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<a class="article-title" href="/2021/02/19/Kotlin-coroutines-on-Android/">Kotlin coroutines on Android</a>
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<a href="/2021/02/19/Kotlin-coroutines-on-Android/" class="archive-article-date">
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<h1 id="What-is-coroutines"><a href="#What-is-coroutines" class="headerlink" title="What is coroutines?"></a>What is coroutines?</h1><h2 id="维基百科的定义"><a href="#维基百科的定义" class="headerlink" title="维基百科的定义"></a>维基百科的定义</h2><p>维基百科上对协程的定义：</p>
<figure class="highlight plain"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line">协程是计算机程序的一类组件，推广了协作式多任务的子程序，允许执行被挂起与被恢复。</span><br></pre></td></tr></table></figure>
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<img src="https://tva1.sinaimg.cn/large/008eGmZEly1gorvgh1hg7j30a00a0dgn.jpg" alt="听上去怎么这么抽象？" />
</div>
<h2 id="从线程说起"><a href="#从线程说起" class="headerlink" title="从线程说起"></a>从线程说起</h2><div align="center">
<img src="https://tva1.sinaimg.cn/large/008eGmZEly1gorvh9k7vjj30hd08x765.jpg" alt="理想中的线程和实际上的线程" />
</div>
<p>我们都知道线程是<strong>抢占式</strong>的，你可以给线程设置优先级，但是 CPU 实际执行的是哪个线程，我们并不能控制，只能通过<strong>锁</strong>的方式来保证程序的逻辑正确。</p>
<p>协程是<strong>协作式</strong>的，是编程语言层级的，而非系统层级。当我们切换协程时，不涉及任何系统调用，因此可以把协程看作是<strong>轻量的线程</strong>。</p>
<a class="article-more-a" href="/2021/02/19/Kotlin-coroutines-on-Android/#more">more >></a>
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<a class="article-more-a" href="/2021/02/19/Kotlin-coroutines-on-Android/">展开全文 >></a>
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<a class="article-title" href="/2020/05/26/from-livedata-to-stream/">从 LiveData 到流</a>
</h1>
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<h1 id="什么是-LiveData"><a href="#什么是-LiveData" class="headerlink" title="什么是 LiveData?"></a>什么是 LiveData?</h1><blockquote>
<p>一个具有生命周期感知能力的可观察的数据存储类。</p>
</blockquote>
<ul>
<li>生命周期感知能力</li>
<li>可观察</li>
<li>数据存储</li>
</ul>
<a class="article-more-a" href="/2020/05/26/from-livedata-to-stream/#more">more >></a>
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<a class="article-title" href="/2019/07/22/Android-build-speed-opt/">Android 项目编译速度优化</a>
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<time datetime="2019-07-22T12:44:27.000Z" itemprop="datePublished"><i class="icon-calendar icon"></i>2019-07-22</time>
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<h3 id="Android-编译这件小事"><a href="#Android-编译这件小事" class="headerlink" title="Android 编译这件小事"></a>Android 编译这件小事</h3><h4 id="什么是编译？"><a href="#什么是编译？" class="headerlink" title="什么是编译？"></a>什么是编译？</h4><p>我们每天都要经历数十次的编译，那么什么是编译呢？简单来说，就是把高级语言转化为机器或虚拟机能识别的低级语言的过程。在 Android 中，就是把我们认识的 Java 或 Kotlin 语言转化为 Android 虚拟机可以执行的 Dalvik 字节码的过程。</p>
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<a class="article-title" href="/2019/06/23/31DaysOfKotlin/">31DaysOfKotlin</a>
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<time datetime="2019-06-23T09:40:39.000Z" itemprop="datePublished"><i class="icon-calendar icon"></i>2019-06-23</time>
