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