问题
In my code below:
I have 5 buttons added into a vertical scrollView. Each button is constrained to the scrollViews's top + 20 ,leading, trailing edges and its height. I have created a b1HeightConstraint
variable. It's there to hold the heightConstraint of the b1
button.
In a button click, I'm trying to remove this constraint. Yet I'm facing an odd issue:
When I log the constraints I only see 2 constraints, even though I've added 4 constraints to it. My the view debug hierarchy is like below:
import UIKit
import Foundation
class ViewController: UIViewController {
var filterView: UIView!
var scrollView: UIScrollView!
var containerView: UIView!
override func loadView() {
filterView = UIView()
view = filterView
view.backgroundColor = #colorLiteral(red: 0.909803926944733, green: 0.47843137383461, blue: 0.643137276172638, alpha: 1.0)
scrollView = UIScrollView()
scrollView.backgroundColor = #colorLiteral(red: 0.474509805440903, green: 0.839215695858002, blue: 0.976470589637756, alpha: 1.0)
view.addSubview(scrollView)
scrollView.translatesAutoresizingMaskIntoConstraints = false
scrollView.topAnchor.constraint(equalTo: view.topAnchor, constant: 0).isActive = true
scrollView.leadingAnchor.constraint(equalTo: view.leadingAnchor).isActive = true
scrollView.widthAnchor.constraint(equalTo: view.widthAnchor).isActive = true
scrollView.heightAnchor.constraint(equalTo: view.heightAnchor, multiplier: 1).isActive = true
scrollView.isScrollEnabled = true
containerView = UIView()
containerView.backgroundColor = #colorLiteral(red: 0.176470592617989, green: 0.498039215803146, blue: 0.756862759590149, alpha: 1.0)
scrollView.addSubview(containerView)
containerView.translatesAutoresizingMaskIntoConstraints = false
// This is key: connect all four edges of the containerView to
// to the edges of the scrollView
containerView.topAnchor.constraint(equalTo: scrollView.topAnchor).isActive = true
containerView.leadingAnchor.constraint(equalTo: scrollView.leadingAnchor).isActive = true
containerView.trailingAnchor.constraint(equalTo: scrollView.trailingAnchor).isActive = true
containerView.bottomAnchor.constraint(equalTo: scrollView.bottomAnchor).isActive = true
// Making containerView and scrollView the same height means the
// content will not scroll vertically
containerView.widthAnchor.constraint(equalTo: scrollView.widthAnchor).isActive = true
}
let b1 = Buttons(titleText: "one")
let b2 = Buttons(titleText: "two")
let b3 = Buttons(titleText: "three")
let b4 = Buttons(titleText: "four")
let b5 = Buttons(titleText: "five")
var b1HeightConstraint : NSLayoutConstraint?
override func viewDidLoad() {
super.viewDidLoad()
let buttonArray = [b1, b2, b3, b4, b5]
b1.button.addTarget(self, action: #selector(ViewController.shrink(_:)), for: .touchUpInside)
var startPoint = containerView.topAnchor
for btn in buttonArray {
let theBtn = btn.button
containerView.addSubview(theBtn)
theBtn.translatesAutoresizingMaskIntoConstraints = false
theBtn.topAnchor.constraint(equalTo: startPoint, constant: 20).isActive = true
theBtn.leadingAnchor.constraint(equalTo: containerView.leadingAnchor).isActive = true
theBtn.trailingAnchor.constraint(equalTo: containerView.trailingAnchor).isActive = true
theBtn.heightAnchor.constraint(equalTo: scrollView.heightAnchor).isActive = true
startPoint = theBtn.bottomAnchor
let btnHeight = theBtn.heightAnchor.constraint(equalTo: scrollView.heightAnchor)
if btn == b1{
b1HeightConstraint = btnHeight
}
}
containerView.bottomAnchor.constraint(equalTo: startPoint, constant: 20).isActive = true
}
@objc func shrink(_ sender: Any){
guard let btn = sender as? UIButton else{
return
}
print("count is: \(btn.constraints.count)")
btn.removeConstraint(b1HeightConstraint!)
