The application I'm writing needs to support iOS5+. Recently, Apple obsoleted ViewDidUnload as we're told there is no significant memory gain in releasing views on memory warning.
In my application, I have a UIViewController that manages a very heavy UIWebView.
This view controller is presented modally and, as a result, often being created and dismissed.
By using Instruments, I found out that the memory taken by UIWebView is not being freed immediately after its controller is dismissed.
I assumed that the controller would eventually get collected by Mono GC, and it would call Dispose on the controller, as well as on its view, which would dispose UIWebView and free underlying native object.
I can't test if this is the case: unfortunately after presenting and dismissing the controller for about ten times, I get a memory warning and the app crashes the next second. I'm not sure if Mono GC gets a chance to run at all.
So what I did was adding GC.Collect call right after the controller has been dismissed.
I also had to add ReleaseDesignerOutlets in ViewDidDisappear.
This seems to free UIWebView.
Update: I already found out that ReleaseDesignerOutlets call in ViewDidDisappear was obviously releasing the web view, but there was no benefit to GC call. In fact, GC never collected my controller because a button click handler was keeping the whole controller alive.
Now, I feel completely lost in some kind of Cargo memory management.
Is it reasonably to force garbage collection in my case?
Why do I have to call ReleaseDesignerOutlets? Surely, if there are no references to the “dead” controller, its views should be considered eligible for collection as well?
From Instruments heapshot diff, it looks like the views created from code “hold on” to the controller as well. Do I have to dispose them? Nullify them?
Do I need to manually call Dispose on the controller I just dismissed?
Do I need to include ReleaseDesignerOutlets call in Dispose method of my controller?
Do I need to null out references to child views in my custom UIView subclasses on Dispose?
You should just call Dispose() on your controller when it is dismissed.
So something like:
private YourModalController modalController;
//When your button is clicked
partial void YourButtonClick() {
modalController = new YourModalController();
PresentViewController(modalController, true, delegate {
modalController.Dispose();
modalController = null;
});
}
In YourModalController, make sure you have:
public override void Dispose(bool disposing) {
ReleaseDesignerOutlets();
base.Dispose(disposing);
}
You don't necessarily have to worry about ViewDidUnload in this case, since this controller is disposed when dismissed.
Prior to iOS 6:
ViewDidUnload was called in a low memory warning for the app
on controllers that are still in memory, but not actively on the screen such as down the stack in a UINavigationController
On this event, you should dispose any views you have C# references to and set them to null
for iOS 6 this doesn't happen any more
Likewise if you have this:
private UIButton buttonIMadeFromCode;
You should check for null, dispose it, and set it to null in Dispose() and ViewDidUnload() (but only mess with ViewDidUnload if you are targeting less than iOS 6).
First: memory management in MonoTouch is a very complex topic, because MonoTouch (which is garbage collected) has to co-exist with ObjectiveC (which is reference counted).
As you have found out by now it is easy to run into cycles, and when these cross the MonoTouch/ObjectiveC boundary, the GC is not able to figure out exactly what's going on and free the entire cycle.
If you're interested in a more in-depth explanation, check this thread out.
Related
I am wondering if it is a good practice to implement a deinit on every view controller to check if it is correctly removed when it disappears and avoiding leaking memory?
By default, you don't have to implement the deinit method in your classes:
Swift automatically deallocates your instances when they are no longer
needed, to free up resources. Swift handles the memory management of
instances through automatic reference counting (ARC), as described in
Automatic Reference Counting. Typically you don’t need to perform
manual cleanup when your instances are deallocated. However, when you
are working with your own resources, you might need to perform some
additional cleanup yourself. For example, if you create a custom class
to open a file and write some data to it, you might need to close the
file before the class instance is deallocated.
Swift Deinitialization Documentation - How Deinitialization Works Section.
Usually, when working with View Controllers it seems that there is no need to do such an implementation. However, as mentioned in #rmaddy's comment, it is still an approach for tracing memory leak or reference cycle with the view controller.
If your purpose is to check if the controller has been removed from the hierarchy (view controller life cycle), you could implement viewWillDisappear(_:) or viewDidDisappear(_:) methods; Note that calling on of these methods does not guarantees that the deinit will be called, i.e it does not mean that disappearing the view controller always leads to deallocate it (related: Deinit never called, explanation for deinit not called).
Also:
these Q&As should be useful:
When should I use deinit?
how to make deinit take effect in swift
Understand deinitialization and inheritance in swift language
Swift automatically deallocates your instances when they are no longer needed, to free up resources. So to add deinit on all your viewControllers seems unnecessary. You should call deinit whenever you need to do some action or cleanup before deallocating an object.
Well during the tests phase it maybe good idea, because you can check if everything is good (eg. if you have a lot of completion handler) but overall it is unnecessary.
I can't seem to find any resolutions to this. I've made all my objects 'weak' in both view controllers in question. I have included dismiss functions and even the 'RemoveFromSuperView' function. I tried them all without luck. I also tried making the button action 'modal', 'push', ect. None made any difference.
