addListener method
- 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);
}