layoutIfNeeded function behaviour - ios

Question

I am using a lot of gradient drawing using this function:
func drawGradient(colors: [CGColor], locations: [NSNumber]) {
let gradientLayer = CAGradientLayer()
gradientLayer.frame.size = self.frame.size
gradientLayer.frame.origin = CGPoint(x: 0.0,y: 0.0)
gradientLayer.colors = colors
gradientLayer.locations = locations
print(self.frame.size)
self.layer.insertSublayer(gradientLayer, at: 0)
}
The problem is that if I don't call self.view.layoutIfNeeded() in viewDidLoad() of UIViewController my gradient doesn't cover whole screen on iPhone X+. But if I call self.view.layoutIfNeeded() it makes my app crash on iOS 9.x and act weird on iPhone 5/5s. I really do not know any workaround and need help to understand how it all works.

Answer 1

You are calling drawGradient in viewDidLoad. That is too early. You need to wait until Auto Layout has sized the frame.
Move the call in viewDidLoad to an override of viewDidLayoutSubviews. Be careful though because viewDidLayoutSubviews is called more than once, so make sure you only call drawGradient once. You can add a property to your viewController called var appliedGradient = false and then check it before applying the gradient and flip it to true.
For your custom subclasses of UITableViewCell and UICollectionViewCell, override layoutSubviews and call drawGradient after super.layoutSubviews(). Again, make sure you only call it once.
Note: If your frame could resize (due to rotation of the phone) or differing cell sizes, you should keep track of the previous gradient layer and replace it with a new one in viewDidLayoutSubviews for your viewController and in layoutSubviews for your cells.
Here I've modified your drawGradient to make a global function called applyGradient that adds a gradient to a view. It replaces a previous gradient layer if there was one:
func applyGradient(colors: [CGColor], locations: [NSNumber], to view: UIView, replacing prior: CALayer?) -> CALayer {
let gradientLayer = CAGradientLayer()
gradientLayer.frame.size = view.frame.size
gradientLayer.frame.origin = CGPoint(x: 0.0,y: 0.0)
gradientLayer.colors = colors
gradientLayer.locations = locations
print(view.frame.size)
if let prior = prior {
view.layer.replaceSublayer(prior, with: gradientLayer)
} else {
view.layer.insertSublayer(gradientLayer, at: 0)
}
return gradientLayer
}
And it is used like this:
class ViewController: UIViewController {
// property to keep track of the gradient layer
var gradient: CALayer?
override func viewDidLayoutSubviews() {
super.viewDidLayoutSubviews()
gradient = applyGradient(colors: [UIColor.red.cgColor, UIColor.yellow.cgColor],
locations: [0.0, 1.0], to: self.view, replacing: gradient)
}
}