Is there a BOOL or some other way of knowing if viewDidLoad: has been called?
I need to change a view property every time my view has entered active, but if the view hasn't ever been loaded, I don't want to prematurely trigger viewDidLoad:. If there isn't way of easily telling if viewDidLoad: has been called, I'll simply add a BOOL called loaded set to NO in the view controller init, then change the view properties after entered active if loaded is YES or in viewWillAppear: if loaded is NO then set loaded to YES.
Use isViewLoaded. Other than that it does exactly what you want, there's not that much to say about it. The documentation is as simple as:
Calling this method reports whether the view is loaded. Unlike the
view property, it does not attempt to load the view if it is not
already in memory.
Perhaps you should init your UIView in viewDidLoad, and then change it in whichever way you need to inside viewWillLayoutSubviews.
Here's the pedantic answer to this question. If you want to know when viewDidLoad has been triggered, you have to implement viewDidLoad in your view controller
- (void)viewDidLoad
{
[super viewDidLoad];
viewDidLoadCalled = YES; // Not actually the best way to do this...
// Set up more view properties
}
But as Tommy says, you actually need to use isViewLoaded. This gets around the problem of doing a check like
if (!self.view) {
// do something
}
which inadvertently loads the view by virtue of asking about it.
Be aware that by the time viewWillAppear: is called, the view will always have loaded. Also, on older (pre-iOS 6 I think) releases, the view can unload and be reloaded many times over a view controller's lifetime. Refer to the very nice Big Nerd Ranch view lifecycle diagram for the old behavior. It's almost the same in iOS 6+, except that the view doesn't unload under low memory conditions and viewDidUnload doesn't get called:
Related
This is driving me nuts. I am using three20's TTTableViewController and when I get a memory warning, the screen goes white. Now, after reading on the three20 google group is seems that the tableView got released. But, I cannot for the life of me figure out a check to see if that is the case, then create it again.
I was using the following because I thought it would fix the issue, but it seems that it doesn't satisfy the if statement:
- (void)viewDidAppear:(BOOL)animated {
[super viewDidAppear:animated];
// If we don't have a datasource we need to reset it
if (!self.dataSource) {
// Create datasource again
}
}//end
Does anyone know what to do when this happens? The google group has been no help.
Are you subclassing TTTableViewController? I haven't used it before, but assuming it's just like UITableViewController...
How does your "viewDidUnload" look like? Are you releasing the tableview here? If so, you need to create tableview in viewDidLoad to match it.
No need to check if dataSource is available in viewDidAppear, because if you read View programming guide, it explains that memory warning will call "viewDidUnload" to give you a chance to clean up data that are created in "viewDidLoad".
i had the same issue and it drove me crazy as well.
Nobody mentions it in the three20 docs, but you shouldn't use UIViewController's initWithNibName function to add subviews. If you do, a memory warning will release these subviews.
Try to move your code from initWithNibName function to viewDidLoad function. I have noticed that some code need to be kept in the initWithNibName, such as navigation styles. However, any subviews added to the controller's view should be in the viewDidLoad function.
In general you should be careful to set up views in viewDidLoad rather than the class constructor. For instance, you should set up your launcher view in viewDidLoad rather than the constructor of your launcher view controller, otherwise your launcher will become empty after a memory warning.
In the case of TTTableViewController however this does not (usually) apply because you don't set up the table view manually. I had the same problem you had, and eventually tracked it down: I had redefined viewWillDisappear: and forgot to call [super viewWillDisappear:animated]. This meant that some of the flags that the Three20 controller maintains about the state of the view were not updated correctly.
I also found that it was beneficial to redefine didReceiveMemoryWarning to call [self setEditing:NO] before calling super; I found that the state of the table view got confused otherwise (this is not relevant if you don't use edit mode for your table).
Finally, there is a bug in Three20 which means that tables in loading/empty/error mode will not be restored properly; see a discussion in the blog post by TwoCentStudios and a proposed fix on github.
Interview question: For example, I have 3 classes A,B & C. I navigate from A -> B -> C, while pushing viewDidLoad function calls automatically and during popping viewWillAppear get called. But would it be possible to call my viewDidLoad function while popping?
No, Its not possible.Only viewDidAppear and viewWillAppear will be called by itself.You can call it manually.
All the best.
The viewDidLoad method is called when the view just loaded. Then viewWillAppear is called by the navigation controller when it's about to display the view.
