Swift Generic Protocol Class Type in Array

Question

We have implemented a protocol, Reusable, to ease UITableView register/dequeue implementations, for our UITableViewCells.

protocol Reusable: class {
    static var defaultIdentifier: String { get }
}

extension Reusable where Self: UITableViewCell {
    static var defaultIdentifier: String {
        return String(describing: self)
    }
}

class TestTableViewCell: UITableViewCell, Reusable { }
class AnotherTestTableViewCell: UITableViewCell, Reusable { }

Then, there's an extension to UITableView like:

extension UITableView {
    func register<T: UITableViewCell & Reusable>(_: T.Type) {
        register(UINib(nibName: T.defaultIdentifier, bundle: nil), forCellReuseIdentifier: T.defaultIdentifier)
    }
}

and as its usage:

let tableView: UITableView = UITableView()
tableView.register(TestTableViewCell.self)
tableView.register(AnotherTableViewCell.self)

Everything works well, but we'd like to move these types to an array, for order. That's where we're stuck, this doesn't work:

let viewCells = [TestTableViewCell.self, AnotherTestTableViewCell.self]
// Without type annotation, it's [UITableViewCell.Type]
// And the error is: Instance method 'register' requires that 'UITableViewCell' conform to 'Reusable'

for viewCell in viewCells {
    tableView.register(viewCell)
}

We've also tried:

let viewCells: [Reusable.Type] = ...
// Error: Cannot invoke 'register' with an argument list of type '(Reusable.Type)'

Also this:

let viewCells: [(UITableViewCell & Reusable).Type] = ...
// Error: Instance method 'register' requires that 'UITableViewCell' conform to 'Reusable'

Is there a way to store the class type info with protocol conformance in an array to make this work?

What's the problem? It doesn't compile? I tried in playground and it compiles — Rico Crescenzio
– Rico Crescenzio, Commented Apr 18, 2019 at 13:25
@RicoCrescenzio yes, I've edited the question, it's because there's only 1 type of class type in the array. It should not compile. — EDUsta
– EDUsta, Commented Apr 18, 2019 at 13:30
"but we'd like to move these types to an array” You’re going to have trouble doing that. Swift doesn’t like the notion of an array of metatypes; a metatype is not really a type in Swift. You might be happier using some other programming language, or some other approach to the overall goal. — matt
– matt, Commented Apr 18, 2019 at 15:40

Tomas Jablonskis · Accepted Answer · 2019-04-24 08:33:29Z

5

Just write an extension for UITableViewCell so it conforms to Reusable protocol and do not cast cells to any type:

extension UITableViewCell: Reusable {}

P.S. you can also check the most popular Reusable protocol implementation on iOS here.

Good luck :)

edited Apr 24, 2019 at 8:33

answered Apr 18, 2019 at 13:04

Tomas Jablonskis

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9 Comments

EDUsta Over a year ago

It doesn't work unfortunately: Value of type ‘UITableViewCell.Type’ does not conform to ‘UITableViewCell & Reusable’ in coercion

Tomas Jablonskis Over a year ago

Instead of discussing in comments lets have a chat.

EDUsta Over a year ago

Summary of chat: It works, but we'd really like not to extend the protocol to every UITableViewCell.

JeremyP Over a year ago

@EDUsta Why don't you want to extend every UITableViewCell to conform to the protocol?

EDUsta Over a year ago

@JeremyP Fair question, there are some UITableViewCell classes that doesn't have a separate nib, or doesn't match with their names, so we're not ready to make that transition yet, instead we're doing one-by-one and get to know the process (which cells are ready/not ready).

|

Vyacheslav · Accepted Answer · 2019-04-18 13:05:46Z

2

protocol Reusable: class {
    static var defaultIdentifier: String { get }
}

extension Reusable where Self: UITableViewCell {
    static var defaultIdentifier: String {
        return String(describing: self)
    }
}

class TestTableViewCell: UITableViewCell, Reusable { }
extension UITableView {
    func register<T: UITableViewCell & Reusable>(_: T.Type) {
        register(UINib(nibName: T.defaultIdentifier, bundle: nil), forCellReuseIdentifier: T.defaultIdentifier)
    }
}
let tableView: UITableView = UITableView()
tableView.register(TestTableViewCell.self)
let viewCells = [TestTableViewCell.self]
// Without type annotation, it's [UITableViewCell.Type]
// And the error is: Instance method 'register' requires that 'UITableViewCell' conform to 'Reusable'

for viewCell in viewCells {
    tableView.register(viewCell)
}

This code successfully executes.

answered Apr 18, 2019 at 13:05

Vyacheslav

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2 Comments

EDUsta Over a year ago

Thanks for the answer, I couldn't see any difference, is it about Swift 4.2/5?

EDUsta Over a year ago

Update: When there's only 1 class, it compiles, but if there's more, it doesn't. Updated the question.

Yonguk Jeong · Accepted Answer · 2019-04-18 15:22:26Z

1

Hope you are happy with this. The key is Self keyword in protocol.

protocol Reusable: class {
    static var defaultIdentifier: String { get }
    static func register(tableView: UITableView)
}

extension Reusable where Self: UITableViewCell {
    static var defaultIdentifier: String {
        return String(describing: self)
    }

    static func register(tableView: UITableView) {
        tableView.register(UINib(nibName: Self.defaultIdentifier, bundle: nil), forCellReuseIdentifier: Self.defaultIdentifier)
    }
}

extension UITableView {
    func register(_ reusable: Reusable.Type) {
        reusable.register(tableView: self)
    }
}

let viewCells: [Reusable.Type] = [TestTableViewCell.self, AnotherTestTableViewCell.self]

for viewCell in viewCells {
    tableView.register(viewCell)
}

I recommend you to read the following links:

answered Apr 18, 2019 at 15:22

Yonguk Jeong

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1 Comment

Yonguk Jeong Over a year ago

@EDUsta Sounds great. Please accept my answer if you don't mind

Aman Pathak · Accepted Answer · 2019-04-18 14:12:30Z

0

Simply use this table view extension method to register the cell.

 extension UITableView {
     func registerCell<T: UITableViewCell>(cellType: T.Type) {
        let nib = UINib(nibName: String(describing: cellType.self), bundle: nil)
        let reuseIdentifier = String(describing: cellType.self)
        self.register(nib, forCellReuseIdentifier: reuseIdentifier)
     }
}

And use it like so

 let tableView: UITableView = UITableView()
    let cellsArray = [Test.self, Test1.self, Test2.self]
    for cell in cellsArray {
        tableView.registerCell(cellType: cell)
    }

answered Apr 18, 2019 at 14:12

Aman Pathak

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1 Comment

EDUsta Over a year ago

Thanks for your response, but it just ignores the protocol and that's the whole reason we're doing this.

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