChildWithMargins</span><span class="params">(View child, <span class="keyword">int</span> widthUsed, <span class="keyword">int</span> heightUsed)</span> </span>{</div><div class="line"> <span class="keyword">final</span> LayoutParams lp = (LayoutParams) child.getLayoutParams();</div><div class="line"> <span class="comment">//通过ItemDecorate获取offset</span></div><div class="line"> <span class="keyword">final</span> Rect insets = mRecyclerView.getItemDecorInsetsForChild(child);</div><div class="line"> widthUsed += insets.left + insets.right;</div><div class="line"> heightUsed += insets.top + insets.bottom;</div><div class="line"></div><div class="line"> <span class="keyword">final</span> <span class="keyword">int</span> widthSpec = getChildMeasureSpec(getWidth(), getWidthMode(), getPaddingLeft() + getPaddingRight() + lp.leftMargin + lp.rightMargin + widthUsed, lp.width, canScrollHorizontally());</div><div class="line"> <span class="keyword">final</span> <span class="keyword">int</span> heightSpec = getChildMeasureSpec(getHeight(), getHeightMode(), getPaddingTop() + getPaddingBottom() + lp.topMargin + lp.bottomMargin + heightUsed, lp.height, canScrollVertically());</div><div class="line"> <span class="keyword">if</span> (shouldMeasureChild(child, widthSpec, heightSpec, lp)) {</div><div class="line"> child.measure(widthSpec, heightSpec);</div><div class="line"> }</div><div class="line">}</div></pre></td></tr></table></figure></p>
<p>该方法中调用<code>mRecyclerView.getItemDecorInsetsForChild(child);</code>获取child的offset，然后对child重新测量绘制：<br><figure class="highlight java"><table><tr><td class="gutter"><pre><div class="line">1</div><div class="line">2</div><div class="line">3</div><div class="line">4</div><div class="line">5</div><div class="line">6</div><div class="line">7</div><div class="line">8</div><div class="line">9</div><div class="line">10</div><div class="line">11</div><div class="line">12</div><div class="line">13</div><div class="line">14</div></pre></td><td class="code"><pre><div class="line"><span class="function">Rect <span class="title">getItemDecorInsetsForChild</span><span class="params">(View child)</span> </span>{</div><div class="line"> ......</div><div class="line"> <span class="keyword">final</span> <span class="keyword">int</span> decorCount = mItemDecorations.size();</div><div class="line"> <span class="keyword">for</span> (<span class="keyword">int</span> i = <span class="number">0</span>; i < decorCount; i++) {</div><div class="line"> mTempRect.set(<span class="number">0</span>, <span class="number">0</span>, <span class="number">0</span>, <span class="number">0</span>);</div><div class="line"> mItemDecorations.get(i).getItemOffsets(mTempRect, child, <span class="keyword">this</span>, mState);</div><div class="line"> insets.left += mTempRect.left;</div><div class="line"> insets.top += mTempRect.top;</div><div class="line"> insets.right += mTempRect.right;</div><div class="line"> insets.bottom += mTempRect.bottom;</div><div class="line"> }</div><div class="line"> lp.mInsetsDirty = <span class="keyword">false</span>;</div><div class="line"> <span class="keyword">return</span> insets;</div><div class="line">}</div></pre></td></tr></table></figure></p>
<p>这个里面的<code>mItemDecorations</code>就是文章开头例子中我通过<code>mRecyclerView.addItemDecoration(new RecyclerItemDecoration(LinearLayoutManager.VERTICAL));</code>添加的Item装饰器<br><figure class="highlight java"><table><tr><td class="gutter"><pre><div class="line">1</div><div class="line">2</div><div class="line">3</div><div class="line">4</div></pre></td><td class="code"><pre><div class="line"><span class="function"><span class="keyword">public</span> <span class="keyword">void</span> <span class="title">layoutDecorated</span><span class="params">(View child, <span class="keyword">int</span> left, <span class="keyword">int</span> top, <span class="keyword">int</span> right, <span class="keyword">int</span> bottom)</span> </span>{</div><div class="line"> <span class="keyword">final</span> Rect insets = ((LayoutParams) child.getLayoutParams()).mDecorInsets;</div><div class="line"> child.layout(left + insets.left, top + insets.top, right - insets.right, bottom - insets.bottom);</div><div class="line">}</div></pre></td></tr></table></figure></p>
<p>可以看到<code>layoutDecorated</code>方法中直接调用了View的layout方法对child视图进行layout布局。</p>
