Memory/resource management using MonoTouch and MonoTouch.Dialog

Question

I have a MonoTouch app that has a UITabBarController, with each of the tabs being a UINavigationController. Some of these wrap a UIViewController which adds a UITableView and a UIToolbar, and others wrap a DialogViewController.

I've not paid much attention to memory / view management thus far (I've been mostly running in the simulator), but as I've started testing on a real device, I've noticed some failures due to low memory conditions (e.g. the app gets terminated, and I discover from my log that DidReceiveMemoryWarning got called prior to this). Other times I notice prolonged pauses in the app's responsiveness that I am assuming are due to a GC cycle.

Thus far I've been assuming that every DialogViewController that I push onto the nav stack will clean up its views and other things it's allocated when I pop it. But I am starting to realize that it's probably not that easy, and that I need to start calling Dispose() on things.

Are there best practices for how to deal with managing resources and memory with MonoTouch and MT.D? Specifically:

• Is it required to call Dispose on a DialogViewController after it's popped? If so, where is it best to do this? (ViewDidUnload? DidReceiveMemoryWarning? destructor?)
• Does the DVC automatically dispose objects like the RootElement that is passed to it or do I need to worry about this? How about UIImages that it loads as part of rendering a table cell (e.g. StyledStringElement)?
• Are there places where I should call GC.Collect() to better space out collections so as to not take a bit hit in responsiveness when a GC does happen?
• Does the generational garbage collector help with the interactivity problems and is it stable enough to use in a production app? (I believe it's still billed as "experimental" in MonoDevelop 3.0.2 / MT 4.3.3)
• What do I need to do in DidReceiveMemoryWarning to reduce the likelihood that iOS will shoot my app? Since each non-visible view controller seems to get this call, I'm assuming that I should clean up that view controller's resources... should I do the same kinds of things I do in ViewDidUnload?
• I don't seem to get my ViewDidUnload called (even after I get a DidReceiveMemoryWarning). In fact I don't recall ever seeing it in my log. If iOS always called my ViewDidUnload after DidReceiveMemoryWarning, I could just do all the cleanup in ViewDidUnload... What is the best way to split cleanup responsibility between ViewDidUnload and DidReceiveMemoryWarning?

I apologize for the general nature of this question - this seems like a good topic for a whitepaper, but I couldn't find any...

Update: to make the question more concrete: after using Instruments and the Xamarin Heapshot profiler, it's clear to me that I'm leaking UIViewControllers when the user pops the navigation stack. Rolf filed a bug for this and it has two dups, so this is a real issue for more than just me. Unfortunately I haven't found a good workaround for the leaked UIViewControllers - I have not found a good place to call Dispose() on them. The natural place to free resources allocated by ViewDidLoad is in the ViewDidUnload message, but it never gets called on the simulator so my memory footprint keeps growing. On the device, I do see DidReceiveMemoryWarning, but I am reluctant to use this as the place to free my viewcontroller and its resources since I am not guaranteed that iOS will actually unload my view, and therefore not guaranteed that my ViewDidLoad will get called again either (leading to a ViewDidAppear which would need to code defensively against situations where its underlying resources were disposed). I'd love to get some advice on how to get out of this mess...

Answer 1

I've spent a couple of days in the MT.D source code and in the profiler. While I am still looking for general guidance on what the best design pattern is for implementing DidReceiveMemoryWarning and ViewDidUnload, I do have some general observations to share that could be useful for someone:

