sequence of auto layout methods get called in rendering cycle - ios

Question

I was reading about auto layout rendering pipelines i mean how auto layout work under the hood. There are some methods which get called at different stages of autoLayout rendering like
layoutIfNeeded()
layoutSubviews()
updateConstraints()
updateConstraintsIfNeeded()
but i don't know which method is called when and what is the significance of that method and if i want to use auto layout then in which order i can use that methods and how can i control the autoLayout rendering pipeline

Answer 1

Usually you don't need to care about the autolayout method chain. You just need to create the constraints for the views to define their sizes and positions. You can add/remove, activate/deactivate constraints anytime in lifecycle of the view, but you want to always have a set of satisfiable (non-conflicting), yet complete set of constraints.
Take an example. You can tell the autolayout that button A should be 50 points wide, 20 points high, with its left top corner positioned at point (0,0) in the viewController's view. Now, this is non-conflicting, yet complete set of constraints for the button A. But lets say you want to expand that button, when the user taps it. So in the tap handler you will add one new constraint saying that the button should be 100 points wide - now you have unsatisfiable constraints - there is a constraint say it should be 50 points wide, and another one saying it shoul be 100 points wide. Therefore, to prevent conflict, before activating the new constraint, you have to deactivate the old one. Incomplete constraints is an opposite case, lets say you deactivate the old width constraint, but never activate the new one. Then autolayout can calculate position (because there are constraints defining it), and height, but not width, which usually ends in undefined behavior (now in case of a UIButton that's not true, because it has intrinsic size, which implicitly defines its width and height, but I hope you get the point).
So when you create those constraints is up to you (in my example you were manipulating them when the user tapped the button). Usually you start in initializer in case of a UIView subclass or in loadView in UIViewController subclass and there you can define and activate the default set of constraints. And then you can use handlers to react to user activity. My recommendation is prepare all the constraints in loadView, keep them in properties, and activate/deactivate them when necessary.
But there are of course some limitation as when and how not to create new constraints - for a more detailed discussion of those cases I really recommend looking at Advanced Autolayout Toolbox by objc.io.
EDIT
See following example of a simple custom SongView that uses autolayout for layout and supports also some dynamic changes in constraints by activating/deactivating them. You can just simply copy paste the whole code into a playground and test it out there, or include it in a project.
Notice there that I don't call any of the autolayout lifecycle methods, except of setNeedsLayout and layoutIfNeeded. setNeedsLayout sets a flag telling the autolayout that constraints have been changed, and layoutIfNeeded then tells it to recalculate frames. Normally, that would happen automatically, but to animate the constraints changes we need to tell it explicitly - see the setExpanded method in SongView. For more detailed explanation of using autolayout in animations, see my different answer.
import UIKit
import PlaygroundSupport
class ViewController: UIViewController {
let songView = SongView()
let button = UIButton()
override func loadView() {
super.loadView()
view.backgroundColor = .white
self.view.addSubview(button)
self.view.addSubview(songView)
button.setTitle("Expand/Collapse", for: .normal)
button.setTitleColor(.blue, for: .normal)
button.addTarget(self, action: #selector(expandCollapse), for: .touchUpInside)
