This code block tries to get to the heart of the problem. If, after dequeuing a cell (via configure), the constraints are changed so that the cell height is changed, then you end up with a broken constraints warning (Unable to simultaneously satisfy constraints...). However, it displays correctly.
import UIKit
class ViewController: UIViewController {
@IBOutlet var tableView: UITableView!
override func viewDidLoad() {
super.viewDidLoad()
tableView.delegate = self
tableView.dataSource = self
tableView.registerClass(Cell.self, forCellReuseIdentifier: "cell")
tableView.estimatedRowHeight = 55.0
tableView.rowHeight = UITableViewAutomaticDimension
}
}
extension ViewController: UITableViewDelegate, UITableViewDataSource {
func tableView(tableView: UITableView, numberOfRowsInSection section: Int) -> Int {
return array.count
}
func numberOfSectionsInTableView(tableView: UITableView) -> Int {
return 1
}
func tableView(tableView: UITableView, cellForRowAtIndexPath indexPath: NSIndexPath) -> UITableViewCell {
let cell = tableView.dequeueReusableCellWithIdentifier("cell") as! Cell
cell.configure(array[indexPath.row])
return cell
}
func tableView(tableView: UITableView, willDisplayCell cell: UITableViewCell, forRowAtIndexPath indexPath: NSIndexPath) {
}
}
enum Position {
case Top
case Middle
case Bottom
}
class Cell: UITableViewCell {
private var topConstraint: NSLayoutConstraint!
private var bottomConstraint: NSLayoutConstraint!
private let label = UILabel()
private var previous: Position?
override init(style: UITableViewCellStyle, reuseIdentifier: String?) {
super.init(style: style, reuseIdentifier: reuseIdentifier)
label.backgroundColor = UIColor.redColor().colorWithAlphaComponent(0.5)
label.textColor = UIColor.blackColor()
label.translatesAutoresizingMaskIntoConstraints = false
contentView.addSubview(label)
label.leadingAnchor.constraintEqualToAnchor(contentView.leadingAnchor, constant: 30).active = true
label.trailingAnchor.constraintEqualToAnchor(contentView.trailingAnchor, constant: -30).active = true
topConstraint = label.topAnchor.constraintEqualToAnchor(contentView.topAnchor)
topConstraint.active = true
bottomConstraint = label.bottomAnchor.constraintEqualToAnchor(contentView.bottomAnchor)
bottomConstraint.active = true
}
required init?(coder aDecoder: NSCoder) {
fatalError("init(coder:) has not been implemented")
}
func configure(position: Position) {
if previous != nil {print("previous \(previous!) new \(position)")}
topConstraint.constant = position == .Top ? 30 : 0
bottomConstraint.constant = position == .Bottom ? -30 : 0
label.text = String(position)
previous = position
}
}
let array: [Position] = [.Top, .Middle, .Middle, .Middle, .Bottom,
.Top, .Middle, .Middle, .Middle, .Middle, .Bottom,
.Top, .Middle, .Middle, .Bottom,
.Top, .Middle, .Middle, .Middle, .Middle, .Middle, .Middle, .Bottom,
.Top, .Middle, .Middle, .Middle, .Bottom,
.Top, .Bottom,
.Top, .Middle, .Middle, .Middle, .Middle, .Middle, .Middle, .Bottom,
.Top, .Middle, .Middle, .Middle, .Bottom,
.Top, .Middle, .Middle, .Bottom,
.Top, .Middle, .Middle, .Middle, .Middle, .Middle, .Middle, .Bottom,
]
Error message is...
2016-04-28 22:56:29.831 test-constraint-change[51059:3776208] Unable to simultaneously satisfy constraints.
Probably at least one of the constraints in the following list is one you don't want.
Try this:
(1) look at each constraint and try to figure out which you don't expect;
(2) find the code that added the unwanted constraint or constraints and fix it.
(
"<NSLayoutConstraint:0x7c117c70 V:|-(30)-[UILabel:0x7c148770'Middle'] (Names: '|':UITableViewCellContentView:0x7c149b70 )>",
"<NSLayoutConstraint:0x7c115bd0 UILabel:0x7c148770'Middle'.bottom == UITableViewCellContentView:0x7c149b70.bottom>",
"<NSLayoutConstraint:0x7c14a3f0 'UIView-Encapsulated-Layout-Height' V:[UITableViewCellContentView:0x7c149b70(20.5)]>"
)
Will attempt to recover by breaking constraint <NSLayoutConstraint:0x7c115bd0 UILabel:0x7c148770'Middle'.bottom == UITableViewCellContentView:0x7c149b70.bottom>
Make a symbolic breakpoint at UIViewAlertForUnsatisfiableConstraints to catch this in the debugger.
The methods in the UIConstraintBasedLayoutDebugging category on UIView listed in <UIKit/UIView.h> may also be helpful.
