图片压缩和缓存高效加载避免oom

最新推荐文章于 2022-01-23 11:23:03 发布

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最新推荐文章于 2022-01-23 11:23:03 发布

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本文介绍了一种通过图片压缩和缓存技术来避免Android应用中出现OOM问题的方法。通过使用BitmapFactory.Options设置inSampleSize来压缩图片，以及利用LruCache进行图片缓存，可以有效地减少内存消耗。

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图片压缩和缓存高效加载规避oom总结

标签： oom

2015-04-15 18:42 1985人阅读评论(0) 收藏举报

 
  分类： 
 
  技术精华（13）

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高效加载大图片

查看每个应用程序最高可用内存是多少：

<code class="hljs avrasm has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;">int maxMemory = (int) (Runtime<span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">.getRuntime</span>()<span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">.maxMemory</span>() / <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">1024</span>)<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">;  </span>
Log<span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">.d</span>(<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"TAG"</span>, <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"Max memory is "</span> + maxMemory + <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"KB"</span>)<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">;  </span></code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li></ul><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li></ul>

BitmapFactory这个类提供了多个解析方法(decodeByteArray, decodeFile, decodeResource等)用于创建Bitmap对象，我们应该根据图片的来源选择合适的方法。比如SD卡中的图片可以使用decodeFile方法，网络上的图片可以使用decodeStream方法，资源文件中的图片可以使用decodeResource方法。
这些方法会尝试为已经构建的bitmap分配内存，这时就会很容易导致OOM出现。为此每一种解析方法都提供了一个可选的BitmapFactory.Options参数，将这个参数的inJustDecodeBounds属性设置为true就可以让解析方法禁止为bitmap分配内存，返回值也不再是一个Bitmap对象，而是null。虽然Bitmap是null了，但是BitmapFactory.Options的outWidth、outHeight和outMimeType属性都会被赋值。这个技巧让我们可以在加载图片之前就获取到图片的长宽值和MIME类型，从而根据情况对图片进行压缩。代码如下：

<code class="hljs avrasm has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;">BitmapFactory<span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">.Options</span> options = new BitmapFactory<span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">.Options</span>()<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">;  </span>
options<span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">.inJustDecodeBounds</span> = true<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">;  </span>
BitmapFactory<span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">.decodeResource</span>(getResources(), R<span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">.id</span><span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">.myimage</span>, options)<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">;  </span>
int imageHeight = options<span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">.outHeight</span><span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">;  </span>
int imageWidth = options<span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">.outWidth</span><span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">;  </span>
String imageType = options<span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">.outMimeType</span><span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">;  </span></code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li></ul><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li></ul>

设置BitmapFactory.Options中inSampleSize的值就可以实现图片压缩，比如我们有一张2048*1536像素的图片，将inSampleSize的值设置为4，就可以把这张图片压缩成512*384像素。原本加载这张图片需要占用12M的内存，压缩后就只需要占用0.75M了(假设图片是ARGB_8888类型，即每个像素点占用4个字节)。下面的方法可以根据传入的宽和高，计算出合适的inSampleSize值：

<code class="hljs java has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">public</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">static</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> <span class="hljs-title" style="box-sizing: border-box;">calculateInSampleSize</span>(BitmapFactory.Options options,  
        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> reqWidth, <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> reqHeight) {  
    <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// 源图片的高度和宽度  </span>
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">final</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> height = options.outHeight;  
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">final</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> width = options.outWidth;  
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> inSampleSize = <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">1</span>;  
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> (height > reqHeight || width > reqWidth) {  
        <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// 计算出实际宽高和目标宽高的比率  </span>
        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">final</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> heightRatio = Math.round((<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">float</span>) height / (<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">float</span>) reqHeight);  
        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">final</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> widthRatio = Math.round((<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">float</span>) width / (<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">float</span>) reqWidth);  
        <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// 选择宽和高中最小的比率作为inSampleSize的值，这样可以保证最终图片的宽和高  </span>
        <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// 一定都会大于等于目标的宽和高。  </span>
        inSampleSize = heightRatio < widthRatio ? heightRatio : widthRatio;  
    }  
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">return</span> inSampleSize;  
}  </code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li><li style="box-sizing: border-box; padding: 0px 5px;">15</li><li style="box-sizing: border-box; padding: 0px 5px;">16</li></ul><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li><li style="box-sizing: border-box; padding: 0px 5px;">15</li><li style="box-sizing: border-box; padding: 0px 5px;">16</li></ul>