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<h2 id="31-天学习-Kotlin-—-第一周回顾"><a href="#31-天学习-Kotlin-—-第一周回顾" class="headerlink" title="#31 天学习 Kotlin — 第一周回顾"></a>#31 天学习 Kotlin — 第一周回顾</h2><p>我们写的 Kotlin 代码越多，我们越喜欢她！Kotlin 的现代语言特性和 <a target="_blank" rel="noopener" href="https://github.com/android/android-ktx">Android KTX</a> 使我们的 Android 代码更加的简洁，清晰和优雅。我们 （<a target="_blank" rel="noopener" href="https://twitter.com/FMuntenescu">@FMuntenescu</a> 和 <a target="_blank" rel="noopener" href="https://twitter.com/objcode">@objcode</a>）启动了 <a target="_blank" rel="noopener" href="https://twitter.com/search?q=%2331DaysOfKotlin">#31DaysOfKotlin</a> 系列作为分享我们最喜爱的 Kotlin 和 Android KTX 特性的一种方式，希望你和我们一样，越来越喜欢她。</p>
<p>在前 7 天的时间里，我们专注于基础知识。</p>
<h2 id="Day-1-Elvis-操作符"><a href="#Day-1-Elvis-操作符" class="headerlink" title="Day 1: Elvis 操作符"></a>Day 1: Elvis 操作符</h2><p>需要处理代码中的空值？可以使用 elvis 操作符，避免您的 “空情况” (null-erplate)。这只是替换空作为值或者返回事件情况的一个小语法。文档： <a target="_blank" rel="noopener" href="https://kotlinlang.org/docs/reference/null-safety.html#elvis-operator">Elvis operator</a>.</p>
<figure class="highlight kotlin"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">val</span> name: String = person.name ?: “unknown”</span><br><span class="line"><span class="keyword">val</span> age = person.age ?: <span class="keyword">return</span></span><br></pre></td></tr></table></figure>
<h2 id="Day-2-字符串模板"><a href="#Day-2-字符串模板" class="headerlink" title="Day 2: 字符串模板"></a>Day 2: 字符串模板</h2><p>格式化字符串？将 $ 符放在变量名的前面表达字符串中的变量和表达式。使用 ${expression} 求表达式的值。文档： <a target="_blank" rel="noopener" href="https://kotlinlang.org/docs/reference/basic-types.html#string-templates">string templates</a>.</p>
<figure class="highlight kotlin"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">val</span> language = “Kotlin”</span><br><span class="line"></span><br><span class="line"><span class="comment">// “Kotlin has 6 characters”</span></span><br><span class="line"><span class="keyword">val</span> text = “$language has ${language.length} characters”</span><br></pre></td></tr></table></figure>
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<a class="article-title" href="/2019/03/02/In-app-resource-opt/">应用内资源文件优化</a>
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<h2 id="使用-lint-检查无用资源"><a href="#使用-lint-检查无用资源" class="headerlink" title="使用 lint 检查无用资源"></a>使用 lint 检查无用资源</h2><p>使用lint检测工具检查无用代码，注意不要使用一键删除，避免以下两个问题:</p>
<ul>
<li>通过反射获取资源，可以将反射资源加入lint.xml，避过lint检测</li>
<li>koltin目前对lint检测支持并不友好</li>
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<article id="post-action-image-capture-issue" class="article article-type-post article-index" itemscope itemprop="blogPost">
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<a class="article-title" href="/2017/09/04/action-image-capture-issue/">使用ACTION_IMAGE_CAPTURE可能存在的风险</a>
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<p>很多的 Android App 中都有使用相机拍摄用户头像的功能。大部分开发者都会使用<code>MediaStore.ACTION_IMAGE_CAPTURE</code>来满足这一需求。这可以节省很多时间，不需要单独开发相机 UI，直接调用系统相机；不需要向系统请求 Camera 权限。正如官方文档里面说的那样，<a target="_blank" rel="noopener" href="https://developer.android.com/training/camera/photobasics.html#TaskCaptureIntent">Taking Photos Simply</a>。然而在最近的一次的测试中，我发现并没有那么简单。因为运行了几年的代码竟然发生了 Crash。具体的 log 如下：</p>
<blockquote>
