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<h1>Android Context理解与陷阱</h1>
<span class="meta">Posted by
<a href="/author/di-wu.html">Di Wu</a>
on 2017年 7月19日 周三
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<h2>Context?</h2>
<p><code>Context</code>在安卓开发时是一个非常常见的组件，我们会在许多地方使用它，举一些例子：</p>
<ul>
<li>启动新的<code>Activity</code> <code>Service</code></li>
<li>发送广播，接收广播</li>
<li>填充<code>View</code></li>
<li>获取资源</li>
</ul>
<p>相信每一个开发者在看见它时都有过这样一些疑问：</p>
<ul>
<li><code>Context</code>是什么</li>
<li><code>Context</code>的作用</li>
<li><code>Context</code>从哪里来</li>
</ul>
<p>同时，我们也经历过需要一个<code>Context</code>但不知道如何去正确获取/传递的情况，事实上不正确地保存一个<code>Context</code>的引用可能会导致部分内存不能被正确GC从而造成事实上的内存泄漏。</p>
<p>本文将着重对上面这些内容进行讲解。</p>
<!--more-->
<h2>Context的定义</h2>
<p>字面上解释，<code>Context</code>意为“环境”，这个解释比较符合它的作用。</p>
<p>官方文档中对<code>Context</code>的解释是：</p>
<blockquote>
<p>Interface to global information about an application environment. This is an abstract class whose implementation is provided by the Android system. It allows access to application-specific resources and classes, as well as up-calls for application-level operations such as launching activities, broadcasting and receiving intents, etc.</p>
<p>关于应用环境的全局信息的接口。它是一个抽象类，具体由安卓系统来实现。它允许我们去访问特定的应用的资源和类，同时也可以经由它去向上请求应用级别的操作例如启动<code>Activity</code>、发送广播、接收<code>intents</code>等等。</p>
</blockquote>
<p>我们可以把它看作是一个连接我们代码与安卓系统的“桥梁”，我们开发的应用是与运行在设备上的操作系统紧密相关的，只能通过操作系统，我们才能去启动一个新的<code>Activity</code>，向其他应用发送广播，启动一个新的<code>Service</code>或是访问我们存放在<code>apk</code>中的资源文件。</p>
<p><code>Context</code>就是系统为我们提供上述功能的一个接口，我们需要使用它去完成与系统的信息交换。</p>
<h2>Context从哪里来</h2>
<p><code>Context</code>作为一个依赖于系统的类，<code>SDK</code>中只给了我们一个抽象类，具体的实现由系统完成，下文举例使用的<code>ContextImpl</code>就是<code>AOSP</code>中安卓源码对于<code>Context</code>的一个实现。</p>
<h2>Context的作用</h2>
<h3>Context中封装的信息</h3>
<p>我们可以看看<code>Context</code>里面包含了哪些东西（部分）。</p>
<div class="highlight"><pre><span></span><span class="kd">private</span> <span class="kd">final</span> <span class="n">String</span> <span class="n">mBasePackageName</span><span class="o">;</span>
<span class="kd">private</span> <span class="kd">final</span> <span class="n">String</span> <span class="n">mOpPackageName</span><span class="o">;</span> <span class="c1">//软件包名</span>
<span class="kd">private</span> <span class="kd">final</span> <span class="n">Resources</span> <span class="n">mResources</span><span class="o">;</span>
<span class="kd">private</span> <span class="kd">final</span> <span class="n">ResourcesManager</span> <span class="n">mResourcesManager</span><span class="o">;</span> <span class="c1">//用于管理资源文件</span>
<span class="kd">private</span> <span class="kd">final</span> <span class="n">Display</span> <span class="n">mDisplay</span><span class="o">;</span> <span class="c1">//为View填充等提供屏幕尺寸、像素密度等信息</span>
<span class="kd">private</span> <span class="kd">final</span> <span class="n">DisplayAdjustments</span> <span class="n">mDisplayAdjustments</span> <span class="o">=</span> <span class="k">new</span> <span class="n">DisplayAdjustments</span><span class="o">();</span>
<span class="kd">private</span> <span class="n">Resources</span><span class="o">.</span><span class="na">Theme</span> <span class="n">mTheme</span> <span class="o">=</span> <span class="kc">null</span><span class="o">;</span> <span class="c1">//主题信息</span>
