Bitmap的mutable屬性引發的血案

　　最近做的一個項目，需要獲取圖片中每個像素點的信息，然后再進行一定順序的重新排列。一開始，將需要處理的圖片放到了Drawable中，通過getPixels()方法獲取了整張圖片的信息，然后處理完之后直接通過setPixels()就得到了新的圖片。很好，然后抽風將圖片放到了sdcard中，用同樣的方法處理后報異常：java.lang.IllegalStateException……

　　最后發現Bitmap中有一個isMutable的字段，我的理解就是是否可變，也不知道對不對。而通過BitmapFactory.decodeResource(Resources res, int id)獲取到的bitmap是mutable的，而BitmapFactory.decodeFile(String path)獲取到的是immutable的，因此導致了前面所說的異常。（查看源碼時看到走了好多方法，並沒有仔細研究為什么兩者生成的bitmap一個是mutable，一個是immutable的，如果哪位大蝦能說清楚些希望指點一下…）

　　通過查找資料，stackoverflow有位大神給出了如下的方法，將immutable的bitmap轉為了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;
}

　　用這個方法轉一下，解決了異常。后來查找每個生成bitmap的方法，發現只有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為true的后者，你懂得），故在進行setPixels前重新通過createBitmap(width, height, Config.ARGB_8888)創建了一個bitmap（他是mutable的），然后再setPixels()，也解決了異常，不過感覺后者的效率能夠高些，看第一個方法還創建了個臨時文件，感覺可能會很卡，但測試是兩個都挺卡……准備做個像ViewPager一樣的緩存，這樣可能不會那么卡吧……

　　參考資料：

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