无法在Swift中创建符合协议的数组

protocol Foo {
    typealias BazType

    func bar(x:BazType) -> BazType
}

class Thing: Foo {
    func bar(x: Int) -> Int {
        return x.successor()
    }
}

class AnotherThing: Foo {
    func bar(x: String) -> String {
        return x
    }
}

var foos: [Foo] = [Thing()]

因为AnotherThing也符合Foo,所以我们也可以把它放在foos中.

foos.append(AnotherThing())

现在我们从foos中随机抓取一个foo.

let foo = foos[Int(arc4random_uniform(UInt32(foos.count - 1)))]

我要调用方法栏,你能告诉我,我应该发送一个字符串或一个整数到bar吗？

foo.bar(“foo”)或foo.bar(1)

斯威夫特不能.

所以它只能用作一般的约束.

什么场景需要这样的协议？

例：

class MyClass<T: Foo> {
        let fooThing: T?

        init(fooThing: T? = nil) {
                self.fooThing = fooThing
        }

        func myMethod() {
                let thing = fooThing as? Thing // ok
                thing?.bar(1) // fine

                let anotherThing = fooThing as? AnotherThing // no problem
                anotherThing?.bar("foo") // you can do it

                // but you can't downcast it to types which doesn't conform to Foo
                let string = fooThing as? String // this is an error
        }
}