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<meta property="og:description" content="经过上一篇介绍的measure过程之后，各个View的尺寸信息已经存储在了每个View中，下面是layout过程，layout过程的目的是根据上一步中计算出的尺寸来正确设置各个View及其后代的位置。这个过程首先被调用的是View的layout()方法，layout()的方...">
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<h1>Android View绘制之layout过程</h1>
<span class="meta">Posted by
<a href="/author/di-wu.html">Di Wu</a>
on 2017年 3月12日 周日
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<p>经过<a href="http://www.viseator.xyz/2017/03/10/android_view_onMeasure/">上一篇</a>介绍的<code>measure</code>过程之后，各个<code>View</code>的尺寸信息已经存储在了每个<code>View</code>中，下面是<code>layout</code>过程，<code>layout</code>过程的目的是根据上一步中计算出的尺寸来正确设置各个<code>View</code>及其后代的位置。这个过程首先被调用的是<code>View</code>的<code>layout()</code>方法，<code>layout()</code>的方法签名是<code>public void layout(int l, int t, int r, int b)</code>，四个参数分别为左边界距父<code>View</code>左边界的距离，上边界距父<code>View</code>上边界的距离，右边界距父<code>View</code>左边界的距离，下边界距父<code>View</code>上边界的距离。</p>
<div class="highlight"><pre><span></span><span class="kt">boolean</span> <span class="n">changed</span> <span class="o">=</span> <span class="n">isLayoutModeOptical</span><span class="o">(</span><span class="n">mParent</span><span class="o">)</span> <span class="o">?</span>
<span class="n">setOpticalFrame</span><span class="o">(</span><span class="n">l</span><span class="o">,</span> <span class="n">t</span><span class="o">,</span> <span class="n">r</span><span class="o">,</span> <span class="n">b</span><span class="o">)</span> <span class="o">:</span> <span class="n">setFrame</span><span class="o">(</span><span class="n">l</span><span class="o">,</span> <span class="n">t</span><span class="o">,</span> <span class="n">r</span><span class="o">,</span> <span class="n">b</span><span class="o">);</span>
</pre></div>
<p><code>changed</code>是用于传递给<code>onLayout()</code>方法的参数，它指示了布局是否被改变。</p>
<p>后面的表达式查看了父<code>View</code>的布局模式是否需要显示边框，如需要，调用的是<code>setOpticalFrame()</code>方法：</p>
<div class="highlight"><pre><span></span><span class="kd">private</span> <span class="kt">boolean</span> <span class="nf">setOpticalFrame</span><span class="o">(</span><span class="kt">int</span> <span class="n">left</span><span class="o">,</span> <span class="kt">int</span> <span class="n">top</span><span class="o">,</span> <span class="kt">int</span> <span class="n">right</span><span class="o">,</span> <span class="kt">int</span> <span class="n">bottom</span><span class="o">)</span> <span class="o">{</span>
<span class="n">Insets</span> <span class="n">parentInsets</span> <span class="o">=</span> <span class="n">mParent</span> <span class="k">instanceof</span> <span class="n">View</span> <span class="o">?</span>
<span class="o">((</span><span class="n">View</span><span class="o">)</span> <span class="n">mParent</span><span class="o">).</span><span class="na">getOpticalInsets</span><span class="o">()</span> <span class="o">:</span> <span class="n">Insets</span><span class="o">.</span><span class="na">NONE</span><span class="o">;</span>
<span class="n">Insets</span> <span class="n">childInsets</span> <span class="o">=</span> <span class="n">getOpticalInsets</span><span class="o">();</span>
<span class="k">return</span> <span class="n">setFrame</span><span class="o">(</span>
<span class="n">left</span> <span class="o">+</span> <span class="n">parentInsets</span><span class="o">.</span><span class="na">left</span> <span class="o">-</span> <span class="n">childInsets</span><span class="o">.</span><span class="na">left</span><span class="o">,</span>
<span class="n">top</span> <span class="o">+</span> <span class="n">parentInsets</span><span class="o">.</span><span class="na">top</span> <span class="o">-</span> <span class="n">childInsets</span><span class="o">.</span><span class="na">top</span><span class="o">,</span>
<span class="n">right</span> <span class="o">+</span> <span class="n">parentInsets</span><span class="o">.</span><span class="na">left</span> <span class="o">+</span> <span class="n">childInsets</span><span class="o">.</span><span class="na">right</span><span class="o">,</span>