<p>java.lang.SecurityException: Permission Denial: starting Intent { act=android.media.action.IMAGE_CAPTURE cat=[android.intent.category.DEFAULT] flg=0x3 cmp=com.google.android.GoogleCamera/com.android.camera.activity.CaptureActivity clip={text/uri-list U:content://com.imzhiqiang.example.fileprovider/imageCache/tmp_avatar.jpg} (has extras) } from ProcessRecord{bf70afd 18107:com.imzhiqiang.example/u0a108} (pid=18107, uid=10108) with revoked permission android.permission.CAMERA</p>
</blockquote>
<p>看上去是因为没有处理运行时权限导致的 Crash。<strong>Interesting!</strong> 我并没有在 manifest 文件中声明 Camera 的权限，为什么会出现没有处理 Camera 运行时权限的问题呢？随后我想到了可能是引用的 library 中声明了该权限。在 Android Studio 中查看了 Merged Manifest，果然是这样。图中深色背景的权限是我自己声明的，下面的权限是第三方的 library 声明的。</p>
<p><img src="http://ww2.sinaimg.cn/large/006tNc79ly1g425ora0u0j30g309egmv.jpg" alt="permission"></p>
<p>然而这和 Intent 又有什么关系？使用 ACTION_IMAGE_CAPTURE 不是可以避免请求 Camera 权限吗？经过几番周折过后，最后终于在官方文档中找到了答案。</p>
<p><img src="http://ww2.sinaimg.cn/large/006tNc79ly1g425pm6qncj30yg0faq63.jpg" alt="action_image_capture"></p>
<p>虽然很难理解 Google 这样的做法，不过总算找到了问题的根本所在。在对 Camera 权限进行正确的处理后，终于正常运行了。</p>
<p><strong>结论</strong></p>
<ol>
<li>如果没有在 manifest 文件中声明 Camera 权限，使用 ACTION_IMAGE_CAPTURE 不需要对 Camera 权限做运行时权限处理，代码正常运行。如果声明了就必须要做权限处理。</li>
<li>使用 Intent 的 action 时，一定要仔细阅读官方文档，避免类似的风险。</li>
<li>引用第三方 library 时，除了熟悉其内部原理和源码外，还要注意它在 manifest 文件中添加的东西。</li>
</ol>
<p><strong>扩展阅读</strong></p>
<ol>
<li><a target="_blank" rel="noopener" href="https://blog.egorand.me/taking-photos-not-so-simply-how-i-got-bitten-by-action_image_capture/">Taking Photos Not So Simply: How I Got Bitten By ACTION_IMAGE_CAPTURE</a></li>
<li><a target="_blank" rel="noopener" href="https://issuetracker.google.com/issues/37063818"> Use intent ACTION_IMAGE_CAPTURE to launch camera app requires CAMERA permission</a></li>
<li><a target="_blank" rel="noopener" href="https://commonsware.com/blog/2015/06/08/action-image-capture-fallacy.html">The ACTION_IMAGE_CAPTURE Fallacy</a></li>
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<a class="article-title" href="/2017/03/05/Cache-in-Glide/">Glide中的缓存</a>
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<p>本文主要介绍了如何配置和管理 Glide 中的缓存，其中大部分内容都可以直接在官方 Wiki 中找到，这里只是进行了整理和汇总。言归正传，Glide 支持图片的二级缓存(并不是三级缓存，因为从网络加载并不属于缓存)，即内存缓存和磁盘缓存。</p>
<h4 id="磁盘缓存"><a href="#磁盘缓存" class="headerlink" title="磁盘缓存"></a>磁盘缓存</h4><p>一般的图片缓存指的就是磁盘缓存，把网络上的图片缓存到本地，这样就不需要每次都从网络加载，既提高了加载速度，又为用户节省了流量。Glide 在默认情况下是开启磁盘缓存的，而且提供了丰富的 API 来让开发者自己配置和管理磁盘缓存。</p>
<p><strong>缓存位置和大小</strong><br>开发者可以通过构建一个自定义的<a target="_blank" rel="noopener" href="https://github.com/bumptech/glide/blob/master/library/src/main/java/com/bumptech/glide/module/GlideModule.java">GlideModule</a>来配置 Glide 磁盘缓存的位置和大小。最简单的方法如下：</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">public</span> <span class="class"><span class="keyword">class</span> <span class="title">DiskCacheMoudle</span> <span class="keyword">implements</span> <span class="title">GlideModule</span> </span>{</span><br><span class="line"> <span class="meta">@Override</span></span><br><span class="line"> <span class="function"><span class="keyword">public</span> <span class="keyword">void</span> <span class="title">applyOptions</span><span class="params">(Context context, GlideBuilder builder)</span> </span>{</span><br><span class="line"> builder.setDiskCache(</span><br><span