SOLVED BELOW
I'm reading this article from raywenderlich blog:
http://www.raywenderlich.com/23037/how-to-use-instruments-in-xcode
to learn about instruments and figure out if I´m doing something wrong in some old projects.
I've seen that in one particular point of my code, when I'm showing a modal view that eventually is closed, the memory allocated remains there. As you can see in the following image.
The execution have 4 marks generated.
Between the 2n and the 3t mark, the view is showed, as you can see, new memory is allocated.
But between the 3t and the 4th, I've called dismissViewController, and the view no longer remains. But the memory remains allocated.
All the properties, are created as strong (probably no the best approach):
And I´ve an NSTimer, that is initialized in viewDidLoad method, and set to nil at viewWillDisappear:
[self.secondTimer invalidate];
self.secondTimer = nil;
So, do you have any idea about what's happening? From what I know, even the properties are declared as strong, when the UIViewController is released, all of them are going to be released to.
EDIT
Thanks to all, with the information I provided, wasn't enough.
As you can see, QRViewController inherits from BaseViewController.
This controller had a delegate defined as strong storage, terrible.
So that's all.
In the view controller hierarchy, self.view holds ALL his subviews with strong, so everything under self.view (Probably all your IBOutlet properties) can switch to weak. That probably won't solve the problem though.
What might help you is the fact that any block you have holds every single object used in that block as a strong, to make sure the block can run it's code at the time being. If nothing holds that block (like a animationWithDuration:) than no worries. But if you have any block that an object is holding (Like and object's "completion-block" or any other creative use of blocks), everything within that block will be strong, and there's a chance you create a retain cycle that way. For example: the presenting view controller is calling the presented view controller with a completion block, and in that block you use self. Now presented VC is holding a block to perform on dismiss, and the block holds the presenting VC. When dismissed you will end up with a VC that holds a block that holds a VC that holds the presented VC....
A simple solution would be to give the block a weak version of self and only when the block executes, make it strong for the time of running the block (To avoid dealloc while running the block):
__weak myViewController *weakself = self;
[self.someObject setBlockHandler:^(BOOL foo){
myViewController *strongself = weakself;
if (strongself) {
// Do whatever...
}
}];
It's difficult to pinpoint precisely the problem, but usually when things like this happen to me, it winds up being one (or a few) "root" culprits -- you find that one, clear it up, and then lots of others clear up too. So one strategy you can try is to sift through the Instruments data looking for any sort of "hierarchy" (think about how your app is structured and how the objects relate to each other) and look for objects closer to the base, then cross-reference against your code to see if they might have a retain cycle or some other such issue.
One immediate change I would make would be to change your IBOutlet declarations from strong to weak. For the most part, IBOutlet properties should be weak, for objects that are within a hierarchy. So if say you've got some UILabel within your xib's main view, that label should be weakly-retained so as to avoid a retain cycle. But if say that UILabel is standing alone as a root item within the xib, then it would need a strong reference. I'm going to guess most (if not all) of your IBOutlets there are within a hierarchy, so make them weak and try again. It may not solve all the leaks, but see if it makes any difference.
This is called Abandoned Memory, check this link.
TIP:
If you are navigating between view controllers, and you perform the navigation inside a closure, you should use a weak or unowned version of self, example:
//Swift 2.1
//Performing naivgation on the main thread for responsiveness:
dispatch_async(dispatch_get_main_queue(), {[weak self] () -> Void in
if let weakSelf = self{
weakSelf.performSegueWithIdentifier("myOtherView", sender: weakSelf)
}
})
Also, when dismissing the view controller is the same:
dispatch_async(dispatch_get_main_queue(), {[weak self] () -> Void in
if let weakSelf = self{
weakSelf.dismissViewControllerAnimated(true, completion: nil)
}
})
The posted link above shows a practical example on how to catch abanodend memory using Xcode Instruments.
Related
In iOS 7 I used to define weak reference to popover controller inside my view controller (which displays in popover). I used this reference to popover controller in order to dismiss popover programmatically. Also, I defined delegate of popover controller in order to track dismissing events (popoverControllerShouldDismissPopover etc).
In iOS 8 it stops working. Investigation shows that weak reference point to nil after some point. The delegate stops working as well (as I understand it's because delegate was defined in popover controller which got destroyed in iOS 8 for some reason after popover displays).
Problem was solved by changed property to be strong reference.
For some popovers (I have bunch of them) I had to add strong reference only for the reason to keep popoverController alive because I need the delegate to work. It's obvious hack. I added property which I don't really need nor use.
