如何从不符合规范的 class 调用协议函数？

Question

我正在使用协议，当我尝试从不符合协议的 class 调用它时，我的应用程序崩溃了。

问题的游乐场代码：

import Cocoa

var str = "Hello, playground"

public protocol example {
    func artihmetic(x: Int, y: Int) -> Int
}

class A : example {
    func artihmetic(x: Int, y: Int) -> Int { return x + y }
}

class B : example {
    func artihmetic(x: Int, y: Int) -> Int { return x * y }
}

class C {

    public init() {}

    func callArithmetic() {
        let x = 2, y = 2
        let z = (self as! example).artihmetic(x: x, y: y)
        print(z)
    }
}

let instC = C()
instC.callArithmetic()

这段代码崩溃了，我得到了这个错误：

Could not cast value of type '__lldb_expr_1.C' (0x112cd91c0) to '__lldb_expr_1.example' (0x7fff993f57e8)

我想知道是否有任何替代方法，或者是否可以从不一致的 class 调用协议函数。

Answer 1

您的错误是认为 Swift 具有某种结构类型，即

Hey, these types are similar enough in their structure (the methods, properties, etc. that they have), I should be able to freely cast between them.

完全不是这样。 Swift 对除元组和函数类型之外的所有内容都有一个 "nominal type system"。两个结构可以具有完全相同的实现，但名称不同，并且它们仍然是不同的、不兼容的类型。

Answer 2

self as! example

当你这样做时，编译器会像，"umm ok let me try that. Let's see the the type of self...ok. self is of type C. Is C a subclass of another type that conforms to example? No. Ok let me trying something else. Does the C class itself ever adopt the example protocol? No.! That's bad. I can't succeed. Oh shoot, they're using ! with as...I have to crash"

C需要采用example协议。你在这样做吗？没有！

另一个更明确的例子是：

protocol Feeder {
    func nurse()
}

class Human {
    func eat() {
        print("eat")
    }
}

class Man: Human{

}

class Woman: Human, Feeder {
    func nurse() {
        print("nurse baby")
    }
}

let human1: Human = Man()
let human2: Human = Woman()

func isFeeder(human: Human) {
    if let feeder = human as? Feeder { // very similar to: `self as! example`
        print("human is feeder!")
    } else {
        print("human is not feeder!")
    }
}

isFeeder(human: human1) // human is not feeder!
isFeeder(human: human2) // human is feeder!

如何从不符合规范的 class 调用协议函数？

How to call a protocol function from a non-conforming class?

casting

protocols

swift