将 Swift 协议传递给 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，但它给了我以下警告：

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

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

知道如何解决该警告而不只是将其静音吗？

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 实现者也实现 NSObject 的接口：@objc protocol Animal : NSObjectProtocol

您还可以将 ObjC 中的变量类型更改为 id<Animal>。

将 Swift 协议传递给 Objective-C 指针

Passing a Swift protocol to an Objective-C pointer

objective-c

compiler-warnings

pointer-conversion

swift

swift-protocols