Could you please clarify if it's right approach. My concern is that strong reference may lead to memory leaks. Also I don't quite understand why popoverController get destroyed in iOS 8 while popover still on the screen.
This is my view controller with weak property. After changing weak to strong to start working well under iOS 8:
#interface TFDSuggestionViewController : UIViewController
...
#property (weak, nonatomic) UIPopoverController *myPopoverController;
...
#end
This is how I assign value to my property and delegate in prepareForSegue method in calling view controller:
- (void)prepareForSegue:(UIStoryboardSegue *)segue sender:(id)sender
{
if ([segue.identifier isEqualToString:#"suggestions"]) {
TFDSuggestionViewController *suggController = ((TFDSuggestionViewController *)[segue destinationViewController]);
suggController.myPopoverController = ((UIStoryboardPopoverSegue *)segue).popoverController;
((UIStoryboardPopoverSegue *)segue).popoverController.delegate = self;
}
}
Thank you for you advice!
In the old days of manual memory management there was something called a reference count. It was essentially the number of times an object was retained (strong reference) by other objects or the App. In the more recent ARC (Automatic reference counting) we no longer need to do [object retain] and [object release]. These actions are handled for us by the compiler.
Now to your situation. A weak reference does not increase the reference count of the object you are referencing. So if you create an object in a scope, assign a weak reference to it, then leave the scope your object's reference count is 0.
-(void)SomeMethod
{
ClassObject *object = [[ClassObject alloc] init];
//object's internal reference count is now 1
self.myPopoverController = object;
//Since myPopoverController is a weak reference, object still has reference count of 1
//Some other code that does things and stuff.
}
//We just closed the scope, so object's reference count is now 0
//ARC is free to release the object to free it's memory, causing any
//weak references to return nil
In the example above it shows a very simple object life cycle. Once you understand the life cycle you can see why a weak reference will do you absolutely no good in this situation.
As to why it worked in iOS7 and not in iOS8 the only answer that I have is that iOS8 is likely much more efficient in garbage collection. If you ran it a million times in iOS7 I'm sure you would find at least one example of the exact same problem happening. It was a flaw in the code that the new OS makes more prevalent.
If you want the object to stay alive you need to have at least one strong reference to it. The only precaution is that when you call dismiss you should nil the strong reference. Then there should be no adverse memory issues to resolve.
Another bit that is very important. The UIPopoverController is not the same object as what is visible on screen. What is visible on screen is the UIPopoverController.view. The view is still retained by the view hierarchy, but the controller needs to be retained by you in order for it not to get released. Once the UIPopoverController is released the view's delegate will be nil since view.delegate is also a weak reference.
Study the object lifecycle. It will help you avoid garbage collection problems that will definitely arise in the future as the OS gets more and more efficient in memory handling.
In my app I'm polling a web service for status updates, using a completionHandler block and making changes to the current view based on returned results when the callback executes.
- (void) tickTimer
{
[MyWebService myWebMethod:param1 completionHandler:^(NSString *result) {
// does view still exist?
[self myUpdateMethod];
// does property still exist?
self.theResult = result;
// does child view still exist?
_txtUpdate.text = result;
}];
}
But in the interim, it's possible that the view may have been unloaded as the user navigates elsewhere.
So a couple of questions:
What happens to a view when a new one is loaded and it gets pushed to the background? I imagine it gets garbage collected at some point, but how do I tell if it's still safe to access by any of the references above, and what would happen if it's not?
If the view does still exist, how do I tell if it is also still the foreground view?
So, blocks create strong references to all objects pointers that are referred to in their closure. Due to this, your block is going to force [self] to stay in memory until the block is destroyed. If this isn't the behavior you want you should create a weak pointer to self and refer to it inside of the block:
__weak typeof(self) weakSelf = self;
So a couple of questions:
What happens to a view when a new one is loaded and it gets pushed to
the background? I imagine it gets garbage collected at some point, but
how do I tell if it's still safe to access by any of the references
above, and what would happen if it's not?
If your view stays in the view hierarchy, it will stay in memory. Once there are no more references to the view it will be dealloced.
If you use a weak pointer like outlined above, then [weakSelf] will be nil if the view has been dealloced
If the view does still exist, how do I tell if it is also still the
foreground view?
I'm not sure what you mean by foreground view, but if you want to see if it's still in the view hierarchy then you can check the property -(UIView *)superview. If superview is nil, then it's not on the screen
If you use ARC right, it will not let you use deallocated viewcontroller.
You can use viewDidAppear and viewDidDisappear methods to know visible yours viewcontroller or not.