<span class="kd">private</span> <span class="n">File</span> <span class="n">mCacheDir</span><span class="o">;</span>
<span class="nd">@GuardedBy</span><span class="o">(</span><span class="s">"mSync"</span><span class="o">)</span>
<span class="kd">private</span> <span class="n">File</span> <span class="n">mCodeCacheDir</span><span class="o">;</span>
<span class="o">...</span>
<span class="nd">@GuardedBy</span><span class="o">(</span><span class="s">"mSync"</span><span class="o">)</span>
<span class="kd">private</span> <span class="n">File</span><span class="o">[]</span> <span class="n">mExternalObbDirs</span><span class="o">;</span>
<span class="nd">@GuardedBy</span><span class="o">(</span><span class="s">"mSync"</span><span class="o">)</span>
<span class="kd">private</span> <span class="n">File</span><span class="o">[]</span> <span class="n">mExternalFilesDirs</span><span class="o">;</span>
<span class="nd">@GuardedBy</span><span class="o">(</span><span class="s">"mSync"</span><span class="o">)</span>
<span class="kd">private</span> <span class="n">File</span><span class="o">[]</span> <span class="n">mExternalCacheDirs</span><span class="o">;</span>
<span class="nd">@GuardedBy</span><span class="o">(</span><span class="s">"mSync"</span><span class="o">)</span>
<span class="kd">private</span> <span class="n">File</span><span class="o">[]</span> <span class="n">mExternalMediaDirs</span><span class="o">;</span> <span class="c1">//各种文件路径</span>
</pre></div>
<p>这些域的存在为功能提供了必要的信息，例如在<code>LayoutInflater</code>填充<code>View</code>时需要一个<code>context</code>作为参数，我们查看这个<code>context</code>如何被使用：</p>
<div class="highlight"><pre><span></span><span class="kd">final</span> <span class="n">XmlResourceParser</span> <span class="n">childParser</span> <span class="o">=</span> <span class="n">context</span><span class="o">.</span><span class="na">getResources</span><span class="o">().</span><span class="na">getLayout</span><span class="o">(</span><span class="n">layout</span><span class="o">);</span>
</pre></div>
<p>我们传入的<code>ResourceId</code>最终会被通过<code>context</code>的<code>getResource()</code>方法获取的<code>Resource</code>对象的<code>getLayout()</code>方法定位到对应的<code>xml</code>文件提供给<code>Inflater</code>进行解析。</p>
<div class="highlight"><pre><span></span><span class="c1">// Apply a theme wrapper, if allowed and one is specified.</span>
<span class="k">if</span> <span class="o">(!</span><span class="n">ignoreThemeAttr</span><span class="o">)</span> <span class="o">{</span>
<span class="kd">final</span> <span class="n">TypedArray</span> <span class="n">ta</span> <span class="o">=</span> <span class="n">context</span><span class="o">.</span><span class="na">obtainStyledAttributes</span><span class="o">(</span><span class="n">attrs</span><span class="o">,</span> <span class="n">ATTRS_THEME</span><span class="o">);</span>
<span class="kd">final</span> <span class="kt">int</span> <span class="n">themeResId</span> <span class="o">=</span> <span class="n">ta</span><span class="o">.</span><span class="na">getResourceId</span><span class="o">(</span><span class="mi">0</span><span class="o">,</span> <span class="mi">0</span><span class="o">);</span>
<span class="k">if</span> <span class="o">(</span><span class="n">themeResId</span> <span class="o">!=</span> <span class="mi">0</span><span class="o">)</span> <span class="o">{</span>
