Table view automatic dimension forcing duplicate constraints - swift

Question

Getting automatic dimension working for UITableView.rowHeight requires a duplicate constraint in my UITableViewCell class.
I'm creating a UITableView programmatically (SwiftUI, no storyboard) and have cells of different heights. I’ve set the table’s rowHeight to UITableView.automaticDimension but can’t find the correct combination of constraints to get the table to actually calculate the correct height for the cells without adding a duplicate constraint.
I would expect to add a width, height, top, and leading constraint to get things working correctly. However, the table does not size the rows properly unless I also add a bottom constraint. Naturally this produces the warning:
2019-10-23 18:06:53.515025-0700 Test[15858:7764405] [LayoutConstraints] 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:0x600001651c20 V:|-(0)-[UIView:0x7f8fc5a08890] (active, names: '|':Aries.TestTableViewCell:0x7f8fc5a084e0'TestTableViewCell' )>",
"<NSLayoutConstraint:0x600001651e50 UIView:0x7f8fc5a08890.bottom == Aries.TestTableViewCell:0x7f8fc5a084e0'TestTableViewCell'.bottom (active)>",
"<NSLayoutConstraint:0x600001651ea0 UIView:0x7f8fc5a08890.height == 40 (active)>",
"<NSLayoutConstraint:0x600001652120 'UIView-Encapsulated-Layout-Height' Aries.TestTableViewCell:0x7f8fc5a084e0'TestTableViewCell'.height == 40.3333 (active)>"
)
Will attempt to recover by breaking constraint
<NSLayoutConstraint:0x600001651ea0 UIView:0x7f8fc5a08890.height == 40 (active)>
If I remove the height constraint or the bottom anchor constraint the duplicate constraint is gone and the warning goes away. However, then the table won’t size the rows properly.
The View:
import SwiftUI
struct TableViewTest: View {
var body: some View {
TestTableView().frame(minWidth: 0, maxWidth: .infinity, minHeight: 0, maxHeight: .infinity, alignment: .center)
}
}
The TableView:
import SwiftUI
struct TestTableView: UIViewRepresentable {
func makeCoordinator() -> Coordinator {
Coordinator()
}
func makeUIView(context: UIViewRepresentableContext<TestTableView>) -> UITableView {
let tableView = UITableView(frame: .zero)
tableView.register(TestTableViewCell.self, forCellReuseIdentifier: "TestTableViewCell")
tableView.rowHeight = UITableView.automaticDimension
let dataSource = UITableViewDiffableDataSource<Section, TestData>(tableView: tableView) { tableView, indexPath, data in
let cell = tableView.dequeueReusableCell(withIdentifier: "TestTableViewCell", for: indexPath) as! TestTableViewCell
cell.data = data
return cell
}
populate(dataSource: dataSource)
context.coordinator.dataSource = dataSource
return tableView
}
func populate(dataSource: UITableViewDiffableDataSource<Section, TestData>) {
let items = [
TestData(color: .red, size: CGSize(width: 40, height: 20)),
TestData(color: .blue, size: CGSize(width: 40, height: 40)),
TestData(color: .green, size: CGSize(width: 40, height: 80))
]
var snapshot = NSDiffableDataSourceSnapshot<Section, TestData>()
snapshot.appendSections([.main])
snapshot.appendItems(items)
dataSource.apply(snapshot)
}
func updateUIView(_ tableView: UITableView, context: UIViewRepresentableContext<TestTableView>) {
guard let dataSource = context.coordinator.dataSource else {
return
}
populate(dataSource: dataSource)
}
class Coordinator {
var dataSource: UITableViewDiffableDataSource<Section, TestData>?
}
enum Section {
case main
}
struct TestData: Hashable {
var id = UUID()
var color: UIColor
var size: CGSize
func hash(into hasher: inout Hasher) {
hasher.combine(id)
}
}
}
The Table View Cell:
import UIKit
class TestTableViewCell: UITableViewCell {
var data: TestTableView.TestData? {
didSet {
if let data = data {
let view = UIView()
view.backgroundColor = data.color
addSubview(view)
// !!! SETTING THE BOTTOM ANCHOR TO NIL OR HEIGHT TO 0 PREVENTS THE TABLE FROM SIZING THE ROWS CORRECTLY
view.anchor(top: topAnchor, left: leftAnchor, bottom: bottomAnchor, right: nil, paddingTop: 0, paddingLeft: 0, paddingBottom: 0, paddingRight: 0, width: data.size.width, height: data.size.height, enableInsets: false)
}
}
}
override init(style: UITableViewCell.CellStyle, reuseIdentifier: String?) {
super.init(style: style, reuseIdentifier: reuseIdentifier)
}
required init?(coder aDecoder: NSCoder) {
fatalError("init(coder:) has not been implemented")
}
}
The UIView Anchor Extension:
func anchor (top: NSLayoutYAxisAnchor?, left: NSLayoutXAxisAnchor?, bottom: NSLayoutYAxisAnchor?, right: NSLayoutXAxisAnchor?, paddingTop: CGFloat, paddingLeft: CGFloat, paddingBottom: CGFloat, paddingRight: CGFloat, width: CGFloat, height: CGFloat, enableInsets: Bool) {
var topInset = CGFloat(0)
var bottomInset = CGFloat(0)
if #available(iOS 11, *), enableInsets {
let insets = self.safeAreaInsets
topInset = insets.top
bottomInset = insets.bottom
}
translatesAutoresizingMaskIntoConstraints = false
if let top = top {
self.topAnchor.constraint(equalTo: top, constant: paddingTop+topInset).isActive = true
}
if let left = left {
self.leftAnchor.constraint(equalTo: left, constant: paddingLeft).isActive = true
}
if let right = right {
rightAnchor.constraint(equalTo: right, constant: -paddingRight).isActive = true
}
if let bottom = bottom {
bottomAnchor.constraint(equalTo: bottom, constant: -paddingBottom-bottomInset).isActive = true
}
if height != 0 {
heightAnchor.constraint(equalToConstant: height).isActive = true
}
if width != 0 {
widthAnchor.constraint(equalToConstant: width).isActive = true
}
}
If I anchor only the top left corner and specify a width and height in the table view cell, the table view will not calculate the heights correctly. However, if I also specify the height, the rows will be sized correctly but a warning is generated about a duplicate constraint. Is there a magic combination that allows correct layout but does not produce the warning?
Bad (but no duplicate constraints):
Good (with duplicate constraints):

Answer 1

To fix this issue, you need to change the priority of your bottom constraint and then the warning will go away.
if let bottom = bottom {
let bottomConstraint = bottomAnchor.constraint(equalTo: bottom, constant: -paddingBottom-bottomInset)
bottomConstraint.priority = UILayoutPriority(750)
bottomConstraint.isActive = true
}