<p>到此RecyclerView列表中Item项视图的measure和layout实际上已经完成。</p>
<h3 id="RecyclerView-onLayout"><a href="#RecyclerView-onLayout" class="headerlink" title="RecyclerView.onLayout"></a>RecyclerView.onLayout</h3><p>看完<code>onMeasure</code>方法，再来看看<code>onLayout</code>方法：<br><figure class="highlight java"><table><tr><td class="gutter"><pre><div class="line">1</div><div class="line">2</div><div class="line">3</div><div class="line">4</div><div class="line">5</div><div class="line">6</div><div class="line">7</div><div class="line">8</div><div class="line">9</div><div class="line">10</div><div class="line">11</div><div class="line">12</div><div class="line">13</div><div class="line">14</div><div class="line">15</div><div class="line">16</div><div class="line">17</div><div class="line">18</div><div class="line">19</div><div class="line">20</div><div class="line">21</div><div class="line">22</div><div class="line">23</div><div class="line">24</div></pre></td><td class="code"><pre><div class="line"> <span class="meta">@Override</span></div><div class="line"><span class="function"><span class="keyword">protected</span> <span class="keyword">void</span> <span class="title">onLayout</span><span class="params">(<span class="keyword">boolean</span> changed, <span class="keyword">int</span> l, <span class="keyword">int</span> t, </span></span></div><div class="line"> <span class="keyword">int</span> r, <span class="keyword">int</span> b) {</div><div class="line"> TraceCompat.beginSection(TRACE_ON_LAYOUT_TAG);</div><div class="line"> dispatchLayout();</div><div class="line"> TraceCompat.endSection();</div><div class="line"> mFirstLayoutComplete = <span class="keyword">true</span>;</div><div class="line">}</div><div class="line"></div><div class="line"><span class="function"><span class="keyword">void</span> <span class="title">dispatchLayout</span><span class="params">()</span> </span>{</div><div class="line"> ......</div><div class="line"> mState.mIsMeasuring = <span class="keyword">false</span>;</div><div class="line"> <span class="keyword">if</span> (mState.mLayoutStep == State.STEP_START) {</div><div class="line"> ......</div><div class="line"> dispatchLayoutStep2();</div><div class="line"> } <span class="keyword">else</span> <span class="keyword">if</span> (mAdapterHelper.hasUpdates() || mLayout.getWidth() != getWidth() || mLayout.getHeight() != getHeight()) {</div><div class="line"> ......</div><div class="line"> dispatchLayoutStep2();</div><div class="line"> } <span class="keyword">else</span> {</div><div class="line"> <span class="comment">// always make sure we sync them (to ensure mode is exact)</span></div><div class="line"> mLayout.setExactMeasureSpecsFrom(<span class="keyword">this</span>);</div><div class="line"> }</div><div class="line"> dispatchLayoutStep3();</div><div class="line">}</div></pre></td></tr></table></figure></p>
<p>通过<code>dispatchLayout</code>方法可以看到onLayout中又执行了我们前面分析过的<code>dispatchLayoutStep2()</code>方法，在最后又执行了一个<code>dispatchLayoutStep3()</code>方法，我们再来看看这个：<br><figure class="highlight java"><table><tr><td class="gutter"><pre><div class="line">1</div><div class="line">2</div><div class="line">3</div><div class="line">4</div><div class="line">5</div><div class="line">6</div><div class="line">7</div><div class="line">8</div><div class="line">9</div><div class="line">10</div><div class="line">11</div><div class="line">12</div><div class="line">13</div><div class="line">14</div><div class="line">15</div><div class="line">16</div><div class="line">17</div><div class="line">18</div><div class="line">19</div><div class="line">20</div><div class="line">21</div><div class="line">22</div><div class="line">23</div><div class="line">24</div><div class="line">25</div><div class="line">26</div><div class="line">27</div><div class="line">28</div><div class="line">29</div><div class="line">30</div><div class="line">31</div><div class="line">32</div><div class="line">33</div><div class="line">34</div></pre></td><td class="code"><pre><div class="line"><span class="function"><span class="keyword">private</span> <span class="keyword">void</span> <span class="title">dispatchLayoutStep3</span><span class="params">()</span> </span>{</div><div class="line"> ......