Bitmap詳解

1.bitmap佔多少內存

getByteCount()方法是在API12加入的，代表存儲Bitmap的色素需要的最少內存。API19開始getAllocationByteCount()方法代替了getByteCount()。
這是API26的

    public final int getByteCount() {
        if (mRecycled) {
            Log.w(TAG, "Called getByteCount() on a recycle()'d bitmap! "
                    + "This is undefined behavior!");
            return 0;
        }
        // int result permits bitmaps up to 46,340 x 46,340
        return getRowBytes() * getHeight();
    }

    public final int getAllocationByteCount() {
        if (mRecycled) {
            Log.w(TAG, "Called getAllocationByteCount() on a recycle()'d bitmap! "
                    + "This is undefined behavior!");
            return 0;
        }
        return nativeGetAllocationByteCount(mNativePtr);
    }

getByteCount()與getAllocationByteCount()的區別
一般情況下兩者是相等的；如果通過複用Bitmap來解碼圖片，如果被複用的Bitmap的內存比待分配內存的Bitmap大,那麼getByteCount()表示新解碼圖片佔用內存的大小（並非實際內存大小,實際大小是複用的那個Bitmap的大小），getAllocationByteCount()表示被複用Bitmap真實佔用的內存大小

2.如何計算Bitmap佔用的內存

通常情況下認爲
bitmap佔用的內存 = width * height * 一個像素所佔的內存。
下面是API26裏面的一個像素所佔的內存

public enum Config {
        ALPHA_8     (1),//With this configuration, each pixel requires 1 byte of memory.
        RGB_565     (3),//Each pixel is stored on 2 bytes
        @Deprecated
        ARGB_4444   (4),//Each pixel is stored on 2 bytes
        ARGB_8888   (5),//Each pixel is stored on 4 bytes   如果沒有指明，默認這個
        RGBA_F16    (6),//Each pixels is stored on 8 bytes   高色域高動態範圍圖像
        HARDWARE    (7);//stored only in graphic memory
    }

通常用的是RGB_565 和 ARGB_8888（默認）
其實這個說法不全對，沒有說明場景，同時也忽略了一個影響項：Density。

public static Bitmap decodeResourceStream(Resources res, TypedValue value,
            InputStream is, Rect pad, Options opts) {
        validate(opts);
        if (opts == null) {
            opts = new Options();
        }

        if (opts.inDensity == 0 && value != null) {
            final int density = value.density;
            if (density == TypedValue.DENSITY_DEFAULT) {
                opts.inDensity = DisplayMetrics.DENSITY_DEFAULT;
            } else if (density != TypedValue.DENSITY_NONE) {
                opts.inDensity = density;
            }
        }

        if (opts.inTargetDensity == 0 && res != null) {
            opts.inTargetDensity = res.getDisplayMetrics().densityDpi;
        }

        return decodeStream(is, pad, opts);
    }

因此，加載一張本地資源圖片，那麼它佔用的內存 = width * height * nTargetDensity/inDensity * nTargetDensity/inDensity * 一個像素所佔的內存。

        // 不做處理，默認縮放。
        BitmapFactory.Options options = new BitmapFactory.Options();
        Bitmap bitmap = BitmapFactory.decodeResource(getResources(), R.mipmap.aaa, options);
        Log.i(TAG, "bitmap：ByteCount = " + bitmap.getByteCount() + ":::bitmap：AllocationByteCount = " + bitmap.getAllocationByteCount());
        Log.i(TAG, "width:" + bitmap.getWidth() + ":::height:" + bitmap.getHeight());
        Log.i(TAG, "inDensity:" + options.inDensity + ":::inTargetDensity:" + options.inTargetDensity);

        Log.i(TAG,"===========================================================================");

        // 手動設置inDensity與inTargetDensity，影響縮放比例。
        BitmapFactory.Options options_setParams = new BitmapFactory.Options();
        options_setParams.inDensity = 320;
        options_setParams.inTargetDensity = 160;
        Bitmap bitmap_setParams = BitmapFactory.decodeResource(getResources(), R.mipmap.aaa, options_setParams);
        Log.i(TAG, "bitmap_setParams：ByteCount = " + bitmap_setParams.getByteCount() + ":::bitmap_setParams：AllocationByteCount = " + bitmap_setParams.getAllocationByteCount());
        Log.i(TAG, "width:" + bitmap_setParams.getWidth() + ":::height:" + bitmap_setParams.getHeight());
        Log.i(TAG, "inDensity:" + options_setParams.inDensity + ":::inTargetDensity:" + options_setParams.inTargetDensity);


07-15 14:49:06.298 8496-8496/com.company.glidetest I/MainActivity: bitmap：ByteCount = 538800:::bitmap：AllocationByteCount = 538800
07-15 14:49:06.298 8496-8496/com.company.glidetest I/MainActivity: width:449:::height:300
07-15 14:49:06.298 8496-8496/com.company.glidetest I/MainActivity: inDensity:320:::inTargetDensity:320
07-15 14:49:06.298 8496-8496/com.company.glidetest I/MainActivity: ===========================================================================
07-15 14:49:06.318 8496-8496/com.company.glidetest I/MainActivity: bitmap_setParams：ByteCount = 135000:::bitmap_setParams：AllocationByteCount = 135000
07-15 14:49:06.318 8496-8496/com.company.glidetest I/MainActivity: width:225:::height:150
07-15 14:49:06.318 8496-8496/com.company.glidetest I/MainActivity: inDensity:320:::inTargetDensity:160

