ViewModel LifeCycle, when does it get disposed?
mvvmcross
Solution
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.
Problem
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 ?