<span class="n">context</span> <span class="o">=</span> <span class="k">new</span> <span class="n">ContextThemeWrapper</span><span class="o">(</span><span class="n">context</span><span class="o">,</span> <span class="n">themeResId</span><span class="o">);</span>
<span class="o">}</span>
<span class="n">ta</span><span class="o">.</span><span class="na">recycle</span><span class="o">();</span>
<span class="o">}</span>
</pre></div>
<p>在这里调用了<code>context</code>的<code>obtainStyledAttributes()</code>方法：</p>
<div class="highlight"><pre><span></span><span class="kd">public</span> <span class="kd">final</span> <span class="n">TypedArray</span> <span class="nf">obtainStyledAttributes</span><span class="o">(</span>
<span class="n">AttributeSet</span> <span class="n">set</span><span class="o">,</span> <span class="nd">@StyleableRes</span> <span class="kt">int</span><span class="o">[]</span> <span class="n">attrs</span><span class="o">)</span> <span class="o">{</span>
<span class="k">return</span> <span class="n">getTheme</span><span class="o">().</span><span class="na">obtainStyledAttributes</span><span class="o">(</span><span class="n">set</span><span class="o">,</span> <span class="n">attrs</span><span class="o">,</span> <span class="mi">0</span><span class="o">,</span> <span class="mi">0</span><span class="o">);</span>
<span class="o">}</span>
</pre></div>
<p>最终使用了<code>context</code>中存放的主题信息为填充的<code>view</code>设置属性。</p>
<p>现在我们知道，我们存放在<code>res</code>文件夹下的内容（布局文件、字符串文件、图片、主题……）都需要通过一个<code>context</code>去向系统获取。</p>
<p>那么为什么在启动<code>activity</code>、启动<code>service</code>、发送广播时都需要使用<code>context</code>呢？因为这些操作与系统是紧密相关的，我们知道启动这些东西都需要使用一个叫<code>intent</code>的东西（关于<code>intent</code>的内容会在另外的文章讲），以<code>startActivity()</code>方法为例，我们一路向上追溯，可以发现启动<code>activity</code>最终是由<code>AcitivityManagerNative.getDefault()</code>的本地方法<code>startActivity()</code>执行的：</p>
<div class="highlight"><pre><span></span><span class="k">try</span> <span class="o">{</span>
<span class="n">intent</span><span class="o">.</span><span class="na">migrateExtraStreamToClipData</span><span class="o">();</span>
<span class="n">intent</span><span class="o">.</span><span class="na">prepareToLeaveProcess</span><span class="o">();</span>
<span class="kt">int</span> <span class="n">result</span> <span class="o">=</span> <span class="n">ActivityManagerNative</span><span class="o">.</span><span class="na">getDefault</span><span class="o">().</span><span class="na">startActivity</span><span class="o">(</span>
<span class="n">whoThread</span><span class="o">,</span> <span class="n">who</span><span class="o">.</span><span class="na">getBasePackageName</span><span class="o">(),</span> <span class="n">intent</span><span class="o">,</span>
<span class="n">intent</span><span class="o">.</span><span class="na">resolveTypeIfNeeded</span><span class="o">(</span><span class="n">who</span><span class="o">.</span><span class="na">getContentResolver</span><span class="o">()),</span> <span class="n">token</span><span class="o">,</span>
<span class="n">target</span> <span class="o">!=</span> <span class="kc">null</span> <span class="o">?</span> <span class="n">target</span><span class="o">.</span><span class="na">mEmbeddedID</span> <span class="o">:</span> <span class="kc">null</span><span class="o">,</span> <span class="n">requestCode</span><span class="o">,</span> <span class="mi">0</span><span class="o">,</span> <span class="kc">null</span><span class="o">,</span> <span class="n">options</span><span class="o">);</span>
