CODEWITHSUNDEEP
listenerdart
Alan Humphrey
Alan Humphrey

Reputation: 553

How do you replace a listener in dart?

I have a class that creates a button and connects a listener to the click event. In a sub class I'd like to replace the superclass handler. This code adds a listener:

row.query("[value='Save']").onClick.listen(handleNewAlert);

How do I remove the existing listener?

Upvotes: 2

Views: 970

Answers (2)

Greg Lowe
Greg Lowe

Reputation: 16261

The Stream.listen() method returns a StreamSubscription object. Call StreamSubscription.cancel() to cancel the event listener.

var subs = element.onClick.listen((e) => print(e));

// Remove the listener.
subs.cancel();

// Add another listener.
element.onClick.listen((e) => print(e));

See this article for more information.

Upvotes: 6

Zdeslav Vojkovic
Zdeslav Vojkovic

Reputation: 14581

Greg's answer describes how to unsubscribe from the event source. This is correct, but IMO a bit awkward, as you need to keep the subscription instance around in case that any derived class wants to cancel it, or you need to provide a method for this on the base class if you want to hide the details.

As you say that you want to replace the handler, which I understand as providing a different implementation, a simpler approach is to simlpy override handleNewAlert method in the derived class.

Of course, it won't work if instead of method name you have specified an anonymous function, as it obviously can't be overridden, but in the scenario shown in your example, this might be an easier approach.

Upvotes: 2

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