Weak, unowned or robust subviews?
I’ve obtained numerous emails and tweets about this matter, so I made a decision to put in writing about it, as a result of it’s actually onerous 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 robust 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 should 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 need to learn this text. This was fairly a giant change and Apple additionally touched their inside view administration. Earlier than iOS6 it was a standard follow to outline weak subviews. As a result of they’d a powerful reference to it they usually weren’t releasing it except you eliminated it from the view hierarchy.
This was about 10 years in the past. Now why are we nonetheless afraid of robust subviews? The primary cause 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 cheap, proper? ðŸ§
Weak subviews
Properly, the issue is that if you wish to outline a weak variable we have now to make use of an non-compulsory, however I do not like the concept of utilizing an non-compulsory variable for the reason that view goes to be all the time there, it is a part of the view hierarchy in some unspecified time in the future in, it isn’t going wherever. It is solely going to be “destroyed” when my view controller is deallocated. Ought to I declare it as an implicitly unwrapped non-compulsory?!? Perhaps.
import UIKit
class ViewController: UIViewController {
weak var foo: UILabel!
weak var bar: UILabel?
override func viewDidLoad() {
tremendous.viewDidLoad()
foo.removeFromSuperview()
foo.textual content = "crash"
}
}
Truly you may go unsuitable with unwrapped weak pointers, as a result of for those who take away your view from the view hiearchy in some unspecified time in the future 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 robust references and your view will probably be deallocated instantly, so if it is an implicitly unwrapped non-compulsory, then we have now a hassle. Your app will crash for those who attempt to entry the property, as a result of it will have a nil worth.
So sure you should utilize implicitly unwrapped non-compulsory variables to retailer subviews, however solely if you’re 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 okay, there might be bugs, however actually that is fairly a vital characteristic and it has been round for a decade by now. 🙃
The opposite different is to make use of a daily weak non-compulsory variable, however in that case you may all the time need to examine 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 definitely worth the effort in any respect and I by no means saved views like this.
Robust subviews
My suggestion is to belief Apple and outline your subviews as robust properties. Okay, this can be problematic you probably have different robust references to the identical stuff, however typically if the view controller has the one reference to that given subview you ought to be completely positive.
Since it is a robust property you additionally need to initialize the view, however that is not a giant deal. You’ll be able to all the time initialize a view with a .zero body and that is it. Alternatively you may create a subclass with a daily init() methodology, that is even higher, becuase you’ll use auto format for certain and this manner can set the translatesAutoresizingMaskIntoConstraints property in a single go.
import UIKit
class Label: UILabel {
init() {
tremendous.init(body: .zero)
self.translatesAutoresizingMaskIntoConstraints = false
}
@accessible(*, 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 assessments and I can affirm you do not have to be afraid of robust views, each the viewcontroller and the view goes to be deallocated if it is wanted. 👻
Unowned subviews
Unowned and weak are roughly equal, I might say that you simply 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 examine for nil values, however after all there might be some circumstances the place you may want an unowned subview reference.
Utilizing loadView and viewDidLoad
The loadView methodology can be utilized to create your individual 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 if you’re 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 while you simply wish to add views to the view hierarchy, it is utterly 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 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 all the pieces else contained in the viewDidLoad perform. 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 perform that returns them based mostly 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.
