Looking to understand the iOS UIViewController lifecycle

Question

Could you explain me the correct manner to manage the UIViewController lifecycle   In particular  I would like to know how to use Initialize  ViewDidLoad  ViewWillAppear  ViewDidAppear  ViewWillDisappear  ViewDidDisappear  ViewDidUnload and Dispose methods in Mono Touch for a UIViewController class

User · Answer

There s a lot of outdated and incomplete information here  For iOS 6 and newer only    loadView a  viewDidLoad a  viewWillAppear viewWillLayoutSubviews is the first time bounds are finalized viewDidLayoutSubviews viewDidAppear   viewWillLayoutSubviews b    viewDidLayoutSubviews b       Footnotes      a  - If you manually nil out your view during didReceiveMemoryWarning  loadView and viewDidLoad will be called again  That is  by default loadView and viewDidLoad only gets called once per view controller instance      b  May be called an additional 0 or more times

User · Answer

As of iOS 6 and onward  The new diagram is as follows

User · Answer

UPDATE  ViewDidUnload was deprecated in iOS 6  so updated the answer accordingly   The UIViewController lifecycle is diagrammed here      The advantage of using Xamarin Native Mono Touch  is that it uses the native APIs  and so it follows the same ViewController lifecycle as you would find in Apple s Documentation

User · Answer

Let s concentrate on methods  which are responsible for the UIViewController s lifecycle    Creation   -  void init  -  void initWithNibName  View creation   -  BOOL isViewLoaded  -  void loadView  -  void viewDidLoad  -  UIView   initWithFrame  CGRect frame  -  UIView   initWithCoder  NSCoder   coder Handling of view state changing    -  void viewDidLoad  -  void viewWillAppear  BOOL animated  -  void viewDidAppear  BOOL animated  -  void viewWillDisappear  BOOL animated  -  void viewDidDisappear  BOOL animated  -  void viewDidUnload Memory warning handling   -  void didReceiveMemoryWarning Deallocation  -  void viewDidUnload  -  void dealloc     For more information please take a look on UIViewController Class Reference

User · Answer

iOS 10 11  Swift 3 1 Swift 4 0   According to UIViewController in UIKit developers   1  loadView    This is where subclasses should create their custom view hierarchy if they aren t using a nib  Should never be called directly   2  loadViewIfNeeded    Loads the view controller s view if it has not already been set   3  viewDidLoad    Called after the view has been loaded  For view controllers created in code  this is after -loadView  For view controllers unarchived from a nib  this is after the view is set   4  viewWillAppear   animated  Bool   Called when the view is about to made visible  Default does nothing  5  viewWillLayoutSubviews    Called just before the view controller s view s layoutSubviews method is invoked  Subclasses can implement as necessary  Default does nothing   6  viewDidLayoutSubviews    Called just after the view controller s view s layoutSubviews method is invoked  Subclasses can implement as necessary  Default does nothing   7  viewDidAppear   animated  Bool   Called when the view has been fully transitioned onto the screen  Default does nothing  8  viewWillDisappear   animated  Bool   Called when the view is dismissed  covered or otherwise hidden  Default does nothing  9  viewDidDisappear   animated  Bool   Called after the view was dismissed  covered or otherwise hidden  Default does nothing  10  viewWillTransition to size  CGSize  with coordinator  UIViewControllerTransitionCoordinator   Called when the view is Transitioning   11  willMove toParentViewController parent  UIViewController    12  didMove toParentViewController parent  UIViewController    These two methods are public for container subclasses to call when transitioning between child controllers  If they are overridden  the overrides should ensure to call the super    The parent argument in both of these methods is nil when a child is being removed from its parent  otherwise it is equal to the new parent view controller   13  didReceiveMemoryWarning    Called when the parent application receives a memory warning  On iOS 6 0 it will no longer clear the view by default

User · Answer

The methods viewWillLayoutSubviews and viewDidLayoutSubviews aren t mentioned in the diagrams  but these are called between viewWillAppear and viewDidAppear   They can be called multiple times

User · Answer

Explaining State Transitions in the official doc  https   developer apple com library ios documentation uikit reference UIViewController Class index html  This image shows the valid state transitions between various view    will    and    did    callback methods  Valid State Transitions    Taken from  https   developer apple com library ios documentation uikit reference UIViewController Class Art UIViewController Class Reference 2x png

User · Answer

Haider s answer is correct for pre-iOS 6   However  as of iOS 6 viewDidUnload and viewWillUnload are never called   The docs state   Views are no longer purged under low-memory conditions and so this method is never called

User · Answer

All these commands are called automatically at the appropriate times by iOS when you load present hide the view controller  It s important to note that these methods are attached to UIViewController and not to UIViews themselves  You won t get any of these features just using a UIView   There s great documentation on Apple s site here  Putting in simply though    ViewDidLoad - Called when you create the class and load from xib  Great for initial setup and one-time-only work  ViewWillAppear - Called right before your view appears  good for hiding showing fields or any operations that you want to happen every time before the view is visible  Because you might be going back and forth between views  this will be called every time your view is about to appear on the screen  ViewDidAppear - Called after the view appears - great place to start an animations or the loading of external data from an API  ViewWillDisappear DidDisappear - Same idea as ViewWillAppear ViewDidAppear  ViewDidUnload ViewDidDispose - In Objective-C  this is where you do your clean-up and release of stuff  but this is handled automatically so not much you really need to do here