containerView.removeConstraint(b1HeightConstraint!)
print("count is: \(btn.constraints.count)")
containerView.updateConstraintsIfNeeded()
containerView.updateConstraints()
scrollView.updateConstraintsIfNeeded()
scrollView.updateConstraints()
}
}
class Buttons : NSObject {
let button = UIButton()
init(titleText: String) {
button.backgroundColor = #colorLiteral(red: 0.976470589637756, green: 0.850980401039124, blue: 0.549019634723663, alpha: 1.0)
button.setTitle(titleText, for: .normal)
}
}
The code is ready to just be dumpped in the ViewController class. Works out of the box. My code is a spinoff of the code written here
回答1:
Here are several comments about your code:
- You never added any constraints to any views, so you shouldn't be removing them. iOS (CocoaTouch) added those constraints to those views, so please don't touch them. (In other words: don't call
removeConstraint
when you didn't calladdConstraint
). Your control over constraints is activating and deactivating them. Leave the adding and removing to iOS. - When you activate a constraint, a constraint is added (by iOS) to the most common ancestor of the two items mentioned in the constraint. So if the two views are siblings, it will be added to the parent. If the two views are parent and child, the constraint will be added to the parent. If the two views are grandparent and grandchild, it will be added to the grandparent. If the two views are first cousins, the constraint will be added to their common grandparent.
These lines of code:
let btnHeight = theBtn.heightAnchor.constraint(equalTo: scrollView.heightAnchor) if btn == b1{ b1HeightConstraint = btnHeight }
are creating a new constraint and assigning it to
b1HeightConstraint
, but you never activated this constraint, so it hasn't have been added to any view at all. So trying to remove it was never going to work, because that constraint exists only in yourb1HeightConstraint
property. Since it was never activated, it isn't actually constraining anything.If you want to shrink a button, you need to do one of these: a) modify the
constant
property of its height constraint OR b) set its height constraint'sisActive
property tofalse
and then give it a new height constraint OR c) modify the priorities of the active constraints to have Auto Layout choose to use different constraints.In your view debug hierarchy, all the constraints shown are active constraints (meaning they are available to be used by Auto Layout). The grayed out ones are the ones Auto Layout chose not to use because a higher priority constraint had precedence over it. This causes no conflict. The
self.height = 34 (content size)
constraint is added by the system to account for content compression and content hugging.UIButton
s resist compression with priority750
and resist expansion with priority250
. Theself.height = 34 (content size)
constraint is grayed out because content hugging has a priority of250
and another higher priority constraint was used instead (the constraint which sets the button's height equal to the scrollView's height has priority1000
).
Updated Code:
Here is your modified code. I changed two things:
- I made sure
b1HeightConstraint
was an activated constraint. - I changed the
shrink
method to deactivate the old height constraint and then create and activate a new one.
Updated code
import UIKit
import Foundation
class ViewController: UIViewController {
var filterView: UIView!
var scrollView: UIScrollView!
var containerView: UIView!