<span class="n">checkStartActivityResult</span><span class="o">(</span><span class="n">result</span><span class="o">,</span> <span class="n">intent</span><span class="o">);</span>
<span class="o">}</span> <span class="k">catch</span> <span class="o">(</span><span class="n">RemoteException</span> <span class="n">e</span><span class="o">)</span> <span class="o">{</span>
<span class="o">}</span>
</pre></div>
<p>这个时候我们发现，传入的<code>context</code>已经变成了上面代码中的<code>who</code>，利用这个 <code>context</code>获取了包名与方法的第四个参数<code>who.getContentResolver()</code>。它的作用是提供信息来解析<code>intent</code>的<a href="https://en.wikipedia.org/wiki/Media_type"><code>MIME type</code></a>，帮助系统决定<code>intent</code>的目标。</p>
<p>可以看到<code>context</code>在这里同样起到了一个提供必要信息的作用。</p>
<h3>Context的作用</h3>
<p>在这里再重复一遍上面说过的话，配合之前的例子，是不是可以更好地理解了呢？</p>
<blockquote>
<p>我们可以把它看作是一个连接我们代码与安卓系统的“桥梁”，我们开发的应用是与运行在设备上的操作系统紧密相关的，只能通过操作系统，我们才能去启动一个新的<code>Activity</code>，向其他应用发送广播，启动一个新的<code>Service</code>或是访问我们存放在<code>apk</code>中的资源文件。</p>
<p><code>Context</code>就是系统为我们提供上述功能的一个接口，我们需要使用它去完成与系统的信息交换。</p>
</blockquote>
<h2>Context的使用</h2>
<h3>Context分类</h3>
<p><code>Context</code>并不是都是相同的，根据获取方式的不同，我们得到的<code>Context</code>的各类也有所不同。</p>
<h4><code>Activity</code>/<code>Service</code></h4>
<p>我们知道<code>Acitivity</code>类继承自<code>ContextThemeWrapper</code>，<code>ContextThemeWrapper</code>继承自<code>ContextWrapper</code>，最后<code>ContextWrapper</code>继承自<code>Context</code>。顾名思义，<code>ContextWrapper</code>与<code>ContextThemeWrapper</code>只是将<code>Context</code>进行了再次的包装，加入了更多的信息，同时对一些方法做了转发。</p>
<p>所以我们在<code>Activity</code>或<code>Service</code>中需要<code>Context</code>时就可以直接使用<code>this</code>，因为它们本身就是<code>Context</code>。</p>
<p>当系统创建一个新的<code>Activity</code>/<code>Service</code>实例时，它也会创建一个新的<code>ContextImpl</code>实例来封装所有的信息。</p>
<p><strong>对于每一个<code>Activity</code>/<code>Service</code>实例，它们的基础<code>Context</code>都是独立的。</strong></p>
<h4><code>Application</code></h4>
<p><code>Application</code>同样继承于<code>ContextWrapper</code>，但是<code>Application</code>本身是以单例模式运行在应用进程中的，它可以被任何<code>Activity</code>/<code>Service</code>用<code>getApplication()</code>或是被任何<code>Context</code>使用<code>getApplicationContext()</code>方法获取。</p>
<p><strong>不管使用什么方法去获取<code>Application</code>，获取的总是同一个<code>Application</code>实例。</strong></p>
<h4><code>BroadcastReciver</code></h4>
<p><code>BroadcastReciver</code>本身并不是一个<code>Context</code>或在内部保存了一个<code>Context</code>，但是系统会在每次调用其<code>onRecive()</code>方法时向它传递一个<code>Context</code>对象，这个<code>Context</code>对象是一个<code>ReceiverRestrictedContext</code>（接收器限定<code>Context</code>），与普通<code>Context</code>不同在它的<code>registerReceiver()</code>与<code>bindSerivce()</code>方法是被禁止使用的，这意味着我们不能在<code>onRecive()</code>方法中调用该<code>Context</code>的这两个方法。</p>
<p><strong>每次调用<code>onReceive()</code>方法传递的<code>Context</code>都是全新的。</strong></p>
<h4><code>ContentProvider</code></h4>
<p>它本身同样不是一个<code>Context</code>，但它在创建时会被赋予一个<code>Context</code>并可以通过<code>getContext()</code>方法获取。</p>
<p><strong>如果这个内容提供器运行在调用它的应用中，将会返回该应用的<code>Application</code>单例，如果它是由其他应用提供的，返回的<code>Context</code>将会是一个新创建的表示其他应用环境的<code>Context</code>。</strong></p>
<h3>使用<code>Context</code>时的陷阱</h3>
