Android ImageResizer：inSampleSize

import android.annotation.TargetApi;
import android.content.Context;
import android.content.res.Resources;
import android.graphics.Bitmap;
import android.graphics.BitmapFactory;
import android.os.Build;

import com.example.android.common.logger.Log;
import com.example.android.displayingbitmaps.BuildConfig;

import java.io.FileDescriptor;

/**
 * A simple subclass of {@link ImageWorker} that resizes images from resources given a target width
 * and height. Useful for when the input images might be too large to simply load directly into
 * memory.
 */
public class ImageResizer extends ImageWorker {
    private static final String TAG = "ImageResizer";
    protected int mImageWidth;
    protected int mImageHeight;

    /**
     * Initialize providing a single target image size (used for both width and height);
     *
     * @param context
     * @param imageWidth
     * @param imageHeight
     */
    public ImageResizer(Context context, int imageWidth, int imageHeight) {
        super(context);
        setImageSize(imageWidth, imageHeight);
    }

    /**
     * Initialize providing a single target image size (used for both width and height);
     *
     * @param context
     * @param imageSize
     */
    public ImageResizer(Context context, int imageSize) {
        super(context);
        setImageSize(imageSize);
    }

    /**
     * Set the target image width and height.
     *
     * @param width
     * @param height
     */
    public void setImageSize(int width, int height) {
        mImageWidth = width;
        mImageHeight = height;
    }

    /**
     * Set the target image size (width and height will be the same).
     *
     * @param size
     */
    public void setImageSize(int size) {
        setImageSize(size, size);
    }

    /**
     * The main processing method. This happens in a background task. In this case we are just
     * sampling down the bitmap and returning it from a resource.
     *
     * @param resId
     * @return
     */
    private Bitmap processBitmap(int resId) {
        if (BuildConfig.DEBUG) {
            Log.d(TAG, "processBitmap - " + resId);
        }
        return decodeSampledBitmapFromResource(mResources, resId, mImageWidth,
                mImageHeight, getImageCache());
    }

    @Override
    protected Bitmap processBitmap(Object data) {
        return processBitmap(Integer.parseInt(String.valueOf(data)));
    }

    /**
     * Decode and sample down a bitmap from resources to the requested width and height.
     *
     * @param res The resources object containing the image data
     * @param resId The resource id of the image data
     * @param reqWidth The requested width of the resulting bitmap
     * @param reqHeight The requested height of the resulting bitmap
     * @param cache The ImageCache used to find candidate bitmaps for use with inBitmap
     * @return A bitmap sampled down from the original with the same aspect ratio and dimensions
     *         that are equal to or greater than the requested width and height
     */
    public static Bitmap decodeSampledBitmapFromResource(Resources res, int resId,
            int reqWidth, int reqHeight, ImageCache cache) {

        // BEGIN_INCLUDE (read_bitmap_dimensions)
        // First decode with inJustDecodeBounds=true to check dimensions
        final BitmapFactory.Options options = new BitmapFactory.Options();
        options.inJustDecodeBounds = true;
        BitmapFactory.decodeResource(res, resId, options);

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

        // If we‘re running on Honeycomb or newer, try to use inBitmap
        if (Utils.hasHoneycomb()) {
            addInBitmapOptions(options, cache);
        }

        // Decode bitmap with inSampleSize set
        options.inJustDecodeBounds = false;
        return BitmapFactory.decodeResource(res, resId, options);
    }

    /**
     * Decode and sample down a bitmap from a file to the requested width and height.
     *
     * @param filename The full path of the file to decode
     * @param reqWidth The requested width of the resulting bitmap
     * @param reqHeight The requested height of the resulting bitmap
     * @param cache The ImageCache used to find candidate bitmaps for use with inBitmap
     * @return A bitmap sampled down from the original with the same aspect ratio and dimensions
     *         that are equal to or greater than the requested width and height
     */
    public static Bitmap decodeSampledBitmapFromFile(String filename,
            int reqWidth, int reqHeight, ImageCache cache) {

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

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

        // If we‘re running on Honeycomb or newer, try to use inBitmap
        if (Utils.hasHoneycomb()) {
            addInBitmapOptions(options, cache);
        }

        // Decode bitmap with inSampleSize set
        options.inJustDecodeBounds = false;
        return BitmapFactory.decodeFile(filename, options);
    }

    /**
     * Decode and sample down a bitmap from a file input stream to the requested width and height.
     *
     * @param fileDescriptor The file descriptor to read from
     * @param reqWidth The requested width of the resulting bitmap
     * @param reqHeight The requested height of the resulting bitmap
     * @param cache The ImageCache used to find candidate bitmaps for use with inBitmap
     * @return A bitmap sampled down from the original with the same aspect ratio and dimensions
     *         that are equal to or greater than the requested width and height
     */
    public static Bitmap decodeSampledBitmapFromDescriptor(
            FileDescriptor fileDescriptor, int reqWidth, int reqHeight, ImageCache cache) {

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

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

        // Decode bitmap with inSampleSize set
        options.inJustDecodeBounds = false;

        // If we‘re running on Honeycomb or newer, try to use inBitmap
        if (Utils.hasHoneycomb()) {
            addInBitmapOptions(options, cache);
        }

        return BitmapFactory.decodeFileDescriptor(fileDescriptor, null, options);
    }

    @TargetApi(Build.VERSION_CODES.HONEYCOMB)
    private static void addInBitmapOptions(BitmapFactory.Options options, ImageCache cache) {

        // inBitmap only works with mutable bitmaps so force the decoder to
        // return mutable bitmaps.
        options.inMutable = true;

        if (cache != null) {
            // Try and find a bitmap to use for inBitmap
            Bitmap inBitmap = cache.getBitmapFromReusableSet(options);

            if (inBitmap != null) {
                options.inBitmap = inBitmap;
            }
        }

    }

    /**
     * Calculate an inSampleSize for use in a {@link android.graphics.BitmapFactory.Options} object when decoding
     * bitmaps using the decode* methods from {@link android.graphics.BitmapFactory}. This implementation calculates
     * the closest inSampleSize that is a power of 2 and will result in the final decoded bitmap
     * having a width and height equal to or larger than the requested width and height.
     *
     * @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) {
        // BEGIN_INCLUDE (calculate_sample_size)
        // 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;

            // Calculate the largest inSampleSize value that is a power of 2 and keeps both
            // height and width larger than the requested height and width.
            while ((halfHeight / inSampleSize) > reqHeight
                    && (halfWidth / inSampleSize) > reqWidth) {
                inSampleSize *= 2;
            }

            // This offers some additional logic in case the image has a strange
            // aspect ratio. For example, a panorama may have a much larger
            // width than height. In these cases the total pixels might still
            // end up being too large to fit comfortably in memory, so we should
            // be more aggressive with sample down the image (=larger inSampleSize).

            long totalPixels = width * height / inSampleSize;

            // Anything more than 2x the requested pixels we‘ll sample down further
            final long totalReqPixelsCap = reqWidth * reqHeight * 2;

            while (totalPixels > totalReqPixelsCap) {
                inSampleSize *= 2;
                totalPixels /= 2;
            }
        }
        return inSampleSize;
        // END_INCLUDE (calculate_sample_size)
    }
}