</div><div class="line"> <span class="keyword">if</span> (mState.mRunSimpleAnimations) {</div><div class="line"> <span class="keyword">for</span> (<span class="keyword">int</span> i = mChildHelper.getChildCount() - <span class="number">1</span>; i >= <span class="number">0</span>; i--) {</div><div class="line"> ViewHolder holder = getChildViewHolderInt(mChildHelper.getChildAt(i));</div><div class="line"> ......</div><div class="line"> <span class="keyword">final</span> ItemHolderInfo animationInfo = mItemAnimator</div><div class="line"> .recordPostLayoutInformation(mState, holder);</div><div class="line"> ......</div><div class="line"> <span class="keyword">if</span> (oldChangeViewHolder != <span class="keyword">null</span> && !oldChangeViewHolder.shouldIgnore()) {</div><div class="line"> ......</div><div class="line"> <span class="keyword">if</span> (oldDisappearing && oldChangeViewHolder == holder) {</div><div class="line"> <span class="comment">//此处会执行动画</span></div><div class="line"> <span class="comment">// run disappear animation instead of change</span></div><div class="line"> mViewInfoStore.addToPostLayout(holder, animationInfo);</div><div class="line"> } <span class="keyword">else</span> {</div><div class="line"> ......</div><div class="line"> <span class="keyword">if</span> (preInfo == <span class="keyword">null</span>) {</div><div class="line"> handleMissingPreInfoForChangeError(key, holder, oldChangeViewHolder);</div><div class="line"> } <span class="keyword">else</span> {</div><div class="line"> <span class="comment">//此方法最终调用DefaultItemAnimate的相关动画</span></div><div class="line"> animateChange(oldChangeViewHolder, holder, preInfo, postInfo, oldDisappearing, newDisappearing);</div><div class="line"> }</div><div class="line"> }</div><div class="line"> } <span class="keyword">else</span> {</div><div class="line"> mViewInfoStore.addToPostLayout(holder, animationInfo);</div><div class="line"> }</div><div class="line"> }</div><div class="line"> <span class="comment">// Step 4: Process view info lists and trigger animations</span></div><div class="line"> mViewInfoStore.process(mViewInfoProcessCallback);</div><div class="line"> }</div><div class="line"></div><div class="line"> ......</div><div class="line">}</div></pre></td></tr></table></figure></p>
<p>上面的方法中调用了<code>ItemAnimation</code>动画类的相关方法</p>
<h3 id="RecyclerView-onDraw"><a href="#RecyclerView-onDraw" class="headerlink" title="RecyclerView.onDraw"></a>RecyclerView.onDraw</h3><figure class="highlight java"><table><tr><td class="gutter"><pre><div class="line">1</div><div class="line">2</div><div class="line">3</div><div class="line">4</div><div class="line">5</div><div class="line">6</div><div class="line">7</div><div class="line">8</div><div class="line">9</div></pre></td><td class="code"><pre><div class="line"> <span class="meta">@Override</span></div><div class="line"><span class="function"><span class="keyword">public</span> <span class="keyword">void</span> <span class="title">onDraw</span><span class="params">(Canvas c)</span> </span>{</div><div class="line"> <span class="keyword">super</span>.onDraw(c);</div><div class="line"></div><div class="line"> <span class="keyword">final</span> <span class="keyword">int</span> count = mItemDecorations.size();</div><div class="line"> <span class="keyword">for</span> (<span class="keyword">int</span> i = <span class="number">0</span>; i < count; i++) {</div><div class="line"> mItemDecorations.get(i).onDraw(c, <span class="keyword">this</span>, mState);</div><div class="line"> }</div><div class="line">}</div></pre></td></tr></table></figure>
<p>可以看到<code>ItemDecoration</code>的<code>onDraw</code>方法是在此处调用<br><figure class="highlight java"><table><tr><td class="gutter"><pre><div class="line">1</div><div class="line">2</div><div class="line">3</div><div class="line">4</div><div class="line">5</div><div class="line">6</div><div class="line">7</div><div class="line">8</div><div class="line">9</div><div class="line">10</div></pre></td><td class="code"><pre><div class="line"><span class="meta">@Override</span></div><div class="line"><span class="function"><span class="keyword">public</span> <span class="keyword">void</span> <span class="title">draw</span><span class="params">(Canvas c)</span> </span>{</div><div class="line"> <span class="keyword">super</span>.draw(c);</div><div class="line"></div><div class="line"> <span class="keyword">final</span> <span class="keyword">int</span> count = mItemDecorations.size();</div><div class="line"> <span class="keyword">for</span> (<span class="keyword">int</span> i = <span class="number">0</span>; i < count; i++) {</div><div class="line"> mItemDecorations.get(i).onDrawOver(c, <span class="keyword">this</span>, mState);</div><div class="line"> }</div><div class="line"> ......</div><div class="line">}</div></pre></td></tr></table></figure></p>