使用这个方法，首先你要将BitmapFactory.Options的inJustDecodeBounds属性设置为true，解析一次图片。然后将BitmapFactory.Options连同期望的宽度和高度一起传递到到calculateInSampleSize方法中，就可以得到合适的inSampleSize值了。之后再解析一次图片，使用新获取到的inSampleSize值，并把inJustDecodeBounds设置为false，就可以得到压缩后的图片了。

整体两次解析代码如下：

<code class="hljs java has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">public</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">static</span> Bitmap <span class="hljs-title" style="box-sizing: border-box;">decodeSampledBitmapFromResource</span>(Resources res, <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> resId,  
        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> reqWidth, <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> reqHeight) {  
    <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// 第一次解析将inJustDecodeBounds设置为true，来获取图片大小  </span>
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">final</span> BitmapFactory.Options options = <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">new</span> BitmapFactory.Options();  
    options.inJustDecodeBounds = <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">true</span>;  
    BitmapFactory.decodeResource(res, resId, options);  
    <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// 调用上面定义的方法计算inSampleSize值  </span>
    options.inSampleSize = calculateInSampleSize(options, reqWidth, reqHeight);  
    <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// 使用获取到的inSampleSize值再次解析图片  </span>
    options.inJustDecodeBounds = <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">false</span>;  
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">return</span> BitmapFactory.decodeResource(res, resId, options);  
}  </code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li></ul><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li></ul>

简单的将任意一张图片压缩成100*100的缩略图，并在ImageView上展示的调用代码如下：

<code class="hljs avrasm has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;">mImageView<span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">.setImageBitmap</span>(  
    decodeSampledBitmapFromResource(getResources(), R<span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">.id</span><span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">.myimage</span>, <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">100</span>, <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">100</span>))<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">;  </span></code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li></ul><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li></ul>

以上的方法适合使用在读取一个未知来源的图片时使用，因为你不知道这个未知来源图片的大小，那么还有一种方法是用在已经载入内存的图片，对已经载入内存的图片做压缩以后重新保存到本地，从而可以把一张原本1M大小的图片变成一张10K的图片。
这种方法的核心思想是首先将图片转成一个输出流，并记录输出流的byte数组大小，通过调用bitmap对象的compress方法，对图片做一次压缩以及格式化，并将byte数组大小与期望压缩的目标大小比对，得出压缩比率，并调用Bitmap的缩放方法，缩放计算出的压缩比率，从而得到压缩后的方法。
代码如下：

