如何在不先读取整个图像的情况下就地缩放流式位图？

我有一个图像密集型的 Android 应用程序。我目前正在使用Bitmap.createScaledBitmap() http://developer.android.com/reference/android/graphics/Bitmap.html#createScaledBitmap%28android.graphics.Bitmap,%20int,%20int,%20boolean%29将图像缩放至所需尺寸。但是，此方法要求内存中已有原始位图，该位图可能相当大。

如何缩放正在下载的位图，而不先将整个位图写入本地内存或文件系统？

此方法将从图像中读取标头信息以确定其大小，然后读取图像并将其缩放到所需的大小，而不为完整的原始大小图像分配内存。

它还使用BitmapFactory.Options.inPurgeable http://developer.android.com/intl/fr/reference/android/graphics/BitmapFactory.Options.html#inPurgeable，这似乎是一个记录很少但在使用大量位图时防止 OoM 异常的理想选择。更新：不再使用 inPurgeable，请参阅这张纸条 https://stackoverflow.com/questions/7068132/why-would-i-ever-not-use-bitmapfactorys-inpurgeable-option/7068403#7068403来自罗曼

它的工作原理是使用 BufferedInputStream 读取图像的标头信息，然后再通过 InputStream 读取整个图像。

/**
 * Read the image from the stream and create a bitmap scaled to the desired
 * size.  Resulting bitmap will be at least as large as the 
 * desired minimum specified dimensions and will keep the image proportions 
 * correct during scaling.
 */
protected Bitmap createScaledBitmapFromStream( InputStream s, int minimumDesiredBitmapWith, int minimumDesiredBitmapHeight ) {

    final BufferedInputStream is = new BufferedInputStream(s, 32*1024);
    try {
        final Options decodeBitmapOptions = new Options();
        // For further memory savings, you may want to consider using this option
        // decodeBitmapOptions.inPreferredConfig = Config.RGB_565; // Uses 2-bytes instead of default 4 per pixel

        if( minimumDesiredBitmapWidth >0 && minimumDesiredBitmapHeight >0 ) {
            final Options decodeBoundsOptions = new Options();
            decodeBoundsOptions.inJustDecodeBounds = true;
            is.mark(32*1024); // 32k is probably overkill, but 8k is insufficient for some jpgs
            BitmapFactory.decodeStream(is,null,decodeBoundsOptions);
            is.reset();

            final int originalWidth = decodeBoundsOptions.outWidth;
            final int originalHeight = decodeBoundsOptions.outHeight;

            // inSampleSize prefers multiples of 2, but we prefer to prioritize memory savings
            decodeBitmapOptions.inSampleSize= Math.max(1,Math.min(originalWidth / minimumDesiredBitmapWidth, originalHeight / minimumDesiredBitmapHeight));

        }

        return BitmapFactory.decodeStream(is,null,decodeBitmapOptions);

    } catch( IOException e ) {
        throw new RuntimeException(e); // this shouldn't happen
    } finally {
        try {
            is.close();
        } catch( IOException ignored ) {}
    }

}