可以看出：
1.不使用Bitmap複用時，getByteCount()與getAllocationByteCount()的值是一致的；
2.默認情況下，華爲手機在xhdpi的文件夾下，inDensity爲320，inTargetDensity爲320，內存大小爲538800；而538800= 449* 300* 4。
3.手動設置inDensity與inTargetDensity，使其比例爲2，內存大小爲135000，而 449* 300* 0.5 * 0.5 * 4 = 134700。

3.Bitmap如何壓縮

inSampleSize
設置inSampleSize之後，Bitmap的寬、高都會縮小inSampleSize倍。例如：一張寬高爲2048x1536的圖片，設置inSampleSize爲4之後，實際加載到內存中的圖片寬高是512x384。佔有的內存就是0.75M而不是12M，足足節省了15倍。

    public static Bitmap decodeSampledBitmapFromResource(Resources res, int resId, int reqWidth, int reqHeight) {
        // 設置inJustDecodeBounds屬性爲true，只獲取Bitmap原始寬高，不分配內存；
        final BitmapFactory.Options options = new BitmapFactory.Options();
        options.inJustDecodeBounds = true;
        BitmapFactory.decodeResource(res, resId, options);// 這裏會找到最適合的inTargetDensity和inDensity
        Log.i(TAG, "inTargetDensity = " + options.inTargetDensity);
        Log.i(TAG, "inDensity = " + options.inDensity);
        Log.i(TAG, "inPreferredConfig = " + options.inPreferredConfig);
        // 計算inSampleSize值；
        options.inSampleSize = calculateInSampleSize(options, reqWidth, reqHeight);
        // 真實加載Bitmap；
        options.inJustDecodeBounds = false;
        return BitmapFactory.decodeResource(res, resId, options);
    }

    public static int calculateInSampleSize(BitmapFactory.Options options, int reqWidth, int reqHeight) {
        // Raw height and width of image
        final int height = options.outHeight;
        final int width = options.outWidth;
        int inSampleSize = 1;
        if (height > reqHeight || width > reqWidth) {
            final int halfHeight = height / 2;
            final int halfWidth = width / 2;
            // 寬和高比需要的寬高大的前提下最大的inSampleSize
            while ((halfHeight / inSampleSize) >= reqHeight
                    && (halfWidth / inSampleSize) >= reqWidth) {
                inSampleSize *= 2;
            }
        }
        Log.i(TAG, "inSampleSize = " + inSampleSize);
        return inSampleSize;
    }

測試代碼及打印結果

Bitmap bitmap1 = decodeSampledBitmapFromResource(getResources(), R.mipmap.aaa, 100, 100);
Log.i(TAG, "bitmap1 = " + bitmap1.getAllocationByteCount());

07-15 15:06:28.988 23977-23977/com.company.glidetest I/MainActivity: inTargetDensity = 320
07-15 15:06:28.988 23977-23977/com.company.glidetest I/MainActivity: inDensity = 320
07-15 15:06:28.988 23977-23977/com.company.glidetest I/MainActivity: inPreferredConfig = ARGB_8888
07-15 15:06:28.988 23977-23977/com.company.glidetest I/MainActivity: inSampleSize = 2
07-15 15:06:28.998 23977-23977/com.company.glidetest I/MainActivity: bitmap1 = 135000

還是之前的圖片（449* 300）正式算的時候應該是把449向上取整數來算的 450/2* 300/2* 1 * 1 * 4 = 135000
（由此可見其實還和inSampleSize有關）

4.Bitmap如何複用

（1）使用LruCache和DiskLruCache做內存和磁盤緩存；
（2）使用Bitmap複用，同時針對版本進行兼容。

        BitmapFactory.Options options = new BitmapFactory.Options();
        // 圖片複用，這個屬性必須設置；
        options.inMutable = true;
        // 手動設置縮放比例，使其取整數，方便計算、觀察數據；
        options.inDensity = 320;
        options.inTargetDensity = 320;
        Bitmap bitmap = BitmapFactory.decodeResource(getResources(), R.mipmap.aaa, options);

        // 使用inBitmap屬性，這個屬性必須設置；
        options.inBitmap = bitmap;
        options.inDensity = 320;
        // 設置縮放寬高爲原始寬高一半；
        options.inTargetDensity = 160;
        options.inMutable = true;
        Bitmap bitmapReuse = BitmapFactory.decodeResource(getResources(), R.mipmap.aaa, options);

        // 複用對象的內存地址；
        Log.i(TAG, "bitmap = " + bitmap);
        Log.i(TAG, "bitmapReuse = " + bitmapReuse);
        Log.i(TAG, "bitmap：AllocationByteCount = " + bitmap.getAllocationByteCount());
        Log.i(TAG, "bitmapReuse：ByteCount = " + bitmapReuse.getByteCount() + ":::bitmapReuse：AllocationByteCount = " + bitmapReuse.getAllocationByteCount());

運行結果
07-15 15:24:40.038 8463-8463/com.company.glidetest I/MainActivity: bitmap = android.graphics.Bitmap@20cffbbc
07-15 15:24:40.038 8463-8463/com.company.glidetest I/MainActivity: bitmapReuse = android.graphics.Bitmap@20cffbbc
07-15 15:24:40.038 8463-8463/com.company.glidetest I/MainActivity: bitmap：AllocationByteCount = 538800
07-15 15:24:40.038 8463-8463/com.company.glidetest I/MainActivity: bitmapReuse：ByteCount = 135000:::bitmapReuse：AllocationByteCount = 538800

可以看到，共用同一塊內存，複用的bitmap因爲inTargetDensity 和 inDensity的原因大約佔到了四分之一的內存（全部分配給了它，但是隻用了四分之一）

1.bitmap佔多少內存

2.如何計算Bitmap佔用的內存

3.Bitmap如何壓縮

4.Bitmap如何複用

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