Class conforming to protocol as function parameter in Swift

Question

In Objective-C, it's possible to specify a class conforming to a protocol as a method parameter. For example, I could have a method that only allows a UIViewController that conforms to UITableViewDataSource:

- (void)foo:(UIViewController<UITableViewDataSource> *)vc;

I can't find a way to do this in Swift (perhaps it's not possible yet). You can specify multiple protocols using func foo(obj: protocol<P1, P2>), but how do you require that the object is of a particular class as well?

Answer 1

You can define foo as a generic function and use type constraints to require both a class and a protocol.

Swift 4

func foo<T: UIViewController & UITableViewDataSource>(vc: T) {
    .....
}

Swift 3 (works for Swift 4 also)

func foo<T: UIViewController>(vc:T) where T:UITableViewDataSource { 
    ....
}

Swift 2

func foo<T: UIViewController where T: UITableViewDataSource>(vc: T) {
    // access UIViewController property
    let view = vc.view
    // call UITableViewDataSource method
    let sections = vc.numberOfSectionsInTableView?(tableView)
}

Answer 2

In Swift 4 you can achieve this with the new & sign:

let vc: UIViewController & UITableViewDataSource

Answer 3

The Swift book documentation suggests that you use type constraints with a where clause:

func someFunction<C1: SomeClass where C1:SomeProtocol>(inParam: C1) {}

This guarantees that "inParam" is of type "SomeClass" with a condition that it also adheres to "SomeProtocol". You even have the power to specify multiple where clauses delimited by a comma:

func itemsMatch<C1: SomeProtocol, C2: SomeProtocol where C1.ItemType == C2.ItemType,    C1.ItemType: SomeOtherProtocol>(foo: C1, bar: C2) -> Bool { return true }

Answer 4

With Swift 3, you can do the following:

func foo(_ dataSource: UITableViewDataSource) {
    self.tableView.dataSource = dataSource
}

func foo(_ delegateAndDataSource: UITableViewDelegate & UITableViewDataSource) { 
    //Whatever
}

Answer 5

Swift 5:

func foo(vc: UIViewController & UITableViewDataSource) {
    ...
}

So essentially Jeroen's answer above.

Answer 6

What about this way?:

protocol MyProtocol {
    func getTableViewDataSource() -> UITableViewDataSource
    func getViewController() -> UIViewController
}

class MyVC : UIViewController, UITableViewDataSource, MyProtocol {

    // ...

    func getTableViewDataSource() -> UITableViewDataSource {
        return self
    }

    func getViewController() -> UIViewController {
        return self
    }
}

func foo(_ vc:MyProtocol) {
    vc.getTableViewDataSource() // working with UITableViewDataSource stuff
    vc.getViewController() // working with UIViewController stuff
}

Answer 7

Note in September 2015: This was an observation in the early days of Swift.

It seems to be impossible. Apple has this annoyance in some of their APIs as well. Here is one example from a newly introduced class in iOS 8 (as of beta 5):

UIInputViewController's textDocumentProxy property:

Defined in Objective-C as follows:

@property(nonatomic, readonly) NSObject<UITextDocumentProxy> *textDocumentProxy;

and in Swift:

var textDocumentProxy: NSObject! { get }

Link to Apple' documentation: https://developer.apple.com/library/prerelease/iOS/documentation/UIKit/Reference/UIInputViewController_Class/index.html#//apple_ref/occ/instp/UIInputViewController/textDocumentProxy

Answer 8

Update for Swift 5:

func yourFun<V: YourClass>(controller: V) where V: YourProtocol