initialData property Null safety
final
The data that will be used to create the initial snapshot.
Providing this value (presumably obtained synchronously somehow when the Stream was created) ensures that the first frame will show useful data. Otherwise, the first frame will be built with the value null, regardless of whether a value is available on the stream: since streams are asynchronous, no events from the stream can be obtained before the initial build.
Implementation
final T? initialData;