ViewModel LifeCycle, when does it get disposed?

Question

In mvvmcross v3 ViewModel

public class TimerViewModel : MvxViewModel
{
    System.Timers.Timer timer;

    public TimerViewModel()
    {
        timer = new System.Timers.Timer(500f);

        timer.Elapsed += HandleTimerElapsed;

        timer.Start();

    }

    void HandleTimerElapsed (object sender, ElapsedEventArgs e)
    {
        Debug.Log( "Time Elapsed" );
    }
}

As MvxViewModel doesn't implement IDisposable, where should I put the following code ?

timer.Stop();
timer.Elapsed += HandleTimerElapsed;

I find that mvvmcross code have some MvxWeakEventSubscription, is it used to solve my problem ?

Answer 1

There's no easy universal way to know when to dispose the ViewModel - especially once you start mixing and matching ViewModel presentation styles to include navigations, tabs, splitviews, flyouts, fragments, lists, etc. and as you include more and more platforms

As a result of this, a couple of ways I have shut things like timers down in the past are:

1. Sometimes I have used a specialised interface on the ViewModel and I ensure this is called appropriately on each client.

For example, I have done some starting/stopping of 'page' level Views using:

• OnPause/OnResume in Android
• OnNavigatedTo/OnNavigatingFrom in Windows
• ViewDidAppear/ViewWillDisappear in iOS

I have thought about adding this as a generalised pattern to do this (it is logged in https://github.com/slodge/MvvmCross/issues/74) - but so far I've not added this to v3 as I think it would lead to too much misunderstanding among users - it's better to let them to do this in the very few situations where it's needed.

Update: I have blogged about this and published a sample - see http://slodge.blogspot.co.uk/2013/11/n42-is-my-viewmodel-visible-can-i-kill.html

2. Sometimes I've just used Event Aggregation through the MvvmCross Messenger - and I've used it's inherent WeakReference-based messaging to make sure the ViewModel can be garbage collected when the view has finished with it.

An example of this is in the InternetMinute sample - which has a single 'Tick generation service' which ViewModels can connect to via messaging for updates - see:

• the service - https://github.com/slodge/MvvmCross-Tutorials/tree/master/InternetMinute/InternetMinute.Core/Services/Tick
• a ViewModel that consumes the service - via https://github.com/slodge/MvvmCross-Tutorials/blob/master/InternetMinute/InternetMinute.Core/ViewModels/HomeViewModel.cs

You might consider this slightly inefficient - as the Tick messages will be generated even if the ViewModel isn't present - but it's only a small inefficiency.

3. I've considered using more final events - things like OnNavigatingFrom(BACK) and 'onDestroy' and some 'final' detection on the UINavigationController delegates ... but I've not had a reason to do this 'for real' on any project yet.