@Override
    public void run() {
        if (allocate()) {
            handler.postDelayed(this, getNextDelay());
        }
    }
Attempts to allocate {@link Bitmap}s and returns {@code true} if there are more
{@link Bitmap}s to allocate and {@code false} otherwise.
core/src/main/java/com/bumptech/glide/load/engine/prefill/BitmapPreFillRunner.java