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.
Related
The [UIViewController viewDidLoad] method is called by the system after loading the associated view (obviously?). A common belief -- which I share -- is that viewDidLoad should not be called directly †. However, I can't find this guidance in the documentation, nor anything else from Apple. Does it exist?
For comparison, the loadView documentation says
You should never call this method directly.
† Excepting [super viewDidLoad] in an overridden method.
I don't think there's any rule that forbids it. I don't mean "there's a rule we all know, it's just not written down." I mean "I don't think there's any rule that forbids it." You are, as best I am aware, free to call viewDidLoad whenever appropriate (including its super).
That doesn't mean you should call it. But then you almost certainly should never call OSCompareAndSwap either. In both cases, if you had a good reason and knew what you were doing, it could be appropriate to call. But it's unlikely to come up.
While I can't think of a time I've had to call viewDidLoad directly, I have had to manually call viewWillAppear and viewDidDisappear to manage view lifecycle in a custom container view controller. There's nothing that forbids calling view lifecycle methods if that's what you mean.
That said, it would be bad practice to call a view lifecycle method if you didn't mean "the view has transitioned into this state." And since it's pretty hard to get into a situation where you have loaded the view, but viewDidLoad won't be called, it's hard to imagine many cases where it would be useful. And you shouldn't call a method uselessly. So that's the only rule at play here as far as I'm aware.
It is not mentioned explicitly but if you would call it, you would somehow violate the "contract" of the method specified in its documentation:
Called after the controller's view is loaded into memory.
In methodA of my view controller VC1 another view controller's (VC2) method methodB is called in which VC1 is deallocated. Then control returns to VC1 methodA which obviously crashes when self is used.
It is not obvious that the instance is deallocated, so developers may use self without knowing that they should not. From that perspective I'd like to fix the deallocation. However, I'd like to get some opinions whether or not such a situation is common or needs to be fixed ASAP a.s.o.
Q1: Is execution of deallocated object's methods somewhat common which one will encounter in typical medium projects?
Q2: Would it be acceptable, if comments are added, which warn the developer?
Q3: Are there any other recommendations / opinions?
The project is of medium complexity (about 200 classes of which 50 are view controllers). I'd like to get a feeling about how much effort I should invest to take care of such "deallocated method calls". If it would be one of my smaller pet projects, it would be rather easy to fix anything. However, with an inherited project which already went through a couple of hands, things are not so trivial any more.
EDIT:
didFinish delegate scenarios:
Thinking about it, I came across didFinish kind of delegate calls. Assume a master view controller (MasterVC) uses a slave view controller (SlaveVC) to do some work, keeps a strong reference to it and sets itself as a delegate for the SlaveVC. When the slave is finished, it calls slaveVcDidFinish. In MasterVC's implementation of slaveVcDidFinish the strong reference to the SlaveVC is set to nil. So when the slaveVcDidFinish returns, it is SlaveVC's responsibility to never use self, as it may have already been deallocated.
- (void) notifyDelegate
{
[self.delegate slaveVcDidFinish];
// From here on, `self` may be invalid...
}
This likely is relevant, when the SlaveVC is dismissed before the call to slaveVcDidFinish, as otherwise it cannot be deallocated because it is part of the view controller hierarchy.
Q4: Is my understanding of this didFinish scenario correct?
[/EDIT]
A few details, in case it is relevant:
VC2 presents VC1 and keeps a strong reference
VC1 does its work and needs to present VC3 for which it dismisses itself before
VC3 does its work and delegates back to VC1's methodA
VC1's methodA delegates to VC2's methodB
methodB releases the strong reference to VC1, VC1 now gets deallocated (dealloc is called), and control returns to methodA
methodA does a little more work and then returns
One way to do the little more work was to call methods on an object passed into methodA as parameter. That should work, as self does not play a role there.
Another way to do that work was to call a method of VC1 using self which obviously causes a crash. So as long as one does not use self, everything should be fine.
Yes, you should fix it, but based on what you are saying your app architecture must be to a greater or lesser extent wrong. You are trying to implement an algorithm distributed across view controllers that really needs an object that lives across the lifetime of the objects you are dealing with. Such needs a mediator object.
Actually I suspect you may not need to be implementing that algorithm at all and another approach that works better with how controllers are instantiated and how control gets passed around the app would work fine, but you haven't posted sufficient info for me to be able to advise on that.
