Android加载长图那些事

    
    
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      **BitmapRegionDecoder主要用于显示图片的某一块矩形区域，如果你需要显示某个图片的指定区域，或者你需要显示一张非常大的图片，那么这个类非常合适。我们可以调用newInstance(…)方法来获得一个BitmapRegionDecoder对象，然后调用decodeRegion()方法就可以获得指定位置的图片了。**
 接下来看一个简单的例子
    

    
    ```
`<span class="hljs-keyword">package</span> oracleen.decoder;

<span class="hljs-keyword">import</span> android.app.Activity;
<span class="hljs-keyword">import</span> android.graphics.Bitmap;
<span class="hljs-keyword">import</span> android.graphics.BitmapFactory;
<span class="hljs-keyword">import</span> android.graphics.BitmapRegionDecoder;
<span class="hljs-keyword">import</span> android.graphics.Rect;
<span class="hljs-keyword">import</span> android.os.Bundle;
<span class="hljs-keyword">import</span> android.widget.ImageView;

<span class="hljs-keyword">import</span> java.io.IOException;
<span class="hljs-keyword">import</span> java.io.InputStream;

<span class="hljs-keyword">public</span> <span class="hljs-class"><span class="hljs-keyword">class</span> <span class="hljs-title">MainActivity</span> <span class="hljs-keyword">extends</span> <span class="hljs-title">Activity</span> {</span>

    <span class="hljs-keyword">private</span> ImageView decoder;

    <span class="hljs-annotation">@Override</span>
    <span class="hljs-keyword">protected</span> <span class="hljs-keyword">void</span> <span class="hljs-title">onCreate</span>(Bundle savedInstanceState) {
        <span class="hljs-keyword">super</span>.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        decoder = (ImageView) findViewById(R.id.decoder);

        <span class="hljs-keyword">try</span> {
            InputStream inputStreamF = getAssets().open(<span class="hljs-string">"datu.png"</span>);

            <span class="hljs-comment">//获得图片的宽、高</span>
            BitmapFactory.Options tmpOptions = <span class="hljs-keyword">new</span> BitmapFactory.Options();
            tmpOptions.inJustDecodeBounds = <span class="hljs-keyword">true</span>;
            BitmapFactory.decodeStream(inputStreamF, <span class="hljs-keyword">null</span>, tmpOptions);
            <span class="hljs-keyword">int</span> width = tmpOptions.outWidth;
            <span class="hljs-keyword">int</span> height = tmpOptions.outHeight;

            InputStream inputStreamS = getAssets().open(<span class="hljs-string">"datu.png"</span>);
            <span class="hljs-comment">//设置显示图片的中心区域</span>
            BitmapRegionDecoder bitmapRegionDecoder = BitmapRegionDecoder.newInstance(inputStreamS, <span class="hljs-keyword">false</span>);
            BitmapFactory.Options options = <span class="hljs-keyword">new</span> BitmapFactory.Options();
            options.inPreferredConfig = Bitmap.Config.RGB_565;
            Bitmap bitmap = bitmapRegionDecoder.decodeRegion(<span class="hljs-keyword">new</span> Rect(<span class="hljs-number">0</span>, <span class="hljs-number">0</span>, width / <span class="hljs-number">2</span>, height), options);
            decoder.setImageBitmap(bitmap);

        } <span class="hljs-keyword">catch</span> (IOException e) {
            e.printStackTrace();
        }
    }

}
`

    
    
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      这个异常，这里需要重新初始化一下我们的输入流，保证它是初始的可读状态。
    

    
    

      ![这里写图片描述](http://img.blog.csdn.net/20160318151945160)
    

    
    ### <a name="t2"></a>小B的总结
    
    

      对于小B的问题有两种解决办法：
 1.把原始的长图切成5张小图，分别设置给5个ImageView，这样每个imageview加载的图都不会超出阈值。
 但是问题是整张图会全部加载出来，可能会出现OOM。
 2.重写HorizontalScrollView，根据左右滑动的距离用BitmapRegionDecoder去分区域显示。
 3.自定义view，在view内部用BitmapRegionDecoder去分区域显示。然后对外提供一个设置图片资源的方法。
    

    
    

      参考链接：
 [http://blog.csdn.net/lmj623565791/article/details/49300989](http://blog.csdn.net/lmj623565791/article/details/49300989)
    

    
    

      [http://developer.android.com/reference/android/graphics/BitmapRegionDecoder.html#newInstance%28java.io.InputStream,%20boolean%29](http://developer.android.com/reference/android/graphics/BitmapRegionDecoder.html#newInstance%28java.io.InputStream,%20boolean%29)
    

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