class="line"> <span class="keyword">new</span> InternalCacheDiskCacheFactory(context, <span class="string">"glide_cache"</span>, <span class="number">100</span> * <span class="number">1024</span> * <span class="number">1024</span>));</span><br><span class="line"> <span class="comment">//builder.setDiskCache(</span></span><br><span class="line"> <span class="comment">// new ExternalCacheDiskCacheFactory(context, "glide_cache", 100 * 1024 * 1024));</span></span><br><span class="line"> }</span><br><span class="line"></span><br><span class="line"> <span class="meta">@Override</span></span><br><span class="line"> <span class="function"><span class="keyword">public</span> <span class="keyword">void</span> <span class="title">registerComponents</span><span class="params">(Context context, Glide glide)</span> </span>{</span><br><span class="line"></span><br><span class="line"> }</span><br><span class="line">}</span><br></pre></td></tr></table></figure>
<p>其中 InternalCache 和 ExternalCache 都最多接收 3 个参数：第一个参数为 Context,没啥好说的；第二个为缓存的目录名称；第三个为缓存大小，单位是 Byte。它们之间唯一的不同就在于 InternalCache 构建的缓存是在应用的内部储存，而 ExternalCache 则是在外部储存。内部储存中的缓存文件是其他应用程序是无法获取到的，更加安全。关于内部储存和外部储存的更多内容，请<a target="_blank" rel="noopener" href="https://developer.android.google.cn/guide/topics/data/data-storage.html">点击这里</a>查看官方文档。</p>
<p>如果不想把缓存放在上面的两个位置怎么办？Glide 当然也支持，具体通过 DiskLruCacheFactory 来实现：</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br></pre></td><td class="code"><pre><span class="line">builder.setDiskCache(</span><br><span class="line"> <span class="keyword">new</span> DiskLruCacheFactory(<span class="keyword">new</span> DiskLruCacheFactory.CacheDirectoryGetter() {</span><br><span class="line"> <span class="meta">@Override</span></span><br><span class="line"> <span class="function"><span class="keyword">public</span> File <span class="title">getCacheDirectory</span><span class="params">()</span> </span>{</span><br><span class="line"> <span class="keyword">return</span> getMyCacheLocationBlockingIO();</span><br><span class="line"> }</span><br><span class="line"> }), <span class="number">100</span> * <span class="number">1024</span> * <span class="number">1024</span>);</span><br></pre></td></tr></table></figure>
<blockquote>
<p>Note: getMyCacheLocationBlockingIO 方法返回的文件不能为空，而且必须是一个已经创建好的文件目录，不可以是文件。</p>
</blockquote>
<p><strong>缓存策略</strong><br>与其他图片加载库的缓存机制不同，Glide 缓存图片时默认只缓存最终加载的那张图片。举个栗子，你要加载的图片分辨率为 1000x1000，但是最终显示该图片的 ImageView 大小只有 500x500，那么 Glide 就会只缓存 500x500 的小图。这也是在从磁盘缓存中加载图片时 Glide 比 Picasso 快的原因。Glide 目前提供了四种缓存策略：</p>
<ol>
<li>DiskCacheStrategy.NONE 不缓存文件</li>
<li>DiskCacheStrategy.SOURCE 只缓存原图</li>
<li>DiskCacheStrategy.RESULT 只缓存最终加载的图（默认的缓存策略）</li>
<li>DiskCacheStrategy.ALL 同时缓存原图和结果图</li>
</ol>
<p><strong>缓存算法</strong><br>在 Glide 中磁盘缓存默认使用的是 LRU（Least Recently Used）算法。如果你想使用其他的缓存算法，就只能通过实现 DiskCache 接口来完成了。</p>
<h4 id="内存缓存"><a href="#内存缓存" class="headerlink" title="内存缓存"></a>内存缓存</h4><p>使用内存缓存可以获得更快的图片加载速度，因为减少了耗时的 IO 操作。众所周知，Bitmap 是 Android 中的内存大户，频繁的创建和回收 Bitmap 必然会引起内存抖动。Glide 中有一个叫做 BitmapPool 的类，可以复用其中的 Bitmap 对象，从而避免 Bitmap 对象的创建，减小内存开销。当配置内存缓存时，我们也应该同时配置 BitmapPool 的大小。具体方法也是通过自定义的 GlideModule 来实现的：</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br></pre></td><td class="code"><pre><span class="line">builder.setMemoryCache(<span class="keyword">new</span> LruResourceCache(yourSizeInBytes));</span><br><span class="line">builder.setBitmapPool(<span class="keyword">new</span> LruBitmapPool(sizeInBytes));</span><br></pre></td></tr></table></figure>