However having said that, implementing an overarching controller could certainly be used to solve your problem. A good way to do this is often to subclass your current root view controller (or better still a subclass that implements a category you define). So for example, if it is a UINavigationController you are using, create UINavigationController subclass, change the class of it's representation in your Storyboard (assuming you are using storyboards) and do the work of coordinating display and dismissal of other view controllers in there.
Also check out the Mediator design pattern http://cocoapatterns.com/ios-view-controller-transitions-mediator-pattern/
As a note, generally these days you can avoid having any object properties for strongly holding view controllers. Usually the root view controller holds strong references to any view controller it needs to present, lazy invocation should be used to present view controllers that are not currently presented or stored in an existing navigation controller stack (which keeps the memory utilisation profile in good shape) and any other child view controllers can be added in to your view controller heirarchy using the addChildViewController: method in which case they are strongly held in the childViewControllers array property.
You may want a convenient name for a view controller, but actually I would recommend writing a small bit of code for as the implementation of any such property that will identify the controller you need from amongst those in the Apple supplied properties, and dynamically retrieve it. This may seem like hard work, but it's worth it and actually, paradoxically, decreases code complexity and helps ensure you stay working with the view controllers the way Apple intended. Doing anything else increases pain.
Using separate object properties for holding references to view controllers will usually only duplicate mechanisms the existing Apple classes already provide you with. Such unnecessary duplication increases code complexity and can introduce bugs and memory management issues (of the kind you are in fact describing).
Sorry this answer is quite general, but you are seeking an answer to the wrong question. From what you have said it's clear that at some level you need to address your app architecture.
I agree the architecture seems flawed but as you say it is inherited code and not trivially small maybe some compromises must be made.
I am thinking since self (or any instance variables or properties) cannot be called anyway perhaps you could make the methods you need to call class methods (+) rather than instance methods (-). Class methods are obviously safe to call without an object instance.
I am really struggling in understanding what to do in didReceiveMemoryWarning. From what I read on StackOverflow and blogs, the following is my understanding -
Generate all data that your view needs in viewDidAppear and destroy those (set to nil) in didReceiveMemoryWarning. This sounds good to me because those properties can be recreated in viewDidAppear.
However, the problem is that didReceiveMemoryWarning is also called for the view which is currently visible. In this case, obviously I would not deallocate data for the view. Shouldn't didReceiveMemoryWarning NOT be called for the view which is currently visible. But that is not case - How can one handle this?
The Idea with didReceiveMemoryWarning is that any Data that you have that can be re-created, should be deallocated and created from that point on whenever the user needs it. Traditionally, this doesn't include UI components.
So unless you have a ton of UIControls on your screen, it's probably not worth it to write the code to re-cycle, or recreate them (especially since this is done for you in UICollection's and UITableView's). That said, if you're getting a didReceiveMemoryWarning, it's probably because you're keeping some stuff in memory around that you don't need.
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 memory is low and views get cleaned up by the OS, my understanding is that viewDidUnload is an appropriate place to clean up objects and memory used by your UIViewController (that otherwise wouldn't get cleaned up as a function of being in the view hierarchy). This data is then re-initialized when loadView is called again to create the view. Can someone give examples of what sort of things might be cleaned up (and likewise initialized in loadView)?
I have some data I initialize in loadView currently which sets the stage for my view controller to run a complex animation script involving captions, images, etc. I figured it would make sense to release and cleanup that data if my view were to be removed by the OS (and viewDidUnload were called), but then I thought to myself, why wouldn't I just initialize that data within init and clean it up in dealloc instead of repeatedly initializing and cleaning up the same data (it doesn't change as a function of when the view is loaded or shown). Would this be a better place for it?
Basically, my thinking is:
yes, i should just initialize it in init and release in dealloc because it never changes
initializing things in loadView (and subsequently cleaning in viewDidUnload) is an appropriate practice when either that data will initialize differently based upon when the view is loaded (or even more appropriately when the view appears in viewWillAppear/viewWillDisappear) or it is a good candidate for freeing memory because it takes up a lot of memory that you'd like to see freed up if the view is not active.
Can anyone give me some clarification on my questions and/or my line of thinking?
if you're going to be going back and forth between that view and another and the viewcontroller will be kept around, you could indeed move the initialisation to init, and clean it up in dealloc. but what you would want to do is also clean it up in - (void)didReceiveMemoryWarning (be careful not to use self.view in didReceiveMemoryWarning otherwise that'll reload the view :) ). then you could use lazy loading to reload it in viewDidLoad (i.e. if it doesn't already exist then initialise the data, otherwise don't).
of course, you can't do any initialisation in init that depends on the view being present.. viewDidLoad is the place for that.