<code class="hljs java has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-javadoc" style="color: rgb(136, 0, 0); box-sizing: border-box;">/**
     * 图片压缩方法：（使用compress的方法）
     * 
     *<span class="hljs-javadoctag" style="color: rgb(102, 0, 102); box-sizing: border-box;"> @explain</span> 如果bitmap本身的大小小于maxSize，则不作处理
     *<span class="hljs-javadoctag" style="color: rgb(102, 0, 102); box-sizing: border-box;"> @param</span> bitmap
     *            要压缩的图片
     *<span class="hljs-javadoctag" style="color: rgb(102, 0, 102); box-sizing: border-box;"> @param</span> maxSize
     *            压缩后的大小，单位kb
     */</span>
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">public</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">static</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">void</span> <span class="hljs-title" style="box-sizing: border-box;">imageZoom</span>(Bitmap bitmap, <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">double</span> maxSize) {
        <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// 将bitmap放至数组中，意在获得bitmap的大小（与实际读取的原文件要大）</span>
        ByteArrayOutputStream baos = <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">new</span> ByteArrayOutputStream();
        <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// 格式、质量、输出流</span>
        bitmap.compress(Bitmap.CompressFormat.JPEG, <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">100</span>, baos);
        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">byte</span>[] b = baos.toByteArray();
        <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// 将字节换成KB</span>
        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">double</span> mid = b.length / <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">1024</span>;
        <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// 获取bitmap大小 是允许最大大小的多少倍</span>
        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">double</span> i = mid / maxSize;
        <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// 判断bitmap占用空间是否大于允许最大空间 如果大于则压缩 小于则不压缩</span>
        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> (i > <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">1</span>) {
            <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// 缩放图片 此处用到平方根 将宽带和高度压缩掉对应的平方根倍</span>
            <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// （保持宽高不变，缩放后也达到了最大占用空间的大小）</span>
            bitmap = scale(bitmap, bitmap.getWidth() / Math.sqrt(i),
                    bitmap.getHeight() / Math.sqrt(i));
        }
    }
<span class="hljs-javadoc" style="color: rgb(136, 0, 0); box-sizing: border-box;">/***
     * 图片的缩放方法
     * 
     *<span class="hljs-javadoctag" style="color: rgb(102, 0, 102); box-sizing: border-box;"> @param</span> src
     *            ：源图片资源
     *<span class="hljs-javadoctag" style="color: rgb(102, 0, 102); box-sizing: border-box;"> @param</span> newWidth
     *            ：缩放后宽度
     *<span class="hljs-javadoctag" style="color: rgb(102, 0, 102); box-sizing: border-box;"> @param</span> newHeight
     *            ：缩放后高度
     */</span>
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">public</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">static</span> Bitmap <span class="hljs-title" style="box-sizing: border-box;">scale</span>(Bitmap src, <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">double</span> newWidth, <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">double</span> newHeight) {
        <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// 记录src的宽高</span>
        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">float</span> width = src.getWidth();
        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">float</span> height = src.getHeight();
        <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// 创建一个matrix容器</span>
        Matrix matrix = <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">new</span> Matrix();
        <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// 计算缩放比例</span>
        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">float</span> scaleWidth = ((<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">float</span>) newWidth) / width;
        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">float</span> scaleHeight = ((<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">float</span>) newHeight) / height;
        <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// 开始缩放</span>
        matrix.postScale(scaleWidth, scaleHeight);
        <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// 创建缩放后的图片</span>
        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">return</span> Bitmap.createBitmap(src, <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span>, <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span>, (<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span>) width, (<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span>) height,
