@Override
    public void clear() {
        Util.assertMainThread();
        if (status == Status.CLEARED) {
            return;
        }
        cancel();
        // Resource must be released before canNotifyStatusChanged is called.
        if (resource != null) {
            releaseResource(resource);
        }
        if (canNotifyStatusChanged()) {
            target.onLoadCleared(getPlaceholderDrawable());
        }
        // Must be after cancel().
        status = Status.CLEARED;
    }
Cancels the current load if it is in progress, clears any resources held onto by the request and replaces
the loaded resource if the load completed with the placeholder.

<p>
    Cleared requests can be restarted with a subsequent call to {@link #begin()}
</p>

@see #cancel()
core/src/main/java/com/bumptech/glide/request/GenericRequest.java