在哪里取消订阅活动？

Question

Eg for a general type, which subscribe to some events in constructor:

class SomeType
{
    public SomeType(...)
    {
        someEvent1 += ...
        someEvent2 += ...
    }
}

Where do I unsubscribe from events?

• Finalizer?
• IDisposable ?
• Some method DontForgetToCallMeSoICanUnsubscribeFromEvents() ?
• Use weak events pattern?

I know it depends . In case of controls (wpf, winforms) there are some events what can be used to subscribe/unsubscribe like Loaded / Unloaded , HandleCreated / HandleDestroyed , etc. But what if parent is a simple object ?

And some more specific example: nested ViewModels, where each level is a List<NextLevelVM> , at any level ViewModel can be deleted, does that means what each ViewModel must implement IDisposable (if eg it is the right way) where it call Dispose for each item in their list? I tried to use weak events, but that doesn't go well .

Answer 1

I've found a really good way to handle this issue is to create two methods in the page's code behind, that calls methods on your ViewModel to start/stop listening to events depending on whether it's visible or not.

Below I'm using the Appearing functions, but depending on the framework you're using it might be slightly different, but the strategy should work.

In Page class:

protected override void OnAppearing()
{
    base.OnAppearing();
    _myViewModel.StartListeningToEvents();
}

protected override void OnDisappearing()
{
    base.OnDisappearing();
    _myViewModel.StopListeningToEvents();
}

Then in my ViewModel, I actually subscribe to the events I require:

public void StartListeningToEvents()
{
    SomeProperty.PropertyChanged += PropertyUpdated;
}

public void StopListeningToEvents()
{
    SomeProperty.PropertyChanged -= PropertyUpdated;
}

void PropertyUpdated(object sender, PropertyChangedEventArgs e)
{
    // do stuff
}

My example is for a property change event. But similar code should work for any event.

In this way you're guaranteed that your page is only listening to events when it's open, and you don't need to worry about disposing anything besides calling the one event when the page is no longer open.