button.translatesAutoresizingMaskIntoConstraints = false
songView.translatesAutoresizingMaskIntoConstraints = false
NSLayoutConstraint.activate([
// button has intrinsic size, no need to define constraints for size, position is enough
button.bottomAnchor.constraint(equalTo: self.view.bottomAnchor, constant: -50),
button.centerXAnchor.constraint(equalTo: self.view.centerXAnchor),
// songView has defined its height (see SongView class), but not width, therefore we need more constraints
songView.centerYAnchor.constraint(equalTo: self.view.centerYAnchor),
songView.leftAnchor.constraint(equalTo: self.view.leftAnchor),
songView.rightAnchor.constraint(equalTo: self.view.rightAnchor),
])
}
#objc func expandCollapse() {
if songView.isExpanded {
songView.setExpanded(to: false, animated: true)
} else {
songView.setExpanded(to: true, animated: true)
}
}
}
class SongView: UIView {
private let numberLabel: UILabel = UILabel()
private let nameLabel: UILabel = UILabel()
private var expandedConstraints: [NSLayoutConstraint] = []
private var collapsedConstraints: [NSLayoutConstraint] = []
// this can be triggered by some event
private(set) var isExpanded: Bool = false
func setExpanded(to expanded: Bool, animated: Bool) {
self.isExpanded = expanded
if animated {
if expanded {
// setup expanded state
NSLayoutConstraint.deactivate(collapsedConstraints)
NSLayoutConstraint.activate(expandedConstraints)
} else {
// setup collapsed
NSLayoutConstraint.deactivate(expandedConstraints)
NSLayoutConstraint.activate(collapsedConstraints)
}
self.setNeedsLayout()
UIView.animate(withDuration: 0.2, animations: {
self.layoutIfNeeded()
})
} else {
// non animated version (no need to explicitly call setNeedsLayout nor layoutIfNeeded)
if expanded {
// setup expanded state
NSLayoutConstraint.deactivate(collapsedConstraints)
NSLayoutConstraint.activate(expandedConstraints)
} else {
// setup collapsed
NSLayoutConstraint.deactivate(expandedConstraints)
NSLayoutConstraint.activate(collapsedConstraints)
}
}
}
var data: (String, String)? {
didSet {
numberLabel.text = data?.0
nameLabel.text = data?.1
}
}
init() {
super.init(frame: CGRect.zero)
setupInitialHierarchy()
setupInitialAttributes()
setupInitialLayout()
}
fileprivate func setupInitialHierarchy() {
self.addSubview(numberLabel)
self.addSubview(nameLabel)
}
fileprivate func setupInitialAttributes() {
numberLabel.font = UIFont.boldSystemFont(ofSize: UIFont.preferredFont(forTextStyle: UIFontTextStyle.body).pointSize)
numberLabel.textColor = UIColor.darkGray
numberLabel.text = "0"
numberLabel.textAlignment = .right
nameLabel.font = UIFont.preferredFont(forTextStyle: UIFontTextStyle.body)
nameLabel.text = "NONE"
nameLabel.textAlignment = .left
self.backgroundColor = UIColor.lightGray
}
fileprivate func setupInitialLayout() {
self.translatesAutoresizingMaskIntoConstraints = false
numberLabel.translatesAutoresizingMaskIntoConstraints = false
nameLabel.translatesAutoresizingMaskIntoConstraints = false
// just randomly selected different layouts for collapsed and expanded states
expandedConstraints = [
numberLabel.widthAnchor.constraint(equalToConstant: 35),
self.heightAnchor.constraint(equalToConstant: 80),
]
collapsedConstraints = [
numberLabel.widthAnchor.constraint(equalToConstant: 50),
self.heightAnchor.constraint(equalToConstant: 40),
]
// activating collapsed as default layout
NSLayoutConstraint.activate(collapsedConstraints)
NSLayoutConstraint.activate([
numberLabel.topAnchor.constraint(equalTo: self.topAnchor, constant: 4),
numberLabel.bottomAnchor.constraint(equalTo: self.bottomAnchor, constant: -4),
numberLabel.leftAnchor.constraint(equalTo: self.leftAnchor, constant: 4),
nameLabel.centerYAnchor.constraint(equalTo: numberLabel.centerYAnchor),
nameLabel.leftAnchor.constraint(equalTo: numberLabel.rightAnchor, constant: 8),
nameLabel.rightAnchor.constraint(equalTo: self.rightAnchor, constant: -4)
])
}
required init?(coder aDecoder: NSCoder) {
super.init(coder: aDecoder)
}
}
PlaygroundPage.current.liveView = ViewController()