<p>这个地方可以看到<code>ItemDecoration</code>的<code>onDrawOver</code>方法是在此处调用</p>
<p>到这里，RecyclerView使用过程中比较常用的几个类(<code>LayoutManager, ItemDecoration, ItemAnimation</code>)的主要作用及使用场景有了个大概的了解。</p>
<p>在RecyclerView中是没有为我们内置Item的单击和长按事件监听接口的，一般为Item设置单击和长按监听都是是直接在Adapter初始化Item视图时，为我们的Item视图直接设置单击监听和长按监听，这种方式与Adapter的耦合度比较高，而且频繁的为view设置监听对象，感觉不太好。其实RecyclerView中为我们提供了一个类<code>OnItemTouchListener</code>通过这个类再结合手势<code>GestureDetector</code>完全可以实现一个耦合度更低复用度更高的单击和长按监听。我们再来看看<code>OnItemTouchListener</code>的实现方式：<br><figure class="highlight java"><table><tr><td class="gutter"><pre><div class="line">1</div><div class="line">2</div><div class="line">3</div><div class="line">4</div><div class="line">5</div><div class="line">6</div><div class="line">7</div><div class="line">8</div><div class="line">9</div><div class="line">10</div><div class="line">11</div><div class="line">12</div><div class="line">13</div><div class="line">14</div><div class="line">15</div><div class="line">16</div><div class="line">17</div><div class="line">18</div><div class="line">19</div><div class="line">20</div><div class="line">21</div><div class="line">22</div><div class="line">23</div><div class="line">24</div><div class="line">25</div><div class="line">26</div><div class="line">27</div><div class="line">28</div><div class="line">29</div><div class="line">30</div><div class="line">31</div><div class="line">32</div><div class="line">33</div><div class="line">34</div><div class="line">35</div><div class="line">36</div><div class="line">37</div><div class="line">38</div><div class="line">39</div><div class="line">40</div><div class="line">41</div><div class="line">42</div><div class="line">43</div><div class="line">44</div><div class="line">45</div><div class="line">46</div><div class="line">47</div><div class="line">48</div><div class="line">49</div><div class="line">50</div><div class="line">51</div><div class="line">52</div><div class="line">53</div><div class="line">54</div><div class="line">55</div><div class="line">56</div><div class="line">57</div><div class="line">58</div><div class="line">59</div><div class="line">60</div><div class="line">61</div><div class="line">62</div><div class="line">63</div><div class="line">64</div><div class="line">65</div><div class="line">66</div><div class="line">67</div><div class="line">68</div><div class="line">69</div></pre></td><td class="code"><pre><div class="line"><span class="meta">@Override</span></div><div class="line"><span class="function"><span class="keyword">public</span> <span class="keyword">boolean</span> <span class="title">onTouchEvent</span><span class="params">(MotionEvent e)</span> </span>{</div><div class="line"> ......</div><div class="line"> <span class="keyword">if</span> (dispatchOnItemTouch(e)) {</div><div class="line"> cancelTouch();</div><div class="line"> <span class="keyword">return</span> <span class="keyword">true</span>;</div><div class="line"> }</div><div class="line"> ......</div><div class="line">}</div><div class="line"></div><div class="line"><span class="function"><span class="keyword">private</span> <span class="keyword">boolean</span> <span class="title">dispatchOnItemTouch</span><span class="params">(MotionEvent e)</span> </span>{</div><div class="line"> <span class="keyword">final</span> <span class="keyword">int</span> action = e.getAction();</div><div class="line"> <span class="keyword">if</span> (mActiveOnItemTouchListener != <span class="keyword">null</span>) {</div><div class="line"> <span class="keyword">if</span> (action == MotionEvent.ACTION_DOWN) {</div><div class="line"> <span class="comment">// Stale state from a previous gesture, we're starting a new one. Clear it.</span></div><div class="line"> mActiveOnItemTouchListener = <span class="keyword">null</span>;</div><div class="line"> } <span class="keyword">else</span> {</div><div class="line"> <span class="comment">//此处即调用OnItemTouchListener的方法</span></div><div class="line"> mActiveOnItemTouchListener.onTouchEvent(<span class="keyword">this</span>, e);</div><div class="line"> <span class="keyword">if</span> (action == MotionEvent.ACTION_CANCEL || action == MotionEvent.ACTION_UP) {</div><div class="line"> <span class="comment">// Clean up for the next gesture.