collectMediaSources function

Set<MediaSource> collectMediaSources(List<Scene> scenes, { GenerativeResolver? generative, })

Gathers every declared MediaSource from scenes before the frame loop — a pure structural walk over the constructor data, with no mounting and no async.

Pre-resolution must finish before the first frame is pumped, so media cannot be collected by mounting and registering: this walks the same tree the user wrote (each scene's Background and its declared children, and the children of the common layout widgets) and reads each MediaCarrier's mediaSource straight off its constructor. Image, Clip, and Background.image/.video are carriers; everything else is transparent.

This collector lives in the composition layer because it walks Scene trees: composition legitimately depends on media and core, so the dependency edge points one way (composition → media), not the other. The result is a Set, so the same declaration across scenes resolves once (the cache deduplicates by value equality). The render harness hands this set to MediaResolver.preResolveAll before frame 0.

When a generative resolver is given (after its generateAll ran), a GenerativeMedia of a visual kind folds in as its produced file-backed MediaSource, so generated images and videos pre-resolve through the same pass as hand-written Image/Clip.

Implementation

Set<MediaSource> collectMediaSources(List<Scene> scenes, {GenerativeResolver? generative}) {
  final sources = <MediaSource>{};
  walkSceneTree(scenes, (widget) {
    if (widget is MediaCarrier) {
      final source = (widget as MediaCarrier).mediaSource;
      if (source != null) sources.add(source);
    } else if (generative != null && widget is GenerativeMedia && widget.source.isVisual) {
      sources.add(generative.mediaFor(widget.source));
    }
  });
  return sources;
}