Android根据指定的尺寸加载Bitmap

因为图片的尺寸千差百异，要想加载不同的图片，又要保证不影响整体布局的美观，我们可以通过计算需要的尺寸来加载图片。

直接上代码：

    
    public static synchronized Bitmap decodeSampledBitmapFromStream(
            InputStream in, int reqWidth, int reqHeight) {

        // First decode with inJustDecodeBounds=true to check dimensions
        final BitmapFactory.Options options = new BitmapFactory.Options();
        options.inJustDecodeBounds = true;
        BitmapFactory.decodeStream(in, null, options);

        // Calculate inSampleSize
        options.inSampleSize = calculateInSampleSize(options, reqWidth,
                reqHeight);

        // Decode bitmap with inSampleSize set
        options.inJustDecodeBounds = false;
        return BitmapFactory.decodeStream(in, null, options);
    }

    /**
     * Calculate an inSampleSize for use in a {@link BitmapFactory.Options}
     * object when decoding bitmaps using the decode* methods from
     * {@link BitmapFactory}. This implementation calculates the closest
     * inSampleSize that will result in the final decoded bitmap having a width
     * and height equal to or larger than the requested width and height. This
     * implementation does not ensure a power of 2 is returned for inSampleSize
     * which can be faster when decoding but results in a larger bitmap which
     * isn't as useful for caching purposes.
     * 
     * @param options
     *            An options object with out* params already populated (run
     *            through a decode* method with inJustDecodeBounds==true
     * @param reqWidth
     *            The requested width of the resulting bitmap
     * @param reqHeight
     *            The requested height of the resulting bitmap
     * @return The value to be used for inSampleSize
     */
    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;

        //先根据宽度进行缩小
        while (width / inSampleSize > reqWidth) {
            inSampleSize++;
        }
        //然后根据高度进行缩小
        while (height / inSampleSize > reqHeight) {
            inSampleSize++;
        }
        return inSampleSize;
    }

 该方法通过指定大小来加载图片，并保证加载的图片宽、高均不超过指定的大小。

来看看inSampleSize的函义：

        /**
         * If set to a value > 1, requests the decoder to subsample the original
         * image, returning a smaller image to save memory. The sample size is
         * the number of pixels in either dimension that correspond to a single
         * pixel in the decoded bitmap. For example, inSampleSize == 4 returns
         * an image that is 1/4 the width/height of the original, and 1/16 the
         * number of pixels. Any value <= 1 is treated the same as 1. Note: the
         * decoder will try to fulfill this request, but the resulting bitmap
         * may have different dimensions that precisely what has been requested.
         * Also, powers of 2 are often faster/easier for the decoder to honor.
         */
        public int inSampleSize;

 当inSampleSize大于1时，解码器会将宽和高分别缩小inSampleSize倍。

上述calculateInSampleSize方法正是通过将宽与高进行比对得出的inSampleSize值。