loadView vs viewDidLoad

For programatic UI setup

July 17, 2015

I was talking to Justin Driscoll on Twitter on setting up the view hierarchy in code, and whether the best place to do that was loadView or viewDidLoad (Update: when you inherit directly from UIViewController).

Developers who choose to setup their view hierarchy in loadView do so like this:

override func loadView() {
    let v = UIView()
    let subview = UIView()
    v.addSubview(subview)
    positionSubviewWithEitherFramesOrConstraints()
    self.view = v
}

Here is one example of this strategy.

Developers who choose to setup their view hierarchy in viewDidLoad do so like this:

override func viewDidLoad() {
    let v = self.view
    let subview = UIView()
    v.addSubview(subview)
    positionSubviewWithEitherFramesOrConstraints()
}

Here is an example of this strategy.

If we setup the view hierarchy in viewDidLoad and we want a custom view as the root view, we should also implement a custom loadView:

override func loadView() {
    self.view = MyCustomView()
}

Both of these strategies work. So which one is better?

What does Apple recommend?

Apple’s documentation does talk about setting up the view hierarchy in code without using Interface Builder.

View Controller Programming Guide:

If you prefer to create views programmatically, instead of using a storyboard, you do so by overriding your view controller’s loadView method. Your implementation of this method should do the following:

1. Create a root view object. …

2. Create additional subviews and add them to the root view.

   For each view, you should:

   1. Create and initialize the view.
   2. Add the view to a parent view using the addSubview: method.

3. If you are using auto layout, assign sufficient constraints to each of the views you just created to control the position and size of your views. Otherwise, …

4. Assign the root view to the view property of your view controller.

UIViewController:

If you cannot define your views in a storyboard or a nib file, override the loadView method to manually instantiate a view hierarchy and assign it to the view property.

…

You can override [the loadView] method in order to create your views manually. If you choose to do so, assign the root view of your view hierarchy to the view property.

…

If you want to perform any additional initialization of your views, do so in the viewDidLoad method.

So, as per Apple’s documentation, the recommended place to setup the view hierarchy is loadView.

Apple suggests we do “any additional initialization” of our views in viewDidLoad. I interpret that as: If you’d like to setup additional views on top of what’s in your storyboard or xib, do it in viewDidLoad.

While it’s not entirely clear what “additional initialization” is meant to be done in viewDidLoad, it’s quite clear that loadView is the recommended override point for setting up the view hierarchy in code.

Practically speaking

I see a lot of code that sets up the view hierarchy in viewDidLoad without setting up the views in Interface Builder. Though that doesn’t strictly follow what Apple recommends we do, that’s perfectly safe, because of the way in which a view controller loads its view.

When the view property of a UIViewController is accessed, a simplified pseudocode for returning a view looks something like this ¹:

1. If there’s already a view set, return that view

2. Call loadView

   The default implementation of loadView in UIViewController does something like this ²:

   • If there’s an associated storyboard / xib,
     • Load the view hierarchy from the storyboard / xib
     • Assign the root view to the view property
   • If there’s no associated storyboard / xib
     • Create a UIView instance with a default frame and autoResizingMask
     • Assign that view to the view property

3. For any layout guides created from a storyboard / xib, setup Auto Layout constraints between the layout guides and the view of the view controller

4. Call viewDidLoad

So, when you’re not using a storyboard or xib, viewDidLoad gets called almost immediately after loadView. In practice, it makes no difference whether you setup the view hierarchy in loadView or viewDidLoad.

I personally prefer doing it in loadView, which is consistent with Apple’s documentation, but I have seen a lot of code that does it in viewDidLoad.

To conclude, we can set up the view hierarchy in either loadView or viewDidLoad, as long as we’re aware of when which method gets called. The most important thing to remember is that if you override loadView, you should set the view property, while if you override viewDidLoad, you should only read the view property, which should have been already set.

(Update: Note that this post applies only when your view is a direct subclass of UIViewController. When you’re inheriting from a subclass of UIViewController, like say UITableViewController, you should build up your view hierarchy in viewDidLoad.)