I pass a completion block to my method, this completion block will be called in the background when a network request is finished. Unfortunately, if the calling object is deallocated in the meantime, the app crashes:
ViewController (which may be deallocated because it's popped from the navigation stack) code:
__unsafe_unretained ViewController *weakSelf = self;
[[URLRequester instance] sendUrl:url successBlock:^(id JSON) {
[weakSelf webserviceCallReturned:JSON];
}];
URLRequester-Code (made simpler, of course):
- (void)sendUrl:(NSString *)urlAfterHost successBlock:(void (^)(id))successBlock {
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
sleep(2);
successBlock(nil);
return;
});
}
If, in this 2 seconds, the ViewController gets popped from the navigation stack, the app crashes. What am I missing?
When you use __unsafe_unretained, then the reference remains around even after the object is deallocated. So if the view controller gets popped, then weakSelf is now pointing to a deallocated object.
If you change it to __weak instead, then when the view controller gets deallocated, it will set weakSelf to nil, and you'll be fine. You don't even need to do a check for if weakSelf is set to anything, because calling a method on nil has no effect.
It seems quite a few people think that 'self' inside a block must always be a weak (or unretained) copy. That's usually not the case**.
In this situation the misunderstanding is causing the crash, by leaving a zombie around. The right thing to do is to refer directly to self in the block (not unsafe_unretained, not weak), just like you'd like normal code to look. The effect will be that the block will retain the instance pointed to by 'self' -- the view controller in this case -- and it won't be destroyed until the block is destroyed (presumably by the url requester).
Will it be damaging to the view controller to process the result of the web request even though it has been popped? Almost certainly not, but if you think it will be, check for that condition in the block.
if (![self.navigationController.viewControllers containsObject:self])
// I must have been popped, ignore the web request result
// Re the discussion in comments, I think a good coder should have misgivings about
// this condition. If you think you need it, ask yourself "why did I design
// my object so that it does something wrong based on whether some other object
// (a navigation vc in this case) contains it?"
// In that sense, the deliberate use of weakSelf is even worse, IMO, because
// it lets the coder ignore _and_obscure_ an important question.
else {
// do whatever i do when the web request completes
}
**The need for weak or unretained pointers in blocks stems from the fact that blocks will retain the objects they refer to. If one of those objects directly or indirectly retains the block, then you get a cycle (A retains B which retains A) and a leak. This can happen with any object to which the block refers, not just 'self'.
But in your case (as in many) the view controller referred to by self does not retain the block.
The good practice in using block (especially delayed ones) is to make a local copy of the block in the calling method. In your case it should be done in -(void)sendUrl:successBlock:
successBlockCopy = [successBlock copy];
and then call
successBlockCopy(nil);
It should retain your viewController for a while until completion.
Also it is better to use __weak instead __unsafe_unretained to avoid problems with suddenly released objects.
An application that I am working on, that uses ARC and needs to support iOS 4.3 and iOS 5, declares every outlet as #property (strong, nonatomic) IBOutlet in the .h file.
e.g.
// myClass.h
#property (strong, nonatomic) IBOutlet UITextView *myTextview;
I know that with ARC only properties which do not hold a strong reference to an object are released.
As a result, the App relies on - (void)viewDidUnload to set the property myTextview to nil.
i.e.
// myClass.m
- (void)viewDidUnload
{
[super viewDidUnload];
self.myTextview = nil;
}
I know, from Apple's Documentation, that Outlets should generally be weak except those from File's Owner ( i.e. A Runtime Object that owns the contents of the iOS Storyboard scene) to Top-Level Objects (my rule of thumb is to use anything that appears in the window with File's Owner, First Responder and View).
Anything I add to the view will be a subview and thus is retained by it's direct superview, meaning a weak reference should be used.
I am also aware that - (void)viewDidUnload is deprecated in iOS 6 and is not called.
1st Question : What are the issues with taking the approach of declaring every outlet as a strong property and setting it to nil in viewDidUnload, apart from the fact that viewDidUnload is deprecated in iOS 6?
My intuition tells me that it is because situations arise where you can set a pointer to nil, before viewDidUnload is called. So you should, to free up memory on the heap. Is there a noticable performance change if this is the case?
2nd Question : Should I go back throughout the project and change strong to weak? Why? Is it worth the time?
3rd Question : If I was to declare the property in a class extension, to 'hide' it, how does this affect my rule of thumb for deciding on when to use strong or weak.
I know there are many threads here that discuss this issue. But many I've found are out of date, and do not address this issue directly. Thanks.
First, a few of your presumptions need addressing:
I know that ARC only releases properties which do not hold a strong
reference to an object. As a result, the App relies on -
(void)viewDidUnload to set the property myTextview to nil.