                matrix, <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">true</span>);
    }</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li><li style="box-sizing: border-box; padding: 0px 5px;">15</li><li style="box-sizing: border-box; padding: 0px 5px;">16</li><li style="box-sizing: border-box; padding: 0px 5px;">17</li><li style="box-sizing: border-box; padding: 0px 5px;">18</li><li style="box-sizing: border-box; padding: 0px 5px;">19</li><li style="box-sizing: border-box; padding: 0px 5px;">20</li><li style="box-sizing: border-box; padding: 0px 5px;">21</li><li style="box-sizing: border-box; padding: 0px 5px;">22</li><li style="box-sizing: border-box; padding: 0px 5px;">23</li><li style="box-sizing: border-box; padding: 0px 5px;">24</li><li style="box-sizing: border-box; padding: 0px 5px;">25</li><li style="box-sizing: border-box; padding: 0px 5px;">26</li><li style="box-sizing: border-box; padding: 0px 5px;">27</li><li style="box-sizing: border-box; padding: 0px 5px;">28</li><li style="box-sizing: border-box; padding: 0px 5px;">29</li><li style="box-sizing: border-box; padding: 0px 5px;">30</li><li style="box-sizing: border-box; padding: 0px 5px;">31</li><li style="box-sizing: border-box; padding: 0px 5px;">32</li><li style="box-sizing: border-box; padding: 0px 5px;">33</li><li style="box-sizing: border-box; padding: 0px 5px;">34</li><li style="box-sizing: border-box; padding: 0px 5px;">35</li><li style="box-sizing: border-box; padding: 0px 5px;">36</li><li style="box-sizing: border-box; padding: 0px 5px;">37</li><li style="box-sizing: border-box; padding: 0px 5px;">38</li><li style="box-sizing: border-box; padding: 0px 5px;">39</li><li style="box-sizing: border-box; padding: 0px 5px;">40</li><li style="box-sizing: border-box; padding: 0px 5px;">41</li><li style="box-sizing: border-box; padding: 0px 5px;">42</li><li style="box-sizing: border-box; padding: 0px 5px;">43</li><li style="box-sizing: border-box; padding: 0px 5px;">44</li><li style="box-sizing: border-box; padding: 0px 5px;">45</li><li style="box-sizing: border-box; padding: 0px 5px;">46</li><li style="box-sizing: border-box; padding: 0px 5px;">47</li><li style="box-sizing: border-box; padding: 0px 5px;">48</li><li style="box-sizing: border-box; padding: 0px 5px;">49</li><li style="box-sizing: border-box; padding: 0px 5px;">50</li><li style="box-sizing: border-box; padding: 0px 5px;">51</li><li style="box-sizing: border-box; padding: 0px 5px;">52</li></ul><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li><li style="box-sizing: border-box; padding: 0px 5px;">15</li><li style="box-sizing: border-box; padding: 0px 5px;">16</li><li style="box-sizing: border-box; padding: 0px 5px;">17</li><li style="box-sizing: border-box; padding: 0px 5px;">18</li><li style="box-sizing: border-box; padding: 0px 5px;">19</li><li style="box-sizing: border-box; padding: 0px 5px;">20</li><li style="box-sizing: border-box; padding: 0px 5px;">21</li><li style="box-sizing: border-box; padding: 0px 5px;">22</li><li style="box-sizing: border-box; padding: 0px 5px;">23</li><li style="box-sizing: border-box; padding: 0px 5px;">24</li><li style="box-sizing: border-box; padding: 0px 5px;">25</li><li style="box-sizing: border-box; padding: 0px 5px;">26</li><li style="box-sizing: border-box; padding: 0px 5px;">27</li><li style="box-sizing: border-box; padding: 0px 5px;">28</li><li style="box-sizing: border-box; padding: 0px 5px;">29</li><li style="box-sizing: border-box; padding: 0px 5px;">30</li><li style="box-sizing: border-box; padding: 0px 5px;">31</li><li style="box-sizing: border-box; padding: 0px 5px;">32</li><li style="box-sizing: border-box; padding: 0px 5px;">33</li><li style="box-sizing: border-box; padding: 0px 5px;">34</li><li style="box-sizing: border-box; padding: 0px 5px;">35</li><li style="box-sizing: border-box; padding: 0px 5px;">36</li><li style="box-sizing: border-box; padding: 0px 5px;">37</li><li style="box-sizing: border-box; padding: 0px 5px;">38</li><li style="box-sizing: border-box; padding: 0px 5px;">39</li><li style="box-sizing: border-box; padding: 0px 5px;">40</li><li style="box-sizing: border-box; padding: 0px 5px;">41</li><li style="box-sizing: border-box; padding: 0px 5px;">42</li><li style="box-sizing: border-box; padding: 0px 5px;">43</li><li style="box-sizing: border-box; padding: 0px 5px;">44</li><li style="box-sizing: border-box; padding: 0px 5px;">45</li><li style="box-sizing: border-box; padding: 0px 5px;">46</li><li style="box-sizing: border-box; padding: 0px 5px;">47</li><li style="box-sizing: border-box; padding: 0px 5px;">48</li><li style="box-sizing: border-box; padding: 0px 5px;">49</li><li style="box-sizing: border-box; padding: 0px 5px;">50</li><li style="box-sizing: border-box; padding: 0px 5px;">51</li><li style="box-sizing: border-box; padding: 0px 5px;">52</li></ul>