<p>一般情况下，开发者是不需要自己去指定它们的大小的，因为 Glide 已经帮我们做好了。默认的内存缓存和 bitmapPool 的大小由<a target="_blank" rel="noopener" href="https://github.com/bumptech/glide/blob/master/library/src/main/java/com/bumptech/glide/load/engine/cache/MemorySizeCalculator.java">MemorySizeCalculator</a>根据当前设备的屏幕大小和可用内存计算得到。同时 Glide 还支持动态的缓存大小调整，在存在大量图片的 Activity/Fragment 中，开发者可以通过 setMemoryCategory 方法来提高 Glide 的内存缓存大小，从而加快图片的加载速度。</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line">Glide.get(context).setMemoryCategory(MemoryCategory.HIGH);</span><br></pre></td></tr></table></figure>
<p>MemoryCategory 有 3 个值可供选择：</p>
<ol>
<li>MemoryCategory.HIGH（初始缓存大小的 1.5 倍）</li>
<li>MemoryCategory.NORMAL（初始缓存大小的 1 倍）</li>
<li>MemoryCategory.LOW（初始缓存大小的 0.5 倍）</li>
</ol>
<p>在有些情况下我们不希望做内存缓存（比如加载 GIF 图片），这个时候可以调用 skipMemoryCache(true)方法跳过内存缓存。</p>
<h4 id="如何缓存动态-Url-的图片"><a href="#如何缓存动态-Url-的图片" class="headerlink" title="如何缓存动态 Url 的图片"></a>如何缓存动态 Url 的图片</h4><p>一般情况下我们从网络上获取到的图片 Url 都是静态的，即一张图片对应一个 Url。那么如果是一张图片对应多个 Url 呢？缓存不就没有意义了。因为图片加载库都是拿图片的 Url 来作为缓存的 key 的，Glide 也不例外，只是会更加复杂一些。如果你开启了 Glide 的 log,就会在控制台看到 Glide 是如何指定缓存 key 的。关于如何打开 log,请参考<a target="_blank" rel="noopener" href="http://www.jianshu.com/p/9bd6efca8724">这篇文章</a>。一般来说，Glide 的 key 由图片的 url、view 的宽和高、屏幕的尺寸大小和 signature 组成。</p>
<p>在什么情况下才会出现动态的 Url 呢？一个很典型的例子就是因为图片的安全问题在原来图片的 Url 后面加上访问凭证。访问凭证与时间关联，这样一来，在不同时间同一图片的 Url 就会不同，缓存就会失效。以七牛的私有空间为例，我们来看看如何去缓存这类图片。从七牛关于私有空间的<a target="_blank" rel="noopener" href="https://developer.qiniu.com/kodo/manual/download-token">文档</a>中可以得到：最终的 Url = 原 Url + ?e=过期时间 + token=下载凭证。那么就只需要在 Glide 缓存时将 Url 中“?”后面的字符串截去就可以了。</p>
<p>首先新建一个叫做 QiNiuImage 的类：</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">public</span> <span class="class"><span class="keyword">class</span> <span class="title">QiNiuImage</span> </span>{</span><br><span class="line"></span><br><span class="line"> <span class="keyword">private</span> <span class="keyword">final</span> String imageUrl;</span><br><span class="line"></span><br><span class="line"> <span class="function"><span class="keyword">public</span> <span class="title">QiNiuImage</span><span class="params">(String imageUrl)</span> </span>{</span><br><span class="line"> <span class="keyword">this</span>.imageUrl = imageUrl;</span><br><span class="line"> }</span><br><span class="line"></span><br><span class="line"> <span class="function"><span class="keyword">public</span> String <span class="title">getImageUrl</span><span class="params">()</span> </span>{</span><br><span class="line"> <span class="keyword">return</span> imageUrl;</span><br><span class="line"> }</span><br><span class="line"></span><br><span class="line"> <span class="function"><span class="keyword">public</span> String <span class="title">getImageId</span><span class="params">()</span> </span>{</span><br><span class="line"> <span class="keyword">if</span> (imageUrl.contains(<span class="string">"?"</span>)) {</span><br><span class="line"> <span class="keyword">return</span> imageUrl.substring(<span class="number">0</span>, imageUrl.lastIndexOf(<span class="string">"?"</span>));</span><br><span class="line"> } <span class="keyword">else</span> {</span><br><span class="line"> <span class="keyword">return</span> imageUrl;</span><br><span class="line"> }</span><br><span class="line"> }</span><br><span class="line">}</span><br></pre></td></tr></table></figure>
