I have a certain code that is executed on the viewDidLoad() of a UIViewController I have. The problem is that it is only loaded for the first time only. After it is loaded the first time and I switch to some other view and come back to this view, it doesn't call the viewDidLoad again. So where should I put this code at so that everytime this view is displayed it executes this code?
i guess in viewWillAppear
good luck
You should put it in - (void)viewDidAppear:(BOOL)animated . This gets called every time your view comes on screen.
Please refer to life cycle of iphone application
- (void)viewDidLoad
should only be called once.
so you should use,since this method is called each & every time the View is called
- (void)viewWillAppear:(BOOL)animated
Related
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:
Please let me know at what times init and loadView method gets called.
To my knowledge init method gets called only once when view is initialized and loadView is called anytime view is loaded. So, even if you are pushing a new view in the view stack and then popping it then also the loadView of the poped up view should get called. But when I am running my code in debugging mode, both of these methods are getting called once, irrespective of how many times I am loading the same screen. Please let me know if I am missing something.
you are right at some points :)
The init method is being called when the ViewController object is instantiated. The loadView method gets called every time a ViewController should load its view into memory. This can happen before the view is displayed for the first time OR when it should be displayed for a second, third,... time but had been removed from memory before. (this might happen if your app runs out of memory.)
If you want to execute some code every time the view becomes visible, you should have a look at the methods viewWillAppear/viewWillDisappear/viewDidAppear/viewDidDisappear.
loadView is called when you access the view property of your view controller and it's nil.
If the view has been unloaded (viewDidUnload has been called for memory purpose) then loadView will be called again. If not it will not be called.
What you want is viewWillAppear: or viewDidAppear:.
In a UIViewController subclass, I have the following methods:
-(void)viewWillAppear:(BOOL)animated {
[super viewWillAppear:animated];
// do something
myTextField.text = #"Default";
}
- (void)viewDidAppear:(BOOL)animated {
[super viewDidAppear:animated];
// do something
[myTextField selectAll:self];
[myTextField becomeFirstResponder];
}
- (void)viewDidLoad {
[super viewDidLoad];
// do something
myTextField.delegate = self;
}
The NIB has been created using Interface Builder. The corresponding view controller object is pushed by the navigation controller through pushViewController.
The inteded behavior is to show a default text entry in a text field, to select the entire text and to set the text field as first responder. [Edit: I've noticed that selecting all and making first responder makes no sense as the selection would dissapear; still, I wonder why the methods behave as described next.]
However, while methods viewDidLoad and viewWillAppear are called, the method viewDidAppear is not called. Can anybody tell me why? Most questions I found on the web and here deal with both viewWillAppear/viewDidAppear are not working; I also understood that in subviews or programmatically created views these methods are not evoked; but this does not apply in case and also I wonder why one of these "lifecycle" methods is evoked and the other not.
Any idea? Thanks!
I had this issue happen to me: viewWillAppear was being called but viewDidAppear was not!
I finally figured out that this was because I had a tabBarController where I overloaded it's own viewDidAppear and forgot the [super viewDidAppear:animated];
It threw off every VC in every tab! adding that line back in fixed it for my other VC's.
Hope this helps someone!
There is one case when viewWillAppear will be called but viewDidAppear will not.
Suppose you have two viewControllers and you push from the first to the second controller. Then, using the swipe, you want to go back to the first one, both controllers are displayed at the same time and at that moment viewWillAppear will be called from the first controller.
This way, you do not finish the swipe and stay on the second controller and viewDidAppear will not be called from the first controller.
I had the same problem.
I had copy/pasted viewDidAppear to create viewWillAppear but had forgotten to change the super.viewDidAppear() call. This then seemed to stop viewDidAppear from being called.
It sounds like somewhere in your code you have missed or messed-up a call to the superclass.
The call to viewDidAppear: should always follow viewWillAppear: unless you are doing something custom, which you say you don't. I don't know why it doesn't work but here are a few ideas:
Could it be that you are doing something strange in one of the delegate methods for UITextFieldDelegate? It's unlikely that it would affect viewDidAppear: being called but it could be a culprit.
Have you loaded a lot of stuff into memory before pushing the view? I'm not sure what would happen if you got a memory warning between viewWillAppear: and viewDidAppear:.
Have you tried to do a Clean? Sometimes that can help.
In cases like these when it should work I usually create a new class and the introduce the functionality one at a the time to see if I can get it work that way. I tried your code in a new Navigation Based project where I added a new UIViewController with an outlet to the text field. Then I pasted the code from the question and it did work as expected.
This can be because you added a child view controller to your parent VC in viewDidLoad or viewWillAppear. The child's appearance prevents the call to viewDidAppear.
This is a crazy thing to do, and I only know because this was a bug in my code. I meant to add the child VC to this VC, not the parent VC.
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];
}
Why in my UIViewController the function
- (void)viewWillAppear:(BOOL)animated
fired at two times.... Is it wrong ?
Can anyone help me?
Thanks in advance..
I used to have the same problem. If you are using Navigation Based project, the IB sets by default your RootViewController as your first view on the NavigationController. So if you push the same view again on the NavigationController in application:didFinishLaunchingWithOptions:, it will fire ViewWillApper for the second time.
If this is not the case, try putting a break point and discover when it fires it.