Does self.view has constraints? - ios

Question

self.view is on the ViewController. Does it have any constraints? I found it had a top and a left constraint. It has a frame for sure. The autolayout system starts from self.view. I mean until self.view the iOS system is still using frame for positioning the views. After that, our views are able to be added with autolayout. So in the viewcontroller, I use self.frame.size.width for the width constraint for my own view is the correct approach to do? Am I right?

Answer 1

Use SCREEN_WIDTH and It's very easy to get your device size and manage your own view properly:
let SCREEN_WIDTH : CGFloat = UIScreen.main.bounds.width
let SCREEN_HEIGHT : CGFloat = UIScreen.main.bounds.height
For more information you can refer this : link
Example :
You should add your own new view programatically.
import UIKit
class ViewController: UIViewController {
override func viewDidLoad() {
super.viewDidLoad()
let SCREEN_WIDTH : CGFloat = UIScreen.main.bounds.width
let SCREEN_HEIGHT : CGFloat = UIScreen.main.bounds.height
let newView = UIView()
newView.translatesAutoresizingMaskIntoConstraints = false
newView.backgroundColor = UIColor.black
newView.frame.size = CGSize(width: SCREEN_WIDTH * 0.5 , height: SCREEN_HEIGHT * 0.5)
self.view.addSubview(newView)
self.view.layoutIfNeeded()
}
}
Here I am giving the frame to the constraints of my customized view. I don't want to give this value in a format of frame.size. I want to use somethings like constraint.constant. I mean pure autolayout.

Answer 2

self.view.frame is only accurate once viewDidLayoutSubviews() is called for the first time, typically just before viewDidAppear meaning if you try and access in viewDidLoad or first time viewWillAppear the frame will be wrong.
So you should just check in viewDidLayoutSubviews() (but note that function is called whenever the layout changes (this does not mean you scroll on a table view, it means the actual constraints change bc of a screen rotation for ex, or otherwise)
If you can't wait until that method or have some code that's not idempotent and don't want to have some like hasSetup bool to make sure you only do it once... Then you can either access the screen size directly via UIScreen.main.bounds
OR
I wouldn't necessarily recommend the following but for the sake of completeness I've also gotten away with tricks to get the view to layout earlier like calling view.layoutIfNeeded() earlier in the view lifecycle or on the view controller that initializes:
let myVC = MyViewController()
_ = myVC.view // fixes frame for some reason
Using AutoLayout correctly here
You want to use constraints but the frame shouldn't matter at all if you make relative constraints. Aka instead of saying height = constant you would say "make this the same height as vc.view" or "make the leading edge of this view 20 pixels right of the leading edge of vc.view".
something like:
let myView = UIView()
self.view.addSubview(myView)
myView.translatesAutoresizingMaskIntoConstraints = false
myView.heightAnchor.constraint(equalTo: self.view.heightAnchor)
myView.leadingAnchor.constraint(equalTo: vc.view.leadingAnchor, constant: 20.0) // 20 pixels to right of vc.view leading