I have recently started working with Swift, and I am trying to go from one UICollectionView on the page (which worked) up to two of them. I am getting the error
"Terminating app due to uncaught exception ' NSInvalidArgumentException', reason: 'UICollectionView must be initialized with a non-nil layout parameter '".
What I've found thus far on Stack Overflow has been incorporated into my code, but I am still getting the error. I am at my wit's end, please help!
Where I've been:
*** Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: 'UICollectionView must be initialized with a non-nil layout parameter'
ViewController.swift :
import UIKit
class ViewController: UIViewController, UICollectionViewDataSource, UICollectionViewDelegate {
var collectionViewLeft = UICollectionView() // changed "let" to "var" so I can assign things to it in viewDidLoad()
var collectionViewRight = UICollectionView()
let collectionViewLeftIdentifier = "CollectionViewLeftCell"
let collectionViewRightIdentifier = "CollectionViewRightCell"
override func viewDidLoad() {
super.viewDidLoad()
let layoutLeft = UICollectionViewFlowLayout() // this is what **I would think** would be my non-nil layout
layoutLeft.itemSize = CGSize(width: 100, height: 100)
let layoutRight = UICollectionViewFlowLayout()
layoutRight.itemSize = CGSize(width: 100, height: 100)
collectionViewLeft = UICollectionView(frame: self.view.frame, collectionViewLayout: layoutLeft)
collectionViewRight = UICollectionView(frame: self.view.frame, collectionViewLayout: layoutRight)
collectionViewLeft.delegate = self
collectionViewRight.delegate = self
collectionViewLeft.dataSource = self
collectionViewRight.dataSource = self
self.view.addSubview(collectionViewLeft)
self.view.addSubview(collectionViewRight)
collectionViewLeft.register(UICollectionViewCell.self, forCellWithReuseIdentifier: collectionViewLeftIdentifier)
collectionViewRight.register(UICollectionViewCell.self, forCellWithReuseIdentifier: collectionViewRightIdentifier)
}
let reuseIdentifierLeft = "cellLeft" // also enter this string as the cell identifier in the storyboard
let reuseIdentifierRight = "cellRight"
var itemsRight = ["1", "2", "3", "4", "5", "6", "7", "8", "9", "10", "11", "12", "13", "14", "15", "16", "17", "18", "19", "20", "21", "22", "23", "24", "25", "26", "27", "28", "29", "30", "31", "32", "33", "34", "35", "36", "37", "38", "39", "40", "41", "42", "43", "44", "45", "46", "47", "48"]
var itemsLeft = ["10", "20", "30", "40", "50", "60"]
// MARK: - UICollectionViewDataSource protocol
// tell the collection view how many cells to make
func collectionView(_ collectionView: UICollectionView, numberOfItemsInSection section: Int) -> Int {
if collectionView == self.collectionViewLeft {
return self.itemsLeft.count
} else if collectionView == self.collectionViewRight {
return self.itemsRight.count
} else {
print("This is very bad")
assert(false, "Passed collectionView is neither collectionViewLeft nor collectionViewRight -- ruh roh!")
return 0
}
}
// make a cell for each cell index path
func collectionView(_ collectionView: UICollectionView, cellForItemAt indexPath: IndexPath) -> UICollectionViewCell {
if collectionView == self.collectionViewLeft {
// get a reference to our storyboard cell
let cellLeft = collectionView.dequeueReusableCell(withReuseIdentifier: reuseIdentifierLeft, for: indexPath as IndexPath) as! MyCollectionViewCell
// Use the outlet in our custom class to get a reference to the UILabel in the cell
cellLeft.myLeftLabel.text = self.itemsLeft[indexPath.item]
cellLeft.backgroundColor = UIColor.red // make cell more visible in our example project
return cellLeft
} else if collectionView == self.collectionViewRight {
let cellRight = collectionView.dequeueReusableCell(withReuseIdentifier: reuseIdentifierRight, for: indexPath as IndexPath) as! MyRightCollectionViewCell
// Use the outlet in our custom class to get a reference to the UILabel in the cell
cellRight.myRightLabel.text = self.itemsRight[indexPath.item]
cellRight.backgroundColor = UIColor.green // make cell more visible in our example project
return cellRight
} else {
print("This is very bad")
assert(false, "Passed collectionView is neither collectionViewLeft nor collectionViewRight -- ruh roh!")
// won't actually execute the following, but to keep the compiler happy...
let cell = collectionView.dequeueReusableCell(withReuseIdentifier: reuseIdentifierLeft, for: indexPath as IndexPath) as! MyCollectionViewCell
return cell
}
}
// MARK: - UICollectionViewDelegate protocol
func collectionView(_ collectionView: UICollectionView, didSelectItemAt indexPath: IndexPath) {
// handle tap events
if collectionView == self.collectionViewLeft {
print("You tapped cell #\(indexPath.item) on the LEFT!")
} else if collectionView == self.collectionViewRight {
print("You tapped cell #\(indexPath.item) on the RIGHT!")
} else {
print("This is very bad")
assert(false, "Passed collectionView is neither collectionViewLeft nor collectionViewRight -- ruh roh!")
}
}
}
MyCollectionViewCell.swift
import UIKit
class MyCollectionViewCell: UICollectionViewCell {
@IBOutlet weak var myLeftLabel: UILabel!
}
class MyRightCollectionViewCell: UICollectionViewCell {
@IBOutlet weak var myRightLabel: UILabel!
}
Don't assign empty UICollectionView
objects to your properties; you are just going to throw them away anyway and it is causing the exception since you aren't providing a layout. This is a situation where you can use an implicitly unwrapped optional:
var collectionViewLeft: UICollectionView!
var collectionViewRight: UICollectionView!
My guess, you declare UICollectionViewController
's with empty default init
constructor as variables:
var collectionViewLeft = UICollectionView()
var collectionViewRight = UICollectionView()
Try to replace your variable declarations without constructors:
Edited code below: As @Paulw11 correctly noted in his answer below, it's better to use an implicitly unwrapped optional ! rather than ?
var collectionViewLeft: UICollectionView!
var collectionViewRight: UICollectionView!
Anyway, you are instantiate them in your viewDidLoad
function with designated initialisers for UICollectionView
.
And just a note, it's better to register custom class / xibs before UICollectionView
is added as a subview. It's not critical right now, but may be a problem after refactoring & etc., because you code can call UICollectionView
delegate
or dataSource
functions before you code registered custom classes for cells. Just move your code with calls to .register()
above self.view.addSubview()
:
collectionViewLeft.register(UICollectionViewCell.self, forCellWithReuseIdentifier: collectionViewLeftIdentifier)
collectionViewRight.register(UICollectionViewCell.self, forCellWithReuseIdentifier: collectionViewRightIdentifier)
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