addListener method Null safety

void addListener(
  1. ImageStreamListener listener
)

Adds a listener callback that is called whenever a new concrete ImageInfo object is available. If a concrete image is already available, this object will call the listener synchronously.

If the assigned completer completes multiple images over its lifetime, this listener will fire multiple times.

The listener will be passed a flag indicating whether a synchronous call occurred. If the listener is added within a render object paint function, then use this flag to avoid calling RenderObject.markNeedsPaint during a paint.

If a duplicate listener is registered N times, then it will be called N times when the image stream completes (whether because a new image is available or because an error occurs). Likewise, to remove all instances of the listener, removeListener would need to called N times as well.

When a listener receives an ImageInfo object, the listener is responsible for disposing of the ImageInfo.image.

Implementation

void addListener(ImageStreamListener listener) {
  if (_completer != null) {
    return _completer!.addListener(listener);
  }
  _listeners ??= <ImageStreamListener>[];
  _listeners!.add(listener);
}