<p>其中 getImageUrl 方法返回真实的 Url，getImageId 方法返回未添加下载凭证前的 Url。<br>然后再自定义一个实现<a target="_blank" rel="noopener" href="https://github.com/bumptech/glide/blob/master/library/src/main/java/com/bumptech/glide/load/model/ModelLoader.java">ModelLoader</a>接口的 QiNiuImageLoader:</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br><span class="line">22</span><br><span class="line">23</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">public</span> <span class="class"><span class="keyword">class</span> <span class="title">QiNiuImageLoader</span> <span class="keyword">implements</span> <span class="title">StreamModelLoader</span><<span class="title">QiNiuImage</span>> </span>{</span><br><span class="line"> <span class="meta">@Override</span></span><br><span class="line"> <span class="function"><span class="keyword">public</span> DataFetcher<InputStream> <span class="title">getResourceFetcher</span><span class="params">(<span class="keyword">final</span> QiNiuImage model, <span class="keyword">int</span> width,</span></span></span><br><span class="line"><span class="function"><span class="params"> <span class="keyword">int</span> height)</span> </span>{</span><br><span class="line"> <span class="keyword">return</span> <span class="keyword">new</span> HttpUrlFetcher(<span class="keyword">new</span> GlideUrl(model.getImageUrl())) {</span><br><span class="line"> <span class="meta">@Override</span></span><br><span class="line"> <span class="function"><span class="keyword">public</span> String <span class="title">getId</span><span class="params">()</span> </span>{</span><br><span class="line"> <span class="keyword">return</span> model.getImageId();</span><br><span class="line"> }</span><br><span class="line"> };</span><br><span class="line"> }</span><br><span class="line"></span><br><span class="line"> <span class="keyword">public</span> <span class="keyword">static</span> <span class="class"><span class="keyword">class</span> <span class="title">Factory</span> <span class="keyword">implements</span> <span class="title">ModelLoaderFactory</span><<span class="title">QiNiuImage</span>, <span class="title">InputStream</span>> </span>{</span><br><span class="line"> <span class="meta">@Override</span></span><br><span class="line"> <span class="function"><span class="keyword">public</span> ModelLoader<QiNiuImage, InputStream> <span class="title">build</span><span class="params">(Context context,</span></span></span><br><span class="line"><span class="function"><span class="params"> GenericLoaderFactory factories)</span> </span>{</span><br><span class="line"> <span class="keyword">return</span> <span class="keyword">new</span> QiNiuImageLoader();</span><br><span class="line"> }</span><br><span class="line"></span><br><span class="line"> <span class="meta">@Override</span></span><br><span class="line"> <span class="function"><span class="keyword">public</span> <span class="keyword">void</span> <span class="title">teardown</span><span class="params">()</span> </span>{ <span class="comment">/* no op */</span> }</span><br><span class="line"> }</span><br><span class="line">}</span><br></pre></td></tr></table></figure>
