Bitmap的mutable属性引发的血案

　　最近做的一个项目，需要获取图片中每个像素点的信息，然后再进行一定顺序的重新排列。一开始，将需要处理的图片放到了Drawable中，通过getPixels()方法获取了整张图片的信息，然后处理完之后直接通过setPixels()就得到了新的图片。很好，然后抽风将图片放到了sdcard中，用同样的方法处理后报异常：java.lang.IllegalStateException……

　　最后发现Bitmap中有一个isMutable的字段，我的理解就是是否可变，也不知道对不对。而通过BitmapFactory.decodeResource(Resources res, int id)获取到的bitmap是mutable的，而BitmapFactory.decodeFile(String path)获取到的是immutable的，因此导致了前面所说的异常。（查看源码时看到走了好多方法，并没有仔细研究为什么两者生成的bitmap一个是mutable，一个是immutable的，如果哪位大虾能说清楚些希望指点一下…）

　　通过查找资料，stackoverflow有位大神给出了如下的方法，将immutable的bitmap转为了mutable的：

/**
 * Converts a immutable bitmap to a mutable bitmap. This operation doesn't allocates
 * more memory that there is already allocated.
 * 
 * @param imgIn - Source image. It will be released, and should not be used more
 * @return a copy of imgIn, but muttable.
 */
public static Bitmap convertToMutable(Bitmap imgIn) {
    try {
        //this is the file going to use temporally to save the bytes. 
        // This file will not be a image, it will store the raw image data.
        File file = new File(Environment.getExternalStorageDirectory() + File.separator + "temp.tmp");

        //Open an RandomAccessFile
        //Make sure you have added uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE"
        //into AndroidManifest.xml file
        RandomAccessFile randomAccessFile = new RandomAccessFile(file, "rw");

        // get the width and height of the source bitmap.
        int width = imgIn.getWidth();
        int height = imgIn.getHeight();
        Config type = imgIn.getConfig();

        //Copy the byte to the file
        //Assume source bitmap loaded using options.inPreferredConfig = Config.ARGB_8888;
        FileChannel channel = randomAccessFile.getChannel();
        MappedByteBuffer map = channel.map(MapMode.READ_WRITE, 0, imgIn.getRowBytes()*height);
        imgIn.copyPixelsToBuffer(map);
        //recycle the source bitmap, this will be no longer used.
        imgIn.recycle();
        System.gc();// try to force the bytes from the imgIn to be released

        //Create a new bitmap to load the bitmap again. Probably the memory will be available. 
        imgIn = Bitmap.createBitmap(width, height, type);
        map.position(0);
        //load it back from temporary 
        imgIn.copyPixelsFromBuffer(map);
        //close the temporary file and channel , then delete that also
        channel.close();
        randomAccessFile.close();

        // delete the temp file
        file.delete();

    } catch (FileNotFoundException e) {
        e.printStackTrace();
    } catch (IOException e) {
        e.printStackTrace();
    } 

    return imgIn;
}

　　用这个方法转一下，解决了异常。后来查找每个生成bitmap的方法，发现只有Bitmap createBitmap(int width, int height, Config config) 和Bitmap createBitmap(int width, int height,Config config, boolean hasAlpha)是Returns a mutable bitmap。（前者就是hasAlpha为true的后者，你懂得），故在进行setPixels前重新通过createBitmap(width, height, Config.ARGB_8888)创建了一个bitmap（他是mutable的），然后再setPixels()，也解决了异常，不过感觉后者的效率能够高些，看第一个方法还创建了个临时文件，感觉可能会很卡，但测试是两个都挺卡……准备做个像ViewPager一样的缓存，这样可能不会那么卡吧……

　　参考资料：

http://stackoverflow.com/questions/4349075/bitmapfactory-decoderesource-returns-a-mutable-bitmap-in-android-2-2-and-an-immu