Not quite. ARC never retained weak properties in the first place. As for strong properties, ARC still releases them, but not until dealloc is called.
viewDidUnload was never used to prevent leaks. It was essentially an optimization, and one that Apple decided was no longer worth the trouble. To understand, consider the standard lifecycle of a pre-iOS6 view controller:
1. Allocated
2a. View Loaded
2b. View Unloaded
3. Deallocated
Where 2a and 2b could be repeated any number of times. For example, a view controller at the bottom of a navigation stack (its view being hidden) could have its view unloaded in low memory situations. It would then be reloaded the next its view became visible.
The method was essentially saying "Hey view controller programmer, we're running low on memory, and nobody can see me anyways, so I'm releasing my view. If you could do the same for your strong properties, that would be great."
That process had subtleties and was generally confusing. As a result of the tediousness, Apple deprecated it. Views are no longer unloaded, so there's no point in implementing it. The key point is that all your strong properties will still be released in ARC's dealloc method.
I know that Outlets should generally be weak...
Why do you know that? There's nothing special about outlets. The 'IBOutlet' keyword is really just for Xcode's benefit when designing things with its visual tools. It has no effect on the compiled code. So, when thinking about strong vs weak outlets, use the same considerations that you do for any other property, namely "do I need this to exists, or am I okay with it disappearing on me?".
What are the issues with taking the approach of
declaring every outlet as a strong property and setting it to nil in
viewDidUnload, apart from the fact that viewDidUnload is deprecated in
iOS 6?
There are no issues with that. If you want your properties to exists as long as your controller, then use strong. viewDidUnload has nothing to do with this. On older iOS versions, you could release strong outlets in viewDidUnload if you want.
Should I go back throughout the project and change
strong to weak? Why? Is it worth the time?
Again, just use whichever qualifier makes sense for your use case. You're almost always safe using strong for you outlets.
If I was to declare the property in a class extension, to 'hide' it,
how does this affect my rule of thumb for deciding on when to use
strong or weak.
There's no difference.
1st question: The outlets are subviews of the main view which is a property of the view controller. If you declare them as weak, they have a retain count of 1, since they are subviews. If the view is released, they are also released. If you declare them as strong, they have a retain count of 2, since they are subviews, and strong properties of the view controller. So they are only released when the view controller is released (which releases also its view and its subviews). To my understanding, the difference is that when the view of a view controller is released, e.g. when it is not presented and memory is low, the outlets still allocate memory when they have been declared as strong.
2nd question: I believe in most circumstances it does not matter.
3rd question: I believe if you declare properties in a class extension, the simply belong to the class, and thus there is no difference in handling them compared to "real" class properties.
My approach would be to make sure not to declare properties as strong that could result in retain cycles, what happens, i.e., if you declare a delegate as strong.
I have two View Controllers, one with a grid of buttons, and the second one is a detailed description based on the button the user presses.
I'm using "Mark Heap" in Instruments allocations tool and finding that my app is increasing in memory after the user sees the detailed View Controller and then presses the back button in the navigation bar. IT should result in a net change of zero for memory... I'm using ARC, xcode 4.2.1, deploying to ios 5.0+
When I load the new ViewController after the button press, I load some data images in a background thread. Is it possible that because I'm pressing the back button quickly, that data is still being loaded in the background thread and is never released from memory?
from the slides from the class i had that described memory cycles problem you may be facing:
what if you had the following property of a class?
#property (strong, nonatomic) NSArray* myBlocks; // array of blocks
and then tried to do the following in one of that class's methods?
[self.myBlocks addObject:^() {
[self doSomething];
}];
all objects referenced inside a block will stay in the heap as long as the block does. (in other words, blocks keep a strong pointer to all objects referenced inside of them.)
in this case, self is an object referenced in this block. thus the block will have a strong pointer to self. but notice that self also has a strong pointer to the block (through its myBlocks proeprty)
THIS IS A SERIOUS PROBLEM!
neither self nor the block can ever escape the heap, now. that's because there will always be a strong pointer to both of them (each other's pointer). this is called a memory "cycle".
solution:
local variables are always strong. that's okay, because when they go out of scope, they disappear, so the strong pointer goes away. but there's a way to declare that a local variable is weak. here's how ...
__weak MyClass* weakSelf = self;
[self.myBlocks addObject:^{
[weakSelf doSomething];
}];
this solves the problem because now the block only has a weak pointer to self. (self still has a strong pointer to block, but that's okay) as long as someone in the univese has a strong pointer to this self, the block's pointer is good. and since the block will not exist if self does not exist (since myBlocks won't exist), all is well!