<span class="n">bottom</span> <span class="o">+</span> <span class="n">parentInsets</span><span class="o">.</span><span class="na">top</span> <span class="o">+</span> <span class="n">childInsets</span><span class="o">.</span><span class="na">bottom</span><span class="o">);</span>
<span class="o">}</span>
</pre></div>
<p>可以看到这个方法读取了设置的边框值， 把原值加上边框值后还是调用了<code>setFrame()</code>方法。</p>
<p><code>setFrame()</code>方法通过传入的参数确定了该<code>View</code>最终的位置以及尺寸。</p>
<p>可以看到，一个<code>View</code>最终显示在什么位置以及它的尺寸是由<code>layout()</code>方法决定的，<code>onMeasure()</code>方法只是将测量出的<code>View</code>期望具有的大小储存在<code>View</code>中。一般情况下，我们会根据储存的这个尺寸来作为设定的依据。</p>
<p>接下来<code>layout()</code>方法会调用<code>onLayout()</code>方法，（如果需要的话）我们需要重写这个方法来调用子<code>View</code>的<code>layout()</code>方法。所以决定子<code>View</code>如何显示的关键步骤就在这里，他们的位置和尺寸完全取决于这里调用它们的<code>layout()</code>方法时传入的参数。当然一般情况下我们会根据子<code>View</code>中的测量结果来设置这个值。这里拿<code>FrameLayout</code>这个需要处理子<code>View</code>的<code>ViewGroup</code>实例来举例：</p>
<div class="highlight"><pre><span></span><span class="nd">@Override</span>
<span class="kd">protected</span> <span class="kt">void</span> <span class="nf">onLayout</span><span class="o">(</span><span class="kt">boolean</span> <span class="n">changed</span><span class="o">,</span> <span class="kt">int</span> <span class="n">left</span><span class="o">,</span> <span class="kt">int</span> <span class="n">top</span><span class="o">,</span> <span class="kt">int</span> <span class="n">right</span><span class="o">,</span> <span class="kt">int</span> <span class="n">bottom</span><span class="o">)</span> <span class="o">{</span>
<span class="n">layoutChildren</span><span class="o">(</span><span class="n">left</span><span class="o">,</span> <span class="n">top</span><span class="o">,</span> <span class="n">right</span><span class="o">,</span> <span class="n">bottom</span><span class="o">,</span> <span class="kc">false</span> <span class="cm">/* no force left gravity */</span><span class="o">);</span>
<span class="o">}</span>
</pre></div>
<p>直接调用了<code>layoutChildren()</code>：（省略部分行）</p>
<div class="highlight"><pre><span></span><span class="kt">void</span> <span class="nf">layoutChildren</span><span class="o">(</span><span class="kt">int</span> <span class="n">left</span><span class="o">,</span> <span class="kt">int</span> <span class="n">top</span><span class="o">,</span> <span class="kt">int</span> <span class="n">right</span><span class="o">,</span> <span class="kt">int</span> <span class="n">bottom</span><span class="o">,</span> <span class="kt">boolean</span> <span class="n">forceLeftGravity</span><span class="o">)</span> <span class="o">{</span>
<span class="kd">final</span> <span class="kt">int</span> <span class="n">count</span> <span class="o">=</span> <span class="n">getChildCount</span><span class="o">();</span>
<span class="kd">final</span> <span class="kt">int</span> <span class="n">parentLeft</span> <span class="o">=</span> <span class="n">getPaddingLeftWithForeground</span><span class="o">();</span>
<span class="kd">final</span> <span class="kt">int</span> <span class="n">parentRight</span> <span class="o">=</span> <span class="n">right</span> <span class="o">-</span> <span class="n">left</span> <span class="o">-</span> <span class="n">getPaddingRightWithForeground</span><span class="o">();</span>
<span class="kd">final</span> <span class="kt">int</span> <span class="n">parentTop</span> <span class="o">=</span> <span class="n">getPaddingTopWithForeground</span><span class="o">();</span>
<span class="kd">final</span> <span class="kt">int</span> <span class="n">parentBottom</span> <span class="o">=</span> <span class="n">bottom</span> <span class="o">-</span> <span class="n">top</span> <span class="o">-</span> <span class="n">getPaddingBottomWithForeground</span><span class="o">();</span>
<span class="k">for</span> <span class="o">(</span><span class="kt">int</span> <span class="n">i</span> <span class="o">=</span> <span class="mi">0</span><span class="o">;</span> <span class="n">i</span> <span class="o"><</span> <span class="n">count</span><span class="o">;</span> <span class="n">i</span><span class="o">++)</span> <span class="o">{</span>