压缩以后就可高效加载bitmap了。这种方法可以有效避免大图造成的oom，但是当你需要在界面上加载一大堆图片的时候，在很多情况下，（比如使用ListView, GridView 或者 ViewPager 这样的组件），屏幕上显示的图片可以通过滑动屏幕等事件不断地增加，最终也会导致OOM。
这就需要缓存技术避免oom了

图片缓存

为了保证内存的使用始终维持在一个合理的范围，通常会把被移除屏幕的图片进行回收处理。此时垃圾回收器也会认为你不再持有这些图片的引用，从而对这些图片进行GC操作。用这种思路来解决问题是非常好的，可是为了能让程序快速运行，在界面上迅速地加载图片，你又必须要考虑到某些图片被回收之后，用户又将它重新滑入屏幕这种情况。这时重新去加载一遍刚刚加载过的图片无疑是性能的瓶颈，你需要想办法去避免这个情况的发生。
这个时候，使用内存缓存技术可以很好的解决这个问题，它可以让组件快速地重新加载和处理图片。下面我们就来看一看如何使用内存缓存技术来对图片进行缓存，从而让你的应用程序在加载很多图片的时候可以提高响应速度和流畅性。

其中最核心的类是LruCache (此类在Android-support-v4的包中提供) 。这个类非常适合用来缓存图片，它的主要算法原理是把最近使用的对象用强引用存储在 LinkedHashMap 中，并且把最近最少使用的对象在缓存值达到预设定值之前从内存中移除。
从 Android 2.3 (API Level 9)开始，垃圾回收器会更倾向于回收持有软引用或弱引用的对象，这让软引用和弱引用变得不再可靠。
另外，Android 3.0 (API Level 11)中，图片的数据会存储在本地的内存当中，因而无法用一种可预见的方式将其释放，这就有潜在的风险造成应用程序的内存溢出并崩溃。
下面是一个使用 LruCache 来缓存图片的例子：

<code class="hljs java has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">private</span> LruCache<String, Bitmap> mMemoryCache;  

<span class="hljs-annotation" style="color: rgb(155, 133, 157); box-sizing: border-box;">@Override</span>  
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">protected</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">void</span> <span class="hljs-title" style="box-sizing: border-box;">onCreate</span>(Bundle savedInstanceState) {  
    <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// 获取到可用内存的最大值，使用内存超出这个值会引起OutOfMemory异常。  </span>
    <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// LruCache通过构造函数传入缓存值，以KB为单位。  </span>
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> maxMemory = (<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span>) (Runtime.getRuntime().maxMemory() / <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">1024</span>);  
    <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// 使用最大可用内存值的1/8作为缓存的大小。  </span>
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> cacheSize = maxMemory / <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">8</span>;  
    mMemoryCache = <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">new</span> LruCache<String, Bitmap>(cacheSize) {  
        <span class="hljs-annotation" style="color: rgb(155, 133, 157); box-sizing: border-box;">@Override</span>  
        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">protected</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> <span class="hljs-title" style="box-sizing: border-box;">sizeOf</span>(String key, Bitmap bitmap) {  
            <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// 重写此方法来衡量每张图片的大小，默认返回图片数量。  </span>
            <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">return</span> bitmap.getByteCount() / <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">1024</span>;  
        }  
    };  
}  

<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">public</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">void</span> <span class="hljs-title" style="box-sizing: border-box;">addBitmapToMemoryCache</span>(String key, Bitmap bitmap) {  
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> (getBitmapFromMemCache(key) == <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">null</span>) {  
        mMemoryCache.put(key, bitmap);  
    }  
}  

