UIKit – loadView vs viewDidLoad

Weak, unowned or sturdy subviews?

I’ve bought various emails and tweets about this matter, so I made a decision to put in writing about it, as a result of it’s actually arduous to discover a correct reply for this query on the web. There are some nice posts and programming guides, some some articles are a bit older, nonetheless many individuals are asking the weak vs sturdy IBOutlet query even on the official boards, however noone actually explains the explanations, even on the boards they solely advocate this WWDC session video. So what is going on on right here? 🤔

I did some research on the subject and the very very first thing that we must always state is that this: Apple eliminated the viewDidUnload methodology in iOS6 and from that model the iOS view controller lifecycle modified a bit. If you do not know a lot concerning the lifecycle strategies (demystified), you must learn this text. This was fairly a giant change and Apple additionally touched their inner view administration. Earlier than iOS6 it was a standard observe to outline weak subviews. As a result of they’d a powerful reference to it and so they weren’t releasing it until you eliminated it from the view hierarchy.

This was about 10 years in the past. Now why are we nonetheless afraid of sturdy subviews? The primary motive was the addSubview methodology. The documentation states that it will create a powerful reference, which robotically triggered my mind and I outlined my views as weak pointers, since they are going have a powerful reference to their mother and father. Appears affordable, proper? 🧠

Weak subviews

Effectively, the issue is that if you wish to outline a weak variable we’ve got to make use of an non-obligatory, however I do not like the thought of utilizing an non-obligatory variable for the reason that view goes to be at all times there, it is a part of the view hierarchy sooner or later in, it isn’t going anyplace. It is solely going to be “destroyed” when my view controller is deallocated. Ought to I declare it as an implicitly unwrapped non-obligatory?!? Possibly.

import UIKit

class ViewController: UIViewController {

    weak var foo: UILabel! 
    weak var bar: UILabel? 
    
    override func viewDidLoad() {
        tremendous.viewDidLoad()

        
        foo.removeFromSuperview()
        foo.textual content = "crash"
    }
}

Really you may go fallacious with unwrapped weak pointers, as a result of if you happen to take away your view from the view hiearchy sooner or later in time earlier than the view controller deallocation then your weak pointer will probably be nil. On this case there will not be any extra sturdy references and your view will probably be deallocated instantly, so if it is an implicitly unwrapped non-obligatory, then we’ve got a hassle. Your app will crash if you happen to attempt to entry the property, as a result of it should have a zero worth.

So sure you should utilize implicitly unwrapped non-obligatory variables to retailer subviews, however solely in case you are certain that you’re not going to take away it from the hiearchy. This additionally implies that you do not belief Apple’s view administration system, which is ok, there could be bugs, however truthfully that is fairly a vital function and it has been round for a decade by now. 🙃

The opposite different is to make use of an everyday weak non-obligatory variable, however in that case you will at all times should test if it is nil or not, which goes to be a ache within the ass, however at the very least you are going to be protected for certain. Private opinion: it will not well worth the effort in any respect and I by no means saved views like this.

Sturdy subviews

My advice is to belief Apple and outline your subviews as sturdy properties. Okay, this can be problematic when you’ve got different sturdy references to the identical stuff, however generally if the view controller has the one reference to that given subview you have to be completely positive.

Since it is a sturdy property you additionally should initialize the view, however that is not a giant deal. You’ll be able to at all times initialize a view with a .zero body and that is it. Alternatively you may create a subclass with an everyday init() methodology, that is even higher, becuase you’re going to use auto format for certain and this fashion can set the translatesAutoresizingMaskIntoConstraints property in a single go.

import UIKit

class Label: UILabel {
    
    init() {
        tremendous.init(body: .zero)

        self.translatesAutoresizingMaskIntoConstraints = false
    }
    
    @obtainable(*, unavailable)
    required init?(coder: NSCoder) {
        fatalError("init(coder:) has not been carried out")
    }
    
    deinit {
        print("deinit Label")
    }
}

class ViewController: UIViewController {

    
    var foo: Label = .init()
    var bar: UILabel = .init(body: .zero)
    
    override func viewDidLoad() {
        tremendous.viewDidLoad()
        
    }
    
    deinit {
        print("deinit ViewController")
    }
    
}

By implementing a customized deinit methodology and even higher, by making a symbolic breakpoint you may simply detect retain cycles and repair reminiscence points. I made some exams and I can verify you do not have to be afraid of sturdy views, each the viewcontroller and the view goes to be deallocated if it is wanted. 👻

Unowned subviews

Unowned and weak are kind of equal, I might say that you just will not must outline views as unowned references, as a result of they are often problematic if it involves initialization. It is often higher to have a weak reference and test for nil values, however in fact there could be some instances the place you would possibly want an unowned subview reference.

Utilizing loadView and viewDidLoad

The loadView methodology can be utilized to create your personal views manually. It is best to by no means name this methodology straight, however it’s save to override it. The opposite factor that you shouldn’t is that in case you are utilizing this methodology to override the foundation view, you then should not name tremendous.loadView().

import UIKit

class ViewController: UIViewController {
    
    override func loadView() {
        view = UIView(body: .zero)

        
            
    }
}

In each different case once you simply wish to add views to the view hierarchy, it is fully positive to name the tremendous methodology. I am often implementing this methodology to setup views and constraints.

import UIKit 

class ViewController: UIViewController {

    var foo: Label = .init()
    
    override func loadView() {
        tremendous.loadView()
        
        view.addSubview(foo)
        
        NSLayoutConstraint.activate([
            view.centerXAnchor.constraint(equalTo: foo.centerXAnchor),
            view.leadingAnchor.constraint(equalTo: foo.leadingAnchor),
            view.trailingAnchor.constraint(equalTo: foo.trailingAnchor),
            foo.heightAnchor.constraint(equalToConstant: 44),
        ])
    }
}

This fashion I can make sure that each single view is prepared by the point the viewDidLoad methodology is known as. It’s doable to configure views contained in the loadView methodology too, however I desire to maintain the hierarchy setup there and I place every little thing else contained in the viewDidLoad operate. I imply controller associated stuff solely, like organising navigation bar buttons and issues like this.

As I discussed this in my earlier article, I desire to make use of subclasses to configure my views, I additionally transfer format constraints there (as a operate that returns them primarily based on some parameters) to maintain the view controller clear. Contained in the viewDidLoad methodology I can carry out extra person interface associated actions, however that is it I do not use it for including or styling views anymore.

Conclusion

Primarily based on my present data, here’s what I like to recommend for contemporary UIKit builders:

• Outline your subviews as sturdy properties
• All the time test for leaks, implement deinit, use breakpoints or devices
• Use weak / unowned references if you must break retain cycles
• Add views to the hierarchy within the loadView methodology
• Use subclasses for styling views, make them reusable
• Outline format constraint getters on the view subclass, activate them inside loadView
• Carry out remaining UI associated operations within the viewDidLoad operate

That is it. I am not saying that is the proper strategy, however for me it is undoubtedly the best way to go ahead with UIKit. I do know for certain that many individuals are nonetheless working with the framework and it’s right here to remain for a very long time. I hope the following tips will provide help to to know UIKit just a bit bit higher. ☺️