    </section>
    
    <section class="summary source-code" id="source">
      <h2><span>Implementation</span></h2>
      <pre class="language-dart"><code class="language-dart">void addRetained(EngineLayer retainedLayer) {
  assert(retainedLayer is _EngineLayerWrapper);
  assert(() {
    final _EngineLayerWrapper layer = retainedLayer as _EngineLayerWrapper;

    void recursivelyCheckChildrenUsedOnce(_EngineLayerWrapper parentLayer) {
      _debugCheckUsedOnce(parentLayer, &#39;retained layer&#39;);
      