Glide 3.6.0 proof of concept for loading a Drawable as Bitmap through Glide

DrawableBitmapResourceDecoder.java

class DrawableBitmapResourceDecoder implements ResourceDecoder<Drawable, Bitmap> {
	private final BitmapPool pool;
	public DrawableBitmapResourceDecoder(Context context) {
		this(Glide.get(context).getBitmapPool());
	}
	public DrawableBitmapResourceDecoder(BitmapPool pool) {
		this.pool = pool;
	}

	@Override public Resource<Bitmap> decode(Drawable drawable, int width, int height) throws IOException {
		Config config = PixelFormat.formatHasAlpha(drawable.getOpacity())? Config.ARGB_8888 : Config.RGB_565;
		Bitmap bitmap = pool.get(drawable.getIntrinsicWidth(), drawable.getIntrinsicHeight(), config);
		if (bitmap == null) {
			bitmap = Bitmap.createBitmap(drawable.getIntrinsicWidth(), drawable.getIntrinsicHeight(), config);
		}
		Canvas canvas = new Canvas(bitmap);
		drawable.setBounds(0, 0, canvas.getWidth(), canvas.getHeight());
		drawable.draw(canvas);
		return new BitmapResource(bitmap, pool);
	}

	@Override public String getId() { return "DrawableBitmapResourceDecoder"; }
}

initialization_snip.java

GenericRequestBuilder<Drawable, Drawable, Bitmap, ?> glide = Glide
	.with(context)
	.using(new PassthroughModelLoader<Drawable, Drawable>(), Drawable.class)
	.from(Drawable.class)
	.as(Bitmap.class)
	// transcode helps if you want a drawable in the end, but also want to apply BitmapTransformations
	// optional .transcode(new BitmapToGlideDrawableTranscoder(context), GlideDrawable.class)
	.decoder(new DrawableBitmapResourceDecoder(context))
	.cacheDecoder(new FileToStreamDecoder<Bitmap>(new StreamBitmapDecoder(context)))
	.encoder(new BitmapEncoder())
	// or .diskCacheStrategy(DiskCacheStrategy.NONE) instead of last 2
;

loading_snip.java

// simulate a drawable input coming from somewhere
Drawable drawable = getResources().getDrawable(android.R.drawable.sym_def_app_icon);

glide
	.clone() // if you use the same glide multiple times (e.g. in Adapter)
	.load(drawable)
	.signature(new StringSignature("android.R.drawable.sym_def_app_icon")) // required for cache to be correct
	// ... add whatever you want here (transformation, animation ...)
	.into(imageView)
;

PassthroughModelLoader.java

class PassthroughModelLoader<Result, Source extends Result> implements ModelLoader<Source, Result> {
	@Override public DataFetcher<Result> getResourceFetcher(final Source model, int width, int height) {
		return new CastingDataFetcher<Source, Result>(model); // upcasting is ok
	}

	/** Extremely unsafe, use with care. */
	private static class CastingDataFetcher<Source, Result> implements DataFetcher<Result> {
		private final Source model;
		public CastingDataFetcher(Source model) {
			this.model = model;
		}

		@SuppressWarnings("unchecked")
		@Override public Result loadData(Priority priority) throws Exception {
			return (Result) model;
		}

		@Override public void cleanup() { }
		@Override public String getId() { return ""; }
		@Override public void cancel() { }
	}
}