If you want some code to be executed when the view is about to be displayed, be it when it's being pushed or when the top one is being popped, it makes more sense to use the appropriate method viewWillAppear for that, instead of trying to call the viewDidLoad method at a time when it isn't appropriate.
Can't you just move whatever code you have in viewDidLoad to viewWillAppear?
If the question is just "how can we call viewDidLoad while popping?", then it's simple:
- (void) viewWillDisappear:(BOOL)animated
{
[self viewDidLoad];
}
Just as a side note, if you have a view controller stack like A->B, it's possible that viewDidLoad will be called automatically on A when popping B if, while B was on top, the navigation controller unloaded A's view (if the app received a memory warning, for example)
You should not call viewDidLoad manually, it's not designed to be handled this way. Use viewWillAppear, as other users already mentioned.
As for the question whether it may happen that viewDidLoad will be called upon popping from a higher view controller: I imagine that may happen when the device got a low memory warning and unloaded the view controllers further down in the navigation hierarchy. Then the OS has to reload the view, I haven't however tested this and it's possible that this will never happen. The OS only unloads views when they don't have a superview, I didn't check whether upon pushing, views down the hierarchy actually get removed from the hierarchy.
ViewDidLoad should be for view creation.
ViewWillAppear - for Data Interaction if Server Request is Asynchronous. like calling API or any functionality.or any functionality we like to call when view going to appear.
ViewDidAppear - for Data Interaction if Server Request is synchronous.
I have a navigation controller. One of the views adds custom subviews in its viewDidAppear:. I notice that the first time I navigate to an instance of this view controller after launching the app, viewDidAppear: invokes twice. If I pop this view off the stack and navigate to it again, viewDidAppear: invokes only once per appearance. All subsequent appearances invoke viewDidAppear: once.
The problem for me is that the first time I get to this view I end up with twice the number of subviews. I work around this problem by introducing a flag variable or some such, but I'd like to understand what is happening and how come I get two invocations in these circumstances.
You should never rely on -viewWillAppear:/-viewDidAppear: being called appropriately balanced with the disappear variants. While the system view controllers will do the best they can to always bracket the calls properly, I don't know if they ever guarantee it, and certainly when using custom view controllers you can find situations where these can be called multiple times.
In short, your -viewWillAppear:/-viewDidAppear: methods should be idempotent, meaning if -viewDidAppear: is called twice in a row on your controller, it should behave properly. If you want to load custom views, you may want to do that in -viewDidLoad instead and then simply put the on-screen (if they aren't already) in -viewDidAppear:.
You could also put a breakpoint in your -viewDidAppear: method to see why it's being called twice the first time it shows up.
maybe you invoke viewDidAppear in viewDidLoad (or some other stuff is going on there), since it's invoked only once during loading the view from the memory. It would match, that it's invoked two times only the first time.
This was not an iOS 5 bug, but a hidden behavior of addChildViewController:. I should file a radar for lack of documentation, I think
https://github.com/defagos/CoconutKit/issues/4
If you have a line like this in your AppDelegate
window = [[UIWindow alloc] initWithFrame:[[UIScreen mainScreen] bounds]];
make sure you DON'T have a "Main nib file base name" property in your plist set to "Window.xib" or whatever your custom window nib is named. If you do, remove that row from your plist and make sure you something like
yourRootVC = [[UIViewController alloc] init];
[window setRootViewController:yourRootVC];
in your AppDelegate after instantiating your window. In most cases, you could then safely delete the Window.xib as well.
You definitely should provide more info.
Is this the root view controller?
Maybe you initiate the navigation controller with this root view controller and then push it to the navigation controller once again?
Another solution that may have been your underlying cause: Be sure you are not presenting any new view controllers from within your viewWillAppear: method.
I was calling:
[appDel.window.rootViewController presentViewController:login animated:YES completion:nil];
from within viewWillAppear and seeing my originating VC's viewDidAppear: method called twice successively with the same stack trace as you mention. And no intermediary call to viewDidDisappear:
Moving presentViewController: to the originating VC's viewDidAppear: method cleared up the double-call issue, and so now the flow is:
Original viewDidAppear: called
Call presentViewController here
Original viewDidDisappear: called
New view is presented and no longer gives me a warning about "unbalanced VC display"
Fixed with help from this answer: https://stackoverflow.com/a/13315360/1143123 while trying to resolve "Unbalanced calls to begin/end appearance transitions for ..."
it's such an annoying problem, you'd think it runs once but then I now found out about this which is causing mayhem... This applies to all 3 (ViewDidAppear, ViewDidLoad, and ViewWillAppear), I am getting this when integrating with a payment terminal; once it finish calling the API, the window is being re-loaded when it's already on-screen and all it's memory is still there (not retained).
