ScenarioAssetBundle class

The bundle a scenario reads its assets through: one that caches values, never futures.

rootBundle is a CachingAssetBundle, which memoizes the future of a read. Under fake time that future belongs to the fake zone — it completes when a pump flushes the fake microtask queue and at no other moment. Hand it to tester.runAsync, where no pump can run, and the scenario stops dead: no error, no output, until something far away times out.

Neither half of that is exotic. A screen showing an SVG puts the key in the cache; a later step reading the same SVG inside runAsync is what the blank-screen hint recommends. One screen and one step is the whole recipe.

So this caches what came back rather than the promise of it. A hit is a SynchronousFuture, which resolves in whichever zone awaits it and schedules nothing; a miss inside runAsync starts its own read, on the real event loop, and completes there. No future here can be completable by only one zone.

The cost of that is the cache being useless until the first read returns: callers that all ask before then each start their own read, where CachingAssetBundle would have handed them one future. That is a few extra platform reads in the first frame of a scenario, and it is the trade the whole class is — sharing an in-flight read is exactly the thing that deadlocks.

A scenario has one, reachable as s.assets and installed over the tree it pumps, so anything reaching for DefaultAssetBundle.of(context) — an Image.asset, an AssetImage, a SvgPicture.asset — is already reading through it.