objective-c - 将 Swift 协议(protocol)传递给 Objective-C 指针

Question

使用 XCode 10.1/Swift 4.2。

我正在尝试将符合 Swift 协议(protocol)的对象分配给 Objective-C 指针。以下代码是按预期编译和工作的最小示例，但它会给我以下警告:

如果分配给局部变量:Incompatible pointer types initializing 'NSObject<Animal> *__strong' with an expression of type 'id<Animal> _Nullable'

如果分配给存储属性: Incompatible pointer types assigning to 'NSObject<Animal> *' from 'id<Animal> _Nullable'

关于如何解决该警告而不只是将其静音的任何想法？

Swift代码:

@objc protocol Animal {
    var name: String { get }
}

@objc class Pig: NSObject, Animal {
    var name: String = "pig"
}

@objc class Cow: NSObject, Animal {
    var name: String = "cow"
}

@objc class Farm: NSObject {
    static func getAnimal(name: String) -> Animal? {
        // return some animal or nil
    }
}

Objective-C 代码:

// This code returns a valid pointer to a Pig object
// that is usable in objective-c, but it also triggers 
// the warning described above
NSObject<Animal>* animal = [Farm getAnimalWithName:@"pig"];

Answer 1

指定每个 Animal实现者还实现了 NSObject的界面:@objc protocol Animal : NSObjectProtocol

您还可以将 ObjC 中的变量类型更改为 id<Animal> .