I resolved it by doing the following to all the routines mentioned above, below is a sample to one of them:
BOOL viewDidLoadProcessed = false;
-(void)viewDidLoad:(BOOL)animated
{
if (!viewDidLoadProcessed)
{
viewDidLoadProcessed = YES;
.
.
.
... do stuff here...
.
.
}
}
Repeat the above for all the other two, this prevents it from running twice. This never occurred before Steve Jobs died !!!
Kind Regards
Heider Sati
Adding [super viewDidAppear:animated]; worked for me:
//Called twice
- (void)viewDidAppear:(BOOL)animated{
}
//Called once
- (void)viewDidAppear:(BOOL)animated{
[super viewDidAppear:animated];
}
I am using UIImagePickerController to take a photo from the camera. However, the I find that randomly my calling controller (the one that is shown before the UIImagePickercontroller is shown) gets unloaded. I logged the viewDidUnload, and indeed it does get called. When the camera is done and dismissed, my controller's viewDidLoad will be called, unfortunately all the state is now gone. Things like text entered or things selected will be gone.
Obviously this is something to do with running out of memory. But is this behavior normal? Should I be handling it? This is NOT typically how modalViewController works. Usually you show it and dismiss it, everything should be intact.
What is a good way to avoid data lost in this case? Should I have to write a bunch of code to save the full state?
what's happening is that your view controller's didReceiveMemoryWarning is being called, and then since your view isn't visible, it's being unloaded.
the solution is that any data that wants to be persistent should be stored in your view controller rather than in your view.
you can prevent this from happening by providing an implementation of didReceiveMemoryWarning in your UIViewController class that doesn't call [super didReceiveMemoryWarning] which is where the unloading of the view happens, but it's still a good thing to try to understand what's going on.
It's really not advisable to override -didReceiveMemoryWarning so that UIViewController can't deallocate the view because then you may run into a "real" problem of low memory and your app will be forced to quit by the system.
What you really should do is store the text and other bits of entered data from the view in your -didReceiveMemoryWarning method (making sure to call super). Then in -viewDidLoad you can put the text and other bits back into the UI.
This will lighten the memory usage and reduce the likelihood of your app being forced to quit because there's no more memory left. All the while, the user won't know it ever disappeared!
My solution to the same issue described above was to declare an instance variable in my view controller which stores the contents of the UITextView should the view get unloaded. Thus in my viewWillDissapear method I save the contents of UITextView.text into my instance variable and in my viewWillAppear method I restore the contents.
- (void)viewWillAppear:(BOOL)animated
{
if (messageTextString != nil)
{
textEdit.text = messageTextString;
}
[super viewWillAppear:animated];
}
(void)viewWillDisappear:(BOOL)animated
{
self.messageTextString = textEdit.text;
[super viewWillDisappear:animated];
}
Background:
I have a tab bar application. Each tab contains navigation controller allowing the user to transition from one view to the other showing a drill down information of the data (each view is being handled by a view controller and each view controller class has didReceiveMemoryWarning method). Lists are populated by pulling the data from web services.
Problem:
When i use "Hardware > Simulate Memory Warning" option of iPhone Simulator, the didReceiveMemoryWarning method is called for ALL my view controllers - even the one which the user is viewing. I don't want to clear any content which is being used by the active view controller. How can I achieve that?
Which method should have the implementation to reload the data after the data was released because of memory warning? (I see that the view controller classes that contain a table view call viewDidLoad method when user comes back to that view, but if the view contains (say UIWebView) then viewDidLoad method is not called. Why is that?)
Edited (Friday 30 January 2009 - 03:10 PM)
(Note: I'm using Interface builder for creating views, and loadView method is commented out.)
So, when a view controller receives a memory warning message, these are the steps that are carried out:
Following method is called:
- (void)didReceiveMemoryWarning {
[super didReceiveMemoryWarning];
}
As a result of call to [super didReceiveMemoryWarning], [self setView:nil] gets automatically called?
If any resources should be cleared, then setView method should be overwritten to clear local resources.
[self setView:nil] is not called if the view is currently active (By default). Right? - I'm really curious which method takes this decision and how?
Can you please confirm. Plus, I was getting an error following this approach but adding myObject = nil after releasing myObject in dealloc method of controller class fixed the issue. Thanks.
