在swift中比较两个自定义对象

Question

I have the following protocol defined in Swift:

protocol RecordingObserver {
    func aFunc()
}

Somewhere I have to compare two objects that implement this protocol, to check if they are the same. The problem I'm facing is that apparently Swift doesn't allow us to do this:

func areEqual(a:RecordingObserver,b:RecordingObserver){
    if a === b {
        println("Equal")
    }
}

Any idea why this is happening? And how I can do this in another way?

Answer 1

=== is the identical to operator and is used to test whether two object references both refer to the same object instance. It can be applied only to reference types (ie instances of a class ).

=== is different from the "equal to" operator == (which is required in the Equatable protocol).

Therefore, assuming that

• the actual observers are instances of a class , and
• your intention is to check if a and b refer to the same instance ,

you have to define the protocol as a class protocol :

protocol RecordingObserver : class {
    // ...
}

Then

func areEqual(a:RecordingObserver,b:RecordingObserver){
    if a === b {
        println("a and b refer to the same object instance")
    }
}

compiles (and works as expected) because the compiler knows that a and b are reference types.

Answer 2

Your class needs to support the Equatable protocol to use ==

https://developer.apple.com/library/ios/documentation/General/Reference/SwiftStandardLibraryReference/Equatable.html

Or if you want to use === something like this...

protocol RecordingObserver {
    func aFunc()
}

class MyClass: RecordingObserver {
    func aFunc() {
        // Do something
    }
}

func areEqual(a: MyClass, b: MyClass){
    if a === b {
        println("Equal")
    }
}

Answer 3

I believe there is an 'isEqual' method on NSObject. If your custom objects are both subclassed from that you should be able to compare a.isEqual(b) .

Answer 4

It is because you said that you objects implement only RecordingObserver . So compiler don't know if he can compare them.

Try this:

func areEqual<T where T: Equatable, T: RecordingObserver>(a: T,b: T) {

}

You can just copy this code into single view project to test:

protocol RecordingObserver {

}

class SomeClass: NSObject, RecordingObserver {
}

class ViewController: UIViewController {

    func areEqual<T where T: Equatable, T: RecordingObserver>(a: T,b: T) -> Bool {
        return true
    }

    override func viewDidLoad() {
        super.viewDidLoad()

        let a = SomeClass()
        let b = SomeClass()

        NSLog("\(areEqual(a, b: b))")
    }

}