LayerRasterizer class

Rasterizes Layers into ExportedLayers outside of a live editor session.

A layer can only be captured while it is mounted — Layer.captureAsPng returns null when the layer's repaint boundary has no context. That makes captured layers unavailable to anyone restoring a session from an exported state history: the layers deserialize fine, but they have never been laid out, so they cannot be baked into a render.

This controller closes that gap. capture mounts the given layers in the LayerRasterizerHost that carries it, waits for them to paint, and returns the result. The host paints them behind its own child, so they never become visible.

Mount exactly one host per rasterizer, above anything that captures:

final rasterizer = LayerRasterizer();

MaterialApp(
  builder: (context, child) => LayerRasterizerHost(
    rasterizer: rasterizer,
    child: child!,
  ),
);

final history = ImportStateHistory.fromMap(persistedHistory);
final captured = await rasterizer.capture(
  layers: history.stateHistory[history.editorPosition].layers,
  editorBodySize: history.lastRenderedImgSize,
  configs: myEditorConfigs,
);

Concurrent capture calls are serialized: only one set of layers is mounted at a time, so captures cannot read each other's repaint boundaries.

The layers passed to capture must not be mounted anywhere else while they are captured — no running editor, no LayerStack preview showing the same Layer instances. Every layer carries GlobalKeys (and a Hero tag) that LayerWidget attaches, so mounting it twice makes Flutter move the existing element into the host: the visible copy loses its content, and release builds do not report it. Layers already on screen need no host at all — call Layer.captureAllLayers on them directly. Both this and the one-host rule are asserted in debug mode.