1. MonoTouch.Dialog is very well behaved. It does not leak any resources under ordinary usage. It keeps a control tree under DVC.Root, and each Element's Dispose method correctly Disposes the underlying UIKit control. You don't even have to worry about disposing an old RootElement if you've replaced DVC.Root - the property setter automatically disposes it for you. Overall, MT.D doesn't appear to suffer from any significant memory issues. There is one exception - see below.
2. When creating your own custom Elements (e.g. MultilineEntryElement), make sure to override the Dispose(bool) method, disposing the underlying UIKit control (e.g. UITextView), and chain the base class Dispose() method. The source code in Miguel's MT.D github project provides plenty of good examples. All the Elements implement the standard Dispose pattern (although they omit a destructor/finalizer that calls Dispose(false)).
3. When implementing custom view controllers, it is generally not necessary to implement Dispose on UIViewController subclasses, nor on TableView DataSource or Delegate classes. When the view controller gets GC'ed, it will correctly call Dispose on its references. All the cells that you allocate in the DataSource will be properly disposed.
4. As an exception to (3) - I encountered a nasty issue when adding my own subview to a TableView's cell. This subview is a control I created called "UICheckbox" that ultimately inherits from UIImageView, which has two UIImages (on and off) and a public event called Clicked. I only experience an issue when an event handler which references members of the DataSource is hooked to this event (if the event handler doesn't reference the DataSource or controller itself, all is well). However, when the conditions above are met, and the controller is dismissed, there is apparently some cycle that the GC can't figure out, and every UICheckbox I put on the TableView is leaked (along with its images). The only way I found to work around this was to add code to ViewDidDisappear to dispose of the ViewController and clean up its state IFF it is no longer anywhere in the navigation stack. It's hacky but it works.

5. In general, I adhere to the following template for allocating objects in my view controllers:

   • allocate nothing in the constructor (use it only to pass state in)
   • create a control tree in ViewDidLoad (and dispose it in ViewDidUnload). think "InitializeComponent" in XAML (if that helps). If the UIViewController is going to push a DialogViewController onto the nav stack, the ViewDidLoad is a good place to create the DVC.
   • initialize values in the control tree in ViewDidAppear. E.g. you can add/delete/replace Elements, Sections, and even the Root of the DVC in this method. But don't create a new DVC.

6. There is a general issue with leaking ViewControllers when the user navigates up the nav stack (I reference the bugzilla link in the "Update" in the question). This also affects MT.D. There is a fairly straightforward workaround - add the following line of code in ViewDidAppear of the parent view controller:

       // HACK: touch the ViewControllers array to refresh it (in case the user popped the nav stack)
       // this is to work around a bug in monotouch (https://bugzilla.xamarin.com/show_bug.cgi?id=1889)
       // where the UINavigationController leaks UIViewControllers when the user pops the nav stack
       int count = this.NavigationController.ViewControllers.Length;
   

Rolf does a great job explaining why this bug happens and why the workaround works in the bugzilla link, so I won't repeat it.

I hope someone finds this useful. I also hope someone smarter than me has some guidance on how to handle DidReceiveMemoryWarning and how to split work up between that method and ViewDidUnload.

Update: A couple more notes:

• I now realize the protocol for DidReceiveMemoryWarning and ViewDidUnload: the former is always delivered to every view controller, while the latter is only sent for view controllers that aren't currently displaying, AND aren't deeper than the root of the navigation stack. In the end, I decided to ignore DidReceiveMemoryWarning because I don't really have images that I cache and can dump (as per the iOS guidance). In ViewDidUnload, I release all the resources I allocated in ViewDidLoad.

• My app has a TabBar where each tab hosts a UINavigationController, most of which push a DialogViewController. One issue I was dealing with was leaking the DialogViewController after the ViewDidUnload let go of the reference to it. I tried Disposing the DVC in ViewDidUnload, but iOS kept on wanting to reinvoke it and I was getting an exception for invoking a selector on a GC'ed object. I discovered the reason - the navigation controller was holding onto the DVC in its ViewControllers array. The solution is to release the array by creating a zero-length array in its place - in ViewDidUnload:

  this.ViewControllers = new UIViewController[0];
  

The old array will now be GC'ed, and so will the DVC because nothing is pointing to it anymore. And iOS won't ever reinvoke the object. Note - no need to call Dispose on the DVC.