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

DrawableDrawableResoureDecoder.java

class DrawableResoureDecoder implements ResourceDecoder<Drawable, Drawable> {
	@Override public Resource<Drawable> decode(Drawable source, int width, int height) throws IOException {
		return new DrawableResource<Drawable>(source) {
			@Override public int getSize() { return 1; }
			@Override public void recycle() { }
		};
	}
	@Override public String getId() { return "DrawableDrawableResourceDecoder"; }
}

intialization_snip.java

GenericRequestBuilder<Drawable, Drawable, Drawable, Drawable> glide = Glide
	.with(context)
	.using(new PassthroughModelLoader<Drawable, Drawable>(), Drawable.class)
	.from(Drawable.class)
	.as(Drawable.class)
	.decoder(new DrawableResoureDecoder())
	.skipMemoryCache(true)
	.diskCacheStrategy(DiskCacheStrategy.NONE) // can't cache because there's now way to write any Drawable to a File
;

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)
	// ... 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() { }
	}
}

See bumptech/glide#122