</span></div><div class="line"> mActiveOnItemTouchListener = <span class="keyword">null</span>;</div><div class="line"> }</div><div class="line"> <span class="keyword">return</span> <span class="keyword">true</span>;</div><div class="line"> }</div><div class="line"> }</div><div class="line"></div><div class="line"> <span class="comment">// Listeners will have already received the ACTION_DOWN via dispatchOnItemTouchIntercept</span></div><div class="line"> <span class="comment">// as called from onInterceptTouchEvent; skip it.</span></div><div class="line"> <span class="keyword">if</span> (action != MotionEvent.ACTION_DOWN) {</div><div class="line"> <span class="keyword">final</span> <span class="keyword">int</span> listenerCount = mOnItemTouchListeners.size();</div><div class="line"> <span class="keyword">for</span> (<span class="keyword">int</span> i = <span class="number">0</span>; i < listenerCount; i++) {</div><div class="line"> <span class="keyword">final</span> OnItemTouchListener listener = mOnItemTouchListeners.get(i);</div><div class="line"> <span class="comment">//此处即调用OnItemTouchListener的方法</span></div><div class="line"> <span class="keyword">if</span> (listener.onInterceptTouchEvent(<span class="keyword">this</span>, e)) {</div><div class="line"> mActiveOnItemTouchListener = listener;</div><div class="line"> <span class="keyword">return</span> <span class="keyword">true</span>;</div><div class="line"> }</div><div class="line"> }</div><div class="line"> }</div><div class="line"> <span class="keyword">return</span> <span class="keyword">false</span>;</div><div class="line">}</div><div class="line"></div><div class="line"><span class="meta">@Override</span></div><div class="line"><span class="function"><span class="keyword">public</span> <span class="keyword">boolean</span> <span class="title">onInterceptTouchEvent</span><span class="params">(MotionEvent e)</span> </span>{</div><div class="line"> ......</div><div class="line"> <span class="keyword">if</span> (dispatchOnItemTouchIntercept(e)) {</div><div class="line"> cancelTouch();</div><div class="line"> <span class="keyword">return</span> <span class="keyword">true</span>;</div><div class="line"> }</div><div class="line"> ......</div><div class="line">}</div><div class="line"></div><div class="line"><span class="function"><span class="keyword">private</span> <span class="keyword">boolean</span> <span class="title">dispatchOnItemTouchIntercept</span><span class="params">(MotionEvent e)</span> </span>{</div><div class="line"> <span class="keyword">final</span> <span class="keyword">int</span> action = e.getAction();</div><div class="line"> <span class="keyword">if</span> (action == MotionEvent.ACTION_CANCEL || action == MotionEvent.ACTION_DOWN) {</div><div class="line"> mActiveOnItemTouchListener = <span class="keyword">null</span>;</div><div class="line"> }</div><div class="line"></div><div class="line"> <span class="keyword">final</span> <span class="keyword">int</span> listenerCount = mOnItemTouchListeners.size();</div><div class="line"> <span class="keyword">for</span> (<span class="keyword">int</span> i = <span class="number">0</span>; i < listenerCount; i++) {</div><div class="line"> <span class="keyword">final</span> OnItemTouchListener listener = mOnItemTouchListeners.get(i);</div><div class="line"> <span class="keyword">if</span> (listener.onInterceptTouchEvent(<span class="keyword">this</span>, e) && action != MotionEvent.ACTION_CANCEL) {</div><div class="line"> mActiveOnItemTouchListener = listener;</div><div class="line"> <span class="keyword">return</span> <span class="keyword">true</span>;</div><div class="line"> }</div><div class="line"> }</div><div class="line"> <span class="keyword">return</span> <span class="keyword">false</span>;</div><div class="line">}</div></pre></td></tr></table></figure></p>
<hr>
<p>原创文章，本文采用<a href="https://creativecommons.org/licenses/by/2.5/cn/" target="_blank" rel="external">知识共享署名 2.5(中国大陆许可协议)</a>进行许可，欢迎转载，但转载请注明来自<a href="ittiger.cn">ittiger.cn</a>，并保证转载后文章内容的完整性。本人(laohu)保留所有版权相关权利。</p>
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