User · Answer

As per Apple s doc     Start Developing iOS Apps  Swift       Work with View Controllers     Understand the View Controller Lifecycle     viewDidLoad     Called when the view controller   s content view  the top of its view hierarchy  is created and loaded from a storyboard          Use this method to perform any additional setup required by your view controller       viewWillAppear     Called just before the view controller   s content view is added to the app   s view hierarchy  Use this method to trigger any operations that need to occur before the content view is presented onscreen      viewDidAppear     Called just after the view controller   s content view has been added to the app   s view hierarchy  Use this method to trigger any operations that need to occur as soon as the view is presented onscreen  such as fetching data or showing an animation       viewWillDisappear     Called just before the view controller   s content view is removed from the app   s view hierarchy  Use this method to perform cleanup tasks like committing changes or resigning the first responder status       viewDidDisappear     Called just after the view controller   s content view has been removed from the app   s view hierarchy  Use this method to perform additional teardown activities

User · Answer

This is for latest iOS Versions Modified with Xcode 9 3  Swift 4 1   Below are all the stages which makes the lifecycle of a UIViewController complete     loadView   loadViewIfNeeded   viewDidLoad   viewWillAppear   animated  Bool  viewWillLayoutSubviews   viewDidLayoutSubviews   viewDidAppear   animated  Bool  viewWillDisappear   animated  Bool  viewDidDisappear   animated  Bool    Let me explain all those stages    1  loadView  This event creates loads the view that the controller manages  It can load from an associated nib file or an empty UIView if null was found  This makes it a good place to create your views in code programmatically       This is where subclasses should create their custom view hierarchy if they aren t using a nib     Should never be called directly    Only override this method when you programmatically create views and assign the root view to the view property   Don t call super method when you override   loadView   2  loadViewIfNeeded  If incase the view of current viewController has not been set yet then this method will load the view but remember  this is only available in iOS   9 0  So if you are supporting iOS  lt 9 0 then don t expect it to come into the picture      Loads the view controller s view if it has not already been set    3  viewDidLoad  The viewDidLoad event is only called when the view is created and loaded into memory but the bounds for the view are not defined yet  This is a good place to initialise the objects that the view controller is going to use       Called after the view has been loaded  For view controllers created in code  this is after -loadView    For view controllers unarchived from a nib  this is after the view is set    4  viewWillAppear   This event notifies the viewController whenever the view appears on the screen  In this step the view has bounds that are defined but the orientation is not set       Called when the view is about to made visible  Default does nothing    5  viewWillLayoutSubviews  This is the first step in the lifecycle where the bounds are finalised  If you are not using constraints or Auto Layout you probably want to update the subviews here  This is only available in iOS   5 0  So if you are supporting iOS  lt 5 0 then don t expect it to come into the picture       Called just before the view controller s view s layoutSubviews method is invoked    Subclasses can implement as necessary  The default is a nop    6  viewDidLayoutSubviews  This event notifies the view controller that the subviews have been setup  It is a good place to make any changes to the subviews after they have been set  This is only available in iOS   5 0  So if you are supporting iOS  lt 5 0 then don t expect it to come into the picture      Called just after the view controller s view s layoutSubviews method is invoked    Subclasses can implement as necessary  The default is a nop    7  viewDidAppear  The viewDidAppear event fires after the view is presented on the screen  Which makes it a good place to get data from a backend service or database      Called when the view has been fully transitioned onto the screen    Default does nothing   8  viewWillDisappear  The viewWillDisappear event fires when the view of presented viewController is about to disappear  dismiss  cover or hide behind other viewController  This is a good place where you can restrict your network calls  invalidate timer or release objects which is bound to that viewController      Called when the view is dismissed  covered or otherwise hidden    9  viewDidDisappear  This is the last step of the lifecycle that anyone can address as this event fires just after the view of presented viewController has been disappeared  dismissed  covered or hidden      Called after the view was dismissed  covered or otherwise hidden     Default does nothing   Now as per Apple when you are implementing this methods you should remember to call super implementation of that specific method      If you subclass UIViewController  you must call the super implementation of this method  even if you aren t using a NIB    As a convenience  the default init method will do this for you  and specify nil for both of this methods arguments   In the specified NIB  the File s Owner proxy should have its class set to your view controller subclass  with the view outlet connected to the main view  If you invoke this method with a nil nib name  then this class  -loadView method will attempt to load a NIB whose name is the same as your view controller s class  If no such NIB in fact exists then you must either call -setView  before -view is invoked  or override the -loadView method to set up your views programatically    Hope this helped  Thanks   UPDATE  -  As  ThomasW pointed inside comment viewWillLayoutSubviews and viewDidLayoutSubviews will also be called at other times when subviews of the main view are loaded  for example when cells of a table view or collection view are loaded   UPDATE  - As  Maria pointed inside comment  description of loadView was updated

[ios] Looking to understand the iOS UIViewController lifecycle

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