<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">public</span> Bitmap <span class="hljs-title" style="box-sizing: border-box;">getBitmapFromMemCache</span>(String key) {  
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">return</span> mMemoryCache.get(key);  
}  </code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li><li style="box-sizing: border-box; padding: 0px 5px;">15</li><li style="box-sizing: border-box; padding: 0px 5px;">16</li><li style="box-sizing: border-box; padding: 0px 5px;">17</li><li style="box-sizing: border-box; padding: 0px 5px;">18</li><li style="box-sizing: border-box; padding: 0px 5px;">19</li><li style="box-sizing: border-box; padding: 0px 5px;">20</li><li style="box-sizing: border-box; padding: 0px 5px;">21</li><li style="box-sizing: border-box; padding: 0px 5px;">22</li><li style="box-sizing: border-box; padding: 0px 5px;">23</li><li style="box-sizing: border-box; padding: 0px 5px;">24</li><li style="box-sizing: border-box; padding: 0px 5px;">25</li><li style="box-sizing: border-box; padding: 0px 5px;">26</li><li style="box-sizing: border-box; padding: 0px 5px;">27</li></ul><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li><li style="box-sizing: border-box; padding: 0px 5px;">15</li><li style="box-sizing: border-box; padding: 0px 5px;">16</li><li style="box-sizing: border-box; padding: 0px 5px;">17</li><li style="box-sizing: border-box; padding: 0px 5px;">18</li><li style="box-sizing: border-box; padding: 0px 5px;">19</li><li style="box-sizing: border-box; padding: 0px 5px;">20</li><li style="box-sizing: border-box; padding: 0px 5px;">21</li><li style="box-sizing: border-box; padding: 0px 5px;">22</li><li style="box-sizing: border-box; padding: 0px 5px;">23</li><li style="box-sizing: border-box; padding: 0px 5px;">24</li><li style="box-sizing: border-box; padding: 0px 5px;">25</li><li style="box-sizing: border-box; padding: 0px 5px;">26</li><li style="box-sizing: border-box; padding: 0px 5px;">27</li></ul>

当向 ImageView 中加载一张图片时,首先会在 LruCache 的缓存中进行检查。如果找到了相应的键值，则会立刻更新ImageView ，否则开启一个后台线程来加载这张图片。

<code class="hljs java has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">public</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">void</span> <span class="hljs-title" style="box-sizing: border-box;">loadBitmap</span>(<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> resId, ImageView imageView) {  
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">final</span> String imageKey = String.valueOf(resId);  
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">final</span> Bitmap bitmap = getBitmapFromMemCache(imageKey);  
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> (bitmap != <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">null</span>) {  
        imageView.setImageBitmap(bitmap);  
    } <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">else</span> {  
        imageView.setImageResource(R.drawable.image_placeholder);  
        BitmapWorkerTask task = <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">new</span> BitmapWorkerTask(imageView);  
        task.execute(resId);  
    }  
}  </code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li></ul><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li></ul>

BitmapWorkerTask 还要把新加载的图片的键值对放到缓存中。

<code class="hljs scala has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-class" style="box-sizing: border-box;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">class</span> <span class="hljs-title" style="box-sizing: border-box; color: rgb(102, 0, 102);">BitmapWorkerTask</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">extends</span> <span class="hljs-title" style="box-sizing: border-box; color: rgb(102, 0, 102);">AsyncTask</span><<span class="hljs-title" style="box-sizing: border-box; color: rgb(102, 0, 102);">Integer</span>, <span class="hljs-title" style="box-sizing: border-box; color: rgb(102, 0, 102);">Void</span>, <span class="hljs-title" style="box-sizing: border-box; color: rgb(102, 0, 102);">Bitmap</span>> {</span>  
    <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// 在后台加载图片。  </span>
    <span class="hljs-annotation" style="color: rgb(155, 133, 157); box-sizing: border-box;">@Override</span>  
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">protected</span> Bitmap doInBackground(Integer... params) {  
        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">final</span> Bitmap bitmap = decodeSampledBitmapFromResource(  
                getResources(), params[<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span>], <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">100</span>, <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">100</span>);  
        addBitmapToMemoryCache(String.valueOf(params[<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span>]), bitmap);  
        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">return</span> bitmap;  
    }  
}  </code>