Android開發之Bitmap的高效載入

標籤：

BitmapFactory類提供了四類方法:decodeFile, decodeResource, decodeStream和decodeByteArray

分別用於支援從檔案系統,資源,輸入資料流以及位元組數組中載入出一個Bitmap對象,前兩者又間接調用了decodeStream

為了避免OOM,可以通過採樣率有效載入圖片

如果擷取採樣率?

1.將BitmapFactory.Options的inJustDecodeBounds的參數設定為true並載入圖片

2.從BitmapFactory.Options中取出圖片的原始寬高,它們對應outWidth和outHeight參數

3.根據採樣率的規則並結合目標View的所需大小計算採樣率inSampleSize

4.將BitmapFactory.Options的inJustDecodeBounds的參數設定為false,並重新載入圖片

上述步驟通過代碼體現:

public static Bitmap decodeSampledBitmapFromResource(Resource res, int resId, int reqWidth, int reqHeight){    final BitmapFactory.Options options = new BitmapFactory.Options();    options.inJustDecodeBounds = true;    BitmapFactory.decodeResource(res, resId, options);        options.inSampleSize = calculateInSampleSize(options, reqWidth, reqHeight);    options.inJustDecodeBounds = false;    return BitmapFactory.decodeResource(res, resId, options);}public static int calculateInSampleSize(BitmapFactory.Options options, int reqWidth, int reqHeight){    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;                while((halfHeight / inSampleSize) >= reqHeight && (halfWidth / inSampleSize) >= reqWidth){            inSampleSize *= 2;                    }    }        return inSampleSize;}

 

Android開發之Bitmap的高效載入