<p>现在我们知道<code>Context</code>的几种分类，其实上面的分类也就是我们获取它的方式。着重标出的内容说明了它们被提供的来源，也暗指了它们的生命周期。</p>
<p>我们常常会在类中保存对<code>Context</code>的引用，但是我们要考虑生命周期的问题：如果被引用的这个<code>Context</code>是一个<code>Acitivity</code>，<strong>如果存放这个引用的类的生命周期大于<code>Activity</code>的生命周期，那么<code>Activity</code>在停止使用之后还被这个类引用着，就会引致无法被GC，造成事实上的内存泄露。</strong></p>
<p>举一个例子，如果使用下面的一个单例来保存<code>Context</code>的引用来加载资源：</p>
<div class="highlight"><pre><span></span><span class="kd">public</span> <span class="kd">class</span> <span class="nc">CustomManager</span> <span class="o">{</span>
<span class="kd">private</span> <span class="kd">static</span> <span class="n">CustomManager</span> <span class="n">sInstance</span><span class="o">;</span>
<span class="kd">public</span> <span class="kd">static</span> <span class="n">CustomManager</span> <span class="nf">getInstance</span><span class="o">(</span><span class="n">Context</span> <span class="n">context</span><span class="o">)</span> <span class="o">{</span>
<span class="k">if</span> <span class="o">(</span><span class="n">sInstance</span> <span class="o">==</span> <span class="kc">null</span><span class="o">)</span> <span class="o">{</span>
<span class="n">sInstance</span> <span class="o">=</span> <span class="k">new</span> <span class="n">CustomManager</span><span class="o">(</span><span class="n">context</span><span class="o">);</span>
<span class="o">}</span>
<span class="k">return</span> <span class="n">sInstance</span><span class="o">;</span>
<span class="o">}</span>
<span class="kd">private</span> <span class="n">Context</span> <span class="n">mContext</span><span class="o">;</span>
<span class="kd">private</span> <span class="nf">CustomManager</span><span class="o">(</span><span class="n">Context</span> <span class="n">context</span><span class="o">)</span> <span class="o">{</span>
<span class="n">mContext</span> <span class="o">=</span> <span class="n">context</span><span class="o">;</span>
<span class="o">}</span>
<span class="o">}</span>
</pre></div>
<p>这段程序的问题在于不知道传入的<code>Context</code>会是什么类型的，可能在初始化的时候传入的是一个<code>Activity</code>/<code>Serivce</code>，那么几乎可以肯定的是，这个<code>Activity</code>/<code>Service</code>将不会在结束以后被垃圾回收。如果是一个<code>Activity</code>，那么这意味着与它相关联的<code>View</code>或是其他庞大的类都将留在内存中而不会被回收。</p>
<p>为了避免这样的问题，我们可以改正这个单例：</p>
<div class="highlight"><pre><span></span><span class="kd">public</span> <span class="kd">class</span> <span class="nc">CustomManager</span> <span class="o">{</span>
<span class="kd">private</span> <span class="kd">static</span> <span class="n">CustomManager</span> <span class="n">sInstance</span><span class="o">;</span>
<span class="kd">public</span> <span class="kd">static</span> <span class="n">CustomManager</span> <span class="nf">getInstance</span><span class="o">(</span><span class="n">Context</span> <span class="n">context</span><span class="o">)</span> <span class="o">{</span>
<span class="k">if</span> <span class="o">(</span><span class="n">sInstance</span> <span class="o">==</span> <span class="kc">null</span><span class="o">)</span> <span class="o">{</span>
<span class="c1">//Always pass in the Application Context</span>
<span class="n">sInstance</span> <span class="o">=</span> <span class="k">new</span> <span class="n">CustomManager</span><span class="o">(</span><span class="n">context</span><span class="o">.</span><span class="na">getApplicationContext</span><span class="o">());</span>