<span class="kd">final</span> <span class="n">View</span> <span class="n">child</span> <span class="o">=</span> <span class="n">getChildAt</span><span class="o">(</span><span class="n">i</span><span class="o">);</span>
<span class="kd">final</span> <span class="kt">int</span> <span class="n">width</span> <span class="o">=</span> <span class="n">child</span><span class="o">.</span><span class="na">getMeasuredWidth</span><span class="o">();</span>
<span class="kd">final</span> <span class="kt">int</span> <span class="n">height</span> <span class="o">=</span> <span class="n">child</span><span class="o">.</span><span class="na">getMeasuredHeight</span><span class="o">();</span>
<span class="kt">int</span> <span class="n">childLeft</span><span class="o">;</span>
<span class="kt">int</span> <span class="n">childTop</span><span class="o">;</span>
<span class="k">switch</span> <span class="o">(</span><span class="n">verticalGravity</span><span class="o">)</span> <span class="o">{</span>
<span class="k">case</span> <span class="n">Gravity</span><span class="o">.</span><span class="na">TOP</span><span class="o">:</span>
<span class="n">childTop</span> <span class="o">=</span> <span class="n">parentTop</span> <span class="o">+</span> <span class="n">lp</span><span class="o">.</span><span class="na">topMargin</span><span class="o">;</span>
<span class="k">break</span><span class="o">;</span>
<span class="k">case</span> <span class="n">Gravity</span><span class="o">.</span><span class="na">CENTER_VERTICAL</span><span class="o">:</span>
<span class="n">childTop</span> <span class="o">=</span> <span class="n">parentTop</span> <span class="o">+</span> <span class="o">(</span><span class="n">parentBottom</span> <span class="o">-</span> <span class="n">parentTop</span> <span class="o">-</span> <span class="n">height</span><span class="o">)</span> <span class="o">/</span> <span class="mi">2</span> <span class="o">+</span>
<span class="n">lp</span><span class="o">.</span><span class="na">topMargin</span> <span class="o">-</span> <span class="n">lp</span><span class="o">.</span><span class="na">bottomMargin</span><span class="o">;</span>
<span class="k">break</span><span class="o">;</span>
<span class="k">case</span> <span class="n">Gravity</span><span class="o">.</span><span class="na">BOTTOM</span><span class="o">:</span>
<span class="n">childTop</span> <span class="o">=</span> <span class="n">parentBottom</span> <span class="o">-</span> <span class="n">height</span> <span class="o">-</span> <span class="n">lp</span><span class="o">.</span><span class="na">bottomMargin</span><span class="o">;</span>
<span class="k">break</span><span class="o">;</span>
<span class="k">default</span><span class="o">:</span>
<span class="n">childTop</span> <span class="o">=</span> <span class="n">parentTop</span> <span class="o">+</span> <span class="n">lp</span><span class="o">.</span><span class="na">topMargin</span><span class="o">;</span>
<span class="o">}</span>
<span class="n">child</span><span class="o">.</span><span class="na">layout</span><span class="o">(</span><span class="n">childLeft</span><span class="o">,</span> <span class="n">childTop</span><span class="o">,</span> <span class="n">childLeft</span> <span class="o">+</span> <span class="n">width</span><span class="o">,</span> <span class="n">childTop</span> <span class="o">+</span> <span class="n">height</span><span class="o">);</span>
<span class="o">}</span>
<span class="o">}</span>
<span class="o">}</span>
</pre></div>
<p>省略了与获取布局属性相关的代码，可以看到：</p>
<ul>
<li>4-8行获取了父<code>View</code>的位置数据并在18-31行用于确定最终的位置数据</li>
<li>10-11行遍历了所有的子<code>View</code></li>
<li>12-13行获取了子<code>View</code>中在上一步骤的测量过程中储存的宽和高，并用于第33行中设置最终的右边界与下边界</li>
<li>第33行调用子<code>View</code>的<code>layout()</code>方法</li>
</ul>
</article>
<div class="tags">
<p>tags: <a href="/tag/android.html">android</a>, <a href="/tag/view.html">view</a></p>
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