第12章 Bitmap的加载和Cache

public class ImageResizer {
    private static final String TAG = "ImageResizer";
    public ImageResizer() {
    }
    public Bitmap decodeSampledBitmapFromResource(Resources res,
            int resId, int reqWidth, int reqHeight) {
        //1、 将BitmapFactory.Options的inJustDecodeBounds设置为true并加载图片
        final BitmapFactory.Options options = new BitmapFactory.Options();
        options.inJustDecodeBounds = true;
        BitmapFactory.decodeResource(res, resId, options);
        // 2、从BitmapFactory.Options取出图片的原始宽高信息
        options.inSampleSize = calculateInSampleSize(options, reqWidth,
                reqHeight);
        // 4、设置injustDecodBounds为false然后重新加载图片
        options.inJustDecodeBounds = false;
        return BitmapFactory.decodeResource(res, resId, options);
    }
    public int calculateInSampleSize(BitmapFactory.Options options,
            int reqWidth, int reqHeight) {
        if (reqWidth == 0 || reqHeight == 0) {
            return 1;
        }
        // Raw height and width of image
        final int height = options.outHeight;
        final int width = options.outWidth;
        Log.d(TAG, "origin, w= " + width + " h=" + height);
        int inSampleSize = 1;
        if (height > reqHeight || width > reqWidth) {
            final int halfHeight = height / 2;
            final int halfWidth = width / 2;
            // Calculate the largest inSampleSize value that is a power of 2 and
            // keeps both
            // height and width larger than the requested height and width.
            //3、结合采样率规则并结合View的所需大小计算采样率inSampleSize
            while ((halfHeight / inSampleSize) >= reqHeight
                    && (halfWidth / inSampleSize) >= reqWidth) {
                inSampleSize *= 2;
            }
        }
        Log.d(TAG, "sampleSize:" + inSampleSize);
        return inSampleSize;
    }
}