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onListen property
ControllerCallback onListenThe callback which is called when the stream is listened to.
May be set to null, in which case no callback will happen.
Source
ControllerCallback get onListen; void onListen=(Source
void set onListen(void onListenHandler());
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https://api.dartlang.org/stable/1.24.3/dart-async/StreamController/onListen.html