<p>其中 HttpUrlFetcher 的 getId 方法就是组成缓存的 key 的重要部分。这也是我们的核心原理。<br>将这个 ModelLoader 注册到 GlideModule 中，并在 AndroidManifest.xml 中注册:</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">public</span> <span class="class"><span class="keyword">class</span> <span class="title">QiNiuModule</span> <span class="keyword">implements</span> <span class="title">GlideModule</span> </span>{</span><br><span class="line"> <span class="meta">@Override</span></span><br><span class="line"> <span class="function"><span class="keyword">public</span> <span class="keyword">void</span> <span class="title">applyOptions</span><span class="params">(Context context, GlideBuilder builder)</span> </span>{</span><br><span class="line"></span><br><span class="line"> }</span><br><span class="line"></span><br><span class="line"> <span class="meta">@Override</span></span><br><span class="line"> <span class="function"><span class="keyword">public</span> <span class="keyword">void</span> <span class="title">registerComponents</span><span class="params">(Context context, Glide glide)</span> </span>{</span><br><span class="line"> glide.register(QiNiuImage.class, InputStream.class, <span class="keyword">new</span> QiNiuImageLoader.Factory());</span><br><span class="line"> }</span><br><span class="line">}</span><br></pre></td></tr></table></figure>
<figure class="highlight xml"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br></pre></td><td class="code"><pre><span class="line"><meta-data</span><br><span class="line"> android:name="com.yourpackagename.QiNiuModule"</span><br><span class="line"> android:value="GlideModule"/></span><br></pre></td></tr></table></figure>
<p>最后只需要在加载此类图片时，使用下面这段代码就可以了。即使图片的 token 更换了也不会重新从网络上下载而是直接读取本地缓存。</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br></pre></td><td class="code"><pre><span class="line">Glide.with(context)</span><br><span class="line"> .load(<span class="keyword">new</span> QiNiuImage(imageUrl)</span><br><span class="line"> .into(imageView);</span><br></pre></td></tr></table></figure>
<p>参考资料：</p>
<ol>
<li><a target="_blank" rel="noopener" href="https://github.com/bumptech/glide/issues/607">https://github.com/bumptech/glide/issues/607</a></li>
<li><a target="_blank" rel="noopener" href="https://github.com/bumptech/glide/issues/501">https://github.com/bumptech/glide/issues/501</a></li>
</ol>
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<article id="post-Several-important-classes-in-Android-media-framework" class="article article-type-post article-index" itemscope itemprop="blogPost">
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<a class="article-title" href="/2016/09/24/Several-important-classes-in-Android-media-framework/">Android多媒体框架中几个重要的类</a>
</h1>
<a href="/2016/09/24/Several-important-classes-in-Android-media-framework/" class="archive-article-date">
<time datetime="2016-09-24T02:10:09.000Z" itemprop="datePublished"><i class="icon-calendar icon"></i>2016-09-24</time>