<span class="o">}</span>
<span class="k">return</span> <span class="n">sInstance</span><span class="o">;</span>
<span class="o">}</span>
<span class="kd">private</span> <span class="n">Context</span> <span class="n">mContext</span><span class="o">;</span>
<span class="kd">private</span> <span class="nf">CustomManager</span><span class="o">(</span><span class="n">Context</span> <span class="n">context</span><span class="o">)</span> <span class="o">{</span>
<span class="n">mContext</span> <span class="o">=</span> <span class="n">context</span><span class="o">;</span>
<span class="o">}</span>
<span class="o">}</span>
</pre></div>
<p>我们只修改了一处，第7行中我们使用<code>context.getApplicationContext()</code>这个方法来获取<code>Application</code>这个单例，而不是直接保存<code>context</code>本身，这样就可以保证不会出现某<code>context</code>因为被这个单例引用而不能回收的情况。而<code>Application</code>本身是单例这个特性保证了生命周期的一致，不会造成内存的浪费。</p>
<h3>为什么不总是使用<code>application</code>作为<code>context</code></h3>
<p>既然它是一个单例，那么我们为什么不直接在任何地方都只使用它呢？</p>
<p>这是因为各种<code>context</code>的能力有所不同：</p>
<p><img alt="" src="/images/context01.png"></p>
<p>（图片出处见文末）</p>
<p>对几个注解的地方作说明：</p>
<ol>
<li>一个<code>application</code>可以启动一个<code>activity</code>，但是需要新建一个<code>task</code>，在特殊情况下可以这么做，但是这不是一个好的行为因为这会导致一个不寻常的返回栈。</li>
<li>虽然这是合法的，但是会导致填充出来的<code>view</code>使用系统默认的主题而不是我们设置的主题。</li>
<li>如果接收器是<code>null</code>的话是被允许的，通常在4.2及以上的版本中用来获取一个粘性广播的当前值。</li>
</ol>
<p>我们可以发现与<code>UI</code>有关的操作除<code>activity</code>之外都不能完成，在其他地方这些<code>context</code>能做的事情都差不多。</p>
<p>但是我们回过头来想，这三个与<code>UI</code>相关的操作一般都不会在一个<code>activity</code>之外进行，这个特性很大程度上就是系统为我们设计成这样的，如果我们试图去用一个<code>Application</code>去显示一个<code>dialog</code>就会导致异常的抛出和应用的崩溃。</p>
<p>对上面的第二点再进一步解释，虽然我们可以使用<code>application</code>作为<code>context</code>去填充一个<code>view</code>，但是这样填充出的<code>view</code>使用的将会是系统默认的主题，这是因为只有<code>acitivity</code>中才会存有我们定义在<code>manifest</code>中的主题信息，其他的<code>context</code>将会使用默认的主题去填充<code>view</code>。</p>
<h3>如何使用正确的<code>Context</code></h3>
<p>既然我们不能将<code>Activity</code>作为<code>context</code>保存在另外一个比该<code>Activity</code>生命周期长的类中，那么如果我们需要在这个类中完成与<code>UI</code>有关的操作（比如显示一个<code>dialog</code>）该怎么办？</p>
<p>如果真的遇到了这样的情况：我们不得不保存一个<code>activity</code>在一个比该<code>Activity</code>生命周期长的类中以进行<code>UI</code>操作，就说明我们的设计是有问题的，系统的设计决定了我们不应该去进行这样的操作。</p>
<p>所以我们可以得出结论：</p>
<p>我们应该在<code>Activity</code>/<code>Service</code>的生命周期范围内直接使用该<code>Activity</code>/<code>Service</code>作为<code>context</code>，在它们的范围之外的类，应该使用<code>Application</code>单例这个<code>context</code>（并且不应该出现<code>UI</code>操作）。</p>
<h2>Reference</h2>
<p><a href="https://possiblemobile.com/2013/06/context/">https://possiblemobile.com/2013/06/context/</a></p>
<p><a href="https://web.archive.org/web/20170621005334/http://levinotik.tumblr.com/post/15783237959/demystifying-context-in-android">https://web.archive.org/web/20170621005334/http://levinotik.tumblr.com/post/15783237959/demystifying-context-in-android</a></p>
<p><a href="http://grepcode.com/file/repository.grepcode.com/java/ext/com.google.android/android/5.1.1_r1/android/app/ContextImpl.java?av=f">http://grepcode.com/file/repository.grepcode.com/java/ext/com.google.android/android/5.1.1_r1/android/app/ContextImpl.java?av=f</a></p>
</article>
<div class="tags">
<p>tags: <a href="/tag/android.html">android</a></p>
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