override func loadView() {
filterView = UIView()
view = filterView
view.backgroundColor = #colorLiteral(red: 0.909803926944733, green: 0.47843137383461, blue: 0.643137276172638, alpha: 1.0)
scrollView = UIScrollView()
scrollView.backgroundColor = #colorLiteral(red: 0.474509805440903, green: 0.839215695858002, blue: 0.976470589637756, alpha: 1.0)
view.addSubview(scrollView)
scrollView.translatesAutoresizingMaskIntoConstraints = false
scrollView.topAnchor.constraint(equalTo: view.topAnchor, constant: 0).isActive = true
scrollView.leadingAnchor.constraint(equalTo: view.leadingAnchor).isActive = true
scrollView.widthAnchor.constraint(equalTo: view.widthAnchor).isActive = true
scrollView.heightAnchor.constraint(equalTo: view.heightAnchor, multiplier: 1).isActive = true
scrollView.isScrollEnabled = true
containerView = UIView()
containerView.backgroundColor = #colorLiteral(red: 0.176470592617989, green: 0.498039215803146, blue: 0.756862759590149, alpha: 1.0)
scrollView.addSubview(containerView)
containerView.translatesAutoresizingMaskIntoConstraints = false
// This is key: connect all four edges of the containerView to
// to the edges of the scrollView
containerView.topAnchor.constraint(equalTo: scrollView.topAnchor).isActive = true
containerView.leadingAnchor.constraint(equalTo: scrollView.leadingAnchor).isActive = true
containerView.trailingAnchor.constraint(equalTo: scrollView.trailingAnchor).isActive = true
containerView.bottomAnchor.constraint(equalTo: scrollView.bottomAnchor).isActive = true
// Making containerView and scrollView the same height means the
// content will not scroll vertically
containerView.widthAnchor.constraint(equalTo: scrollView.widthAnchor).isActive = true
}
let b1 = Buttons(titleText: "one")
let b2 = Buttons(titleText: "two")
let b3 = Buttons(titleText: "three")
let b4 = Buttons(titleText: "four")
let b5 = Buttons(titleText: "five")
var b1HeightConstraint : NSLayoutConstraint?
override func viewDidLoad() {
super.viewDidLoad()
let buttonArray = [b1, b2, b3, b4, b5]
b1.button.addTarget(self, action: #selector(ViewController.shrink(_:)), for: .touchUpInside)
var startPoint = containerView.topAnchor
for btn in buttonArray {
let theBtn = btn.button
containerView.addSubview(theBtn)
theBtn.translatesAutoresizingMaskIntoConstraints = false
theBtn.topAnchor.constraint(equalTo: startPoint, constant: 20).isActive = true
theBtn.leadingAnchor.constraint(equalTo: containerView.leadingAnchor).isActive = true
theBtn.trailingAnchor.constraint(equalTo: containerView.trailingAnchor).isActive = true
//theBtn.heightAnchor.constraint(equalTo: scrollView.heightAnchor).isActive = true
startPoint = theBtn.bottomAnchor
let btnHeight = theBtn.heightAnchor.constraint(equalTo: scrollView.heightAnchor)
btnHeight.isActive = true
if btn == b1{
b1HeightConstraint = btnHeight
}
}
containerView.bottomAnchor.constraint(equalTo: startPoint, constant: 20).isActive = true
}
@objc func shrink(_ sender: UIButton) {
b1HeightConstraint?.isActive = false
b1HeightConstraint = sender.heightAnchor.constraint(equalToConstant: 20)
b1HeightConstraint?.isActive = true
}
}
class Buttons : NSObject {
let button = UIButton()
init(titleText: String) {
button.backgroundColor = #colorLiteral(red: 0.976470589637756, green: 0.850980401039124, blue: 0.549019634723663, alpha: 1.0)
button.setTitle(titleText, for: .normal)
}
}
Options for shrinking the button's height
Setting the
constant
property of the height constraint// shrink button's height by 200 points b1HeightConstraint?.constant -= 200
Deactivate the old constraint and create and activate a new one
// make button height 20 points b1HeightConstraint?.isActive = false b1HeightConstraint = sender.heightAnchor.constraint(equalToConstant: 20) b1HeightConstraint?.isActive = true
Change the priority of the height constraint
// Set b1HeightConstraint's priority to less than 250, and the // *content hugging* with priority 250 will take over and resize // the button to its intrinsic height b1HeightConstraint?.priority = UILayoutPriority(rawValue: 100)
Note: For this to work, you have to give the height constraint an initial priority less than
1000
(999
works nicely) because Auto Layout will not let you change the priority of an active constraint if it is required (priority1000
).OR
// Just deactivate the buttonHeight constraint and the *content // hugging* will take over and it will set the button's height // to its intrinsic height b1HeightConstraint?.isActive = false
回答2:
This is because a constraint between a view and its superView is added to the superView , you only see height/width constraint if they are static added to the UIButton
, look to this diagram from Vandad IOS Book
see this Demo
来源:https://stackoverflow.com/questions/51793420/why-isnt-uibutton-returning-correct-constraints