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<p>与 iOS 中强大的 AVFoundation 框架相比，Android framework 中提供的有关多媒体处理的类可谓屈指可数，但总比没有好吧。今天我们就来谈谈这几个类。</p>
<blockquote>
<p>这里提到的多媒体处理主要是指音视频处理。包括音视频的裁剪、合并；视频画面的各种变换，旋转、缩放、翻转；视频滤镜；音视频的播放，快速、慢速、倒序播放等等。</p>
</blockquote>
<p>主要涉及到的类有：</p>
<ol>
<li>MediaExtractor：媒体提取器</li>
<li>MediaCodec：编解码器</li>
<li>MediaMuxer：媒体混合器</li>
<li>MediaMetadataRetriever： 获取音视频信息的类</li>
<li>MediaFormat: 包含音视频帧信息的类</li>
<li>GLSurfaceView: 展示 openGL 渲染的 View</li>
<li>OpenGL 相关的类</li>
</ol>
<p>MediaExtractor 在 Android4.1（API16）加入。可以从一段音视频中提取出一帧一帧的数据，与 MediaMuxer 配合使用可以完成视频的裁剪和合并，与 MediaCodec、GLSurfaceView 配合使用可以完成视频的播放。</p>
<p>MediaCodec 在 Android4.1（API16）加入。在 Android4.3(API18)提供输入可以为 Surface。在 Android5.0（API21）又增加了异步处理模式。它是一个低等级的媒体编解码器，可以作为编码器，也可以作为解码器。 可攻可受，嘿嘿嘿。是音视频处理中最为核心的类。</p>
<p>由于相关文档在以前不是很完善，来自 Android 媒体团队的 fadden(现已不在)维护了一个网站 <a target="_blank" rel="noopener" href="http://bigflake.com/mediacodec/">http://bigflake.com/mediacodec/</a> 。上面有大量的相关资源。此外在 stackoverflow 上面只要是 MeidaCodec 相关的问题随处可见 fadden 的身影。感谢 fadden。现在官方文档已经相当详细了，不过都是英文的，对于阅读困难的人，国内也有人进行了翻译。地址在[这里]（<a target="_blank" rel="noopener" href="http://www.cnblogs.com/xiaoshubao/archive/2016/04/11/5368183.html%EF%BC%89">http://www.cnblogs.com/xiaoshubao/archive/2016/04/11/5368183.html）</a> 。</p>
<p>MediaMuxer 在 Android4.3(API18)。可以合成 MP4 格式的视频，输入源通常为从 MediaExtractor 或者 MediaCodec 提供的已编码的数据。</p>
<p>MediaMetadataRetriever 主要用来获取视频的方向信息。在合成视频时，可纠正视频方向。此外还可以获取视频某一帧画面的 bitmap,前提必须是 android 支持的视频格式。</p>
<p>MediaFormat 内部持有一个包含音视频帧信息的 map。</p>
<p>GLSurfaceView 可与 MediaPlayer 配合，完成视频变换的各种效果实时预览。</p>
<p>OpenGL 是一门单独的技术，然而关于其在 Android 中使用的文档和列子实在太少。唯一的一本书《OpenGL ES 应用开发实践指南 Android 卷》在网上也买不到，只能去淘宝买复印版。。。</p>
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<a class="article-title" href="/2015/12/01/How-to-debug-glide/">如何调试Glide加载图片</a>
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<h4 id="前言"><a href="#前言" class="headerlink" title="前言"></a>前言</h4><p>与其他图片加载库不同，在 Glide 加载图片的过程中默认是没有任何 log 输出的。这样使得加载失败的原因难以调试。到底是网络错误还是图片根本就不存在亦或者解码出错，我们不得而知。当然官方也给出了调试的方法，这篇文章就来介绍下如何调试 Glide 加载图片，内容主要是对官方 wiki 的翻译。</p>
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<a class="article-title" href="/2015/11/19/An-accident-about-SwipeRefreshLayout-in-support-v4-package/">记录一次v4包中SwipeRefreshLayout的“坑爹”事件</a>
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<a href="/2015/11/19/An-accident-about-SwipeRefreshLayout-in-support-v4-package/" class="archive-article-date">
<time datetime="2015-11-19T05:29:16.000Z" itemprop="datePublished"><i class="icon-calendar icon"></i>2015-11-19</time>
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<p>当 android support v4 包和 v7 包版本不一样时，v4 包中的 SwipeRefreshLayout 和 v7 包中的 RecyclerView 不能很好地一起工作，会导致 SwipeRefreshLayout 下拉刷新时动画卡住的情况，类似下面的情况：</p>
<p><img src="http://ww1.sinaimg.cn/large/006tNc79ly1g425li2eiwg30dc0a0u0x.gif" alt="sample"></p>
<p>详情请参考:</p>
<p><a target="_blank" rel="noopener" href="http://stackoverflow.com/questions/33032036/swiperefreshlayout-freezes-on-api-4-2-2%E3%80%82">http://stackoverflow.com/questions/33032036/swiperefreshlayout-freezes-on-api-4-2-2。</a></p>
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