This is an old question, but I don't see a proper answer, so here goes:
When a memory warning is received, -didReceiveMemoryWarning gets called in ALL view controllers, whether they are the "current" one or not. The view controllers are simply listening for the memory warning event broadcast.
If the view controller's view isn't being used at the time of the memory warning, the controller will unload it by setting the property to nil. How does it know if the the view is used? By the view's -superview property. If view.superview is nil, the view isn't part of any tree and can be unloaded safely.
Once that happens, the controller's -viewDidUnload gets called. This is the correct place to unload any outlets, and anything that will get re-created in -viewDidLoad.
So what is -didReceiveMemoryWarning for? Your controller might have objects that don't get instanced until accessed. For example, you could have a controller that sometimes needs a big chunk of data from a file, but not always. You could have a property set for it like this:
- (NSData*)bigChunkOfData {
// Get data from our instance variable _data, read from disk if necessary
if (_data == nil) {
_data = [[NSData alloc] initWithContentsOfFile:#"/path/to/data"];
}
return _data;
}
This will read the data from disk this first time, then keep it in an instance variable. Since the _data variable is created on demand, it's safe for us to unloaded it in low-memory situations: it'll just get created again next time we need it.
- (void)didReceiveMemoryWarning {
[super didReceiveMemoryWarning];
[_data release];
_data = nil; // <-- Very important: don't leave strong references dangling.
}
I do my clean up like this:
-(void)setView:(UIView*)view
{
[super setView:view];
if(view == nil)
{
// Our view has been cleared, therefore we should clean up everything
// we are not currently using
....
setView:nil is called by UIViewController in response to a memory warning, if that view is not currently visible - which is basically what you want to know.
EDITED
In answer to the follow ups:
Correct.
That's what I do, and it works for me.
Correct. The implementation of didReceiveMemoryWarning in UIViewController is what does this. If you don't override didReceiveMemoryWarning, then the base class implementation in UIViewController will be called - if you do override it, obviously you should call:
[super didReceiveMemoryWarning]
To ensure that I dont have to handle this for every single viewcontroller I write.. I have just made a Xcode ViewController template which provides guidelines on which objects to release and when..
more explanation here http://iphone2020.wordpress.com/2010/05/30/efficient-memory-handling-in-uiviewcontroller-part-1/
Hope it finds useful.
In regard to the view management and memory warnings:
UIKit doesn’t only allow navigation back from a view controller, but also allows navigation to other view controllers from existing ones.
In such a case, a new UIViewController will be allocated, and then loaded into view.
The old view controller will go off-screen and becomes inactive, but still owns many objects – some in custom properties and variables and others in the view property/hierarchy.
And so does the new visible view controller, in regard to its view objects.
Due to the limited amount of memory of mobile devices, owning the two sets of objects – one in the off-screen view controller and another in the on-screen view controller – might be too much to handle.
If UIKit deems it necessary, it can reclaim some of the off-screen view controller’s memory, which is not shown anyway; UIKit knows which view controller is on-screen and which is off-screen, as after all, it is the one managing them (when you call presentModalViewController:animated: or dismissModalViewControllerAnimated:).
So, every time it feels pressured, UIKit generates a memory warning, which unloads and releases your off-screen view from the view hierarchy, then call your custom viewDidUnload method for you to do the same for your properties and variables.
UIKit releases self.view automatically, allowing us then to manually release our variables and properties in our viewDidUnload code.
It does so for all off-screen view controllers.
When the system is running out of memory, it fires a didReceiveMemoryWarning.
Off-screen views will be reclaimed and released upon memory warning, but your on-screen view will not get released – it is visible and needed.
In case your class owns a lot of memory, such as caches, images, or the like, didReceiveMemoryWarning is where you should purge them, even if they are on-screen; otherwise, your app might be terminated for glutting system resources.
You need to override this method to make sure you clean up your memory; just remember you call [super didReceiveMemoryWarning];.
An even more elaborate explanation is available here: http://myok12.wordpress.com/2010/11/30/custom-uiviewcontrollers-their-views-and-their-memory-management/
Fortunately, the simulator has a handy function that allows you to put low-memory situations to the test. Put some NSLog() statements in both viewDidLoad and didReceiveMemoryWarning, like this:

- (void)viewDidLoad {
NSLog(#"viewDidLoad");
...
}
- (void)didReceiveMemoryWarning {
NSLog(#"didReceiveMemoryWarning");
}