Essentially as I move from controllers, memory seems to just accumulate more and more. On both controllers I'm simply using WebView's. I rack up over 100MB of memory usage after some time navigating between views. Eventually the app runs out of memory and crashes.
How do I either clear all memory accumulated by the app or properly dismiss/kill all inactive View Controllers and clear all memory associated?
This is how you override dealloc method
- (void)dealloc {
[_object release];
[super dealloc];
}
where _object is whatever property you initialized.
I'm having a problem with a view controller that's dismissed and not referenced but still in memory, just wondering in general when is the object actually released in memory when no one references it?
The way I used to test is that I installed the PVC tool from Facebook and use it to print out the view hierarchy when the view controller is presented, after it's dismissed, I make sure no one's referencing it and paused the execution so I can po the memory address of the view controller from the previous PVC tool, but I can still see the view controller instance there.
Thanks!
You appear to be confusing being released and being cleared from memory. When the class is destroyed, the memory it occupied is not zeroed, just like when you delete a file in the filesystem, the disk blocks are not zeroed either.
This would simply take up too much time and have very little benefit.
Being released simply means the memory the class occupied can now be re-used.
One way to see if the class has been destroyed is to add a log in the dealloc method:
- (void)dealloc
{
NSLog(#"I'm being destroyed");
}
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...
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:
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.
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)).
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.
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.
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.
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.
When my iPhone app receives a memory warning the views of UIViewControllers that are not currently visible get unloaded. In one particular controller unloading the view and the outlets is rather fatal.
I'm looking for a way to prevent this view from being unloaded. I find this behavior rather stupid - I have a cache mechanism, so when a memory warning comes - I unload myself tons of data and I free enough memory, but I definitely need this view untouched.
I see UIViewController has a method unloadViewIfReloadable, which gets called when the memory warning comes. Does anybody know how to tell Cocoa Touch that my view is not reloadable?
Any other suggestions how to prevent my view from being unloaded on memory warning?
Thanks in advance
Apple docs about the view life cycle of a view controller says:
didReceiveMemoryWarning - The default
implementation releases the view only
if it determines that it is safe to do
so
Now ... I override the didReceiveMemoryWarning with an empty function which just calls NSLog to let me know a warning was received. However - the view gets unloaded anyway. Plus, on what criteria exactly is decided whether a view is safe to unload ... oh ! so many questions!
According to the docs, the default implementation of didReceiveMemoryWarning: releases the view if it is safe to do (ie: superview==nil).
To prevent the view from being released you could override didReceiveMemoryWarning: but in your implementation do not call [super didReceiveMemoryWarning]. That's where the view is released by default (if not visible).
The default didReceiveMemoryWarning releases the view by calling [viewcontroller setView:nil], so you could override that instead.
What appears to be working for me was to override setView: to ignore setting to nil. It's kludgy, but then, this is a kludgy issue, and this did the trick:
-(void)setView:(UIView*)view {
if(view != nil || self.okayToUnloadView) {
[super setView:view];
}
}
Could it be so simple?
Even though nowhere in the documentation this is mentioned, it seems that if I exclusively retain my view in viewDidLoad, then it does not get released on Memory Warning. I tried with several consecutive warnings in the simulator and all still seem good.
So ... the trick for the moment is "retain" in viewDidLoad, and a release in dealloc - this way the viewcontroller is "stuck" with the view until the time it needs to be released.
I'll test some more, and write about the results
I don't think any of these ideas work. I tried overriding [didReceiveMemoryWarning], and that worked for some phones, but found one phone unloaded the view BEFORE that method was even called (must have been in extremely low memory or something). Overriding [setView] produces loads of log warnings so I wouldn't risk that by Apple. Retaining the view will just leak that view - it'll prevent crashes but not really work - the view will replaced next time the controllers UI is loaded.
So really you've just got to plan on your views being unloaded any time they're off-screen, which is not ideal but there you go. The best patterns I've found to work with this are immediate commit so your UI is always up-to-date, or copy-edit-copy, where you copy your model to a temporary instance, populate your views and use immediate commit with that instance, then copy the changes back to your original model when the user hits 'save' or whatever.
Because the accepted solution has problems with viewDidUnload still getting called even though the view was blocked from being cleared, I'm using a different though still fragile approach. The system unloads the view using an unloadViewForced: message to the controller so I'm intercepting that to block the message. This prevents the confused call to viewDidUnload. Here's the code:
#interface UIViewController (Private)
- (void)unloadViewForced:(BOOL)forced;
#end
- (void)unloadViewForced:(BOOL)forced {
if (!_safeToUnloadView) {
return;
}
[super unloadViewForced:forced];
}
This has obvious problems since it's intercepting an undocumented message in UIViewController.
progrmr posted an answer above which recommends intercepting didReceiveMemoryWarning instead. Based on the stack traces I've seen, intercepting that should also work. I haven't tried that route though because I'm concerned there may be other memory cleanup which would also be blocked (such as causing it to not call child view controllers with the memory warning message).