我可以在Swift中为通用类型T分配默认类型吗? [英] Can I assign a default type to generic type T in Swift?

问题描述

我有一个通用类，希望能够与默认类型一起使用.现在，我可以使用任何类型对其进行初始化，但必须明确.

I have a generic class that I want to be able to use with a default type. Right now I can initialize it with any type, but I have to be explicit.

//Initialize with a type
MyManager<MyCustomerObject>()

// Initialize with NSObject (what I want to be my default type)
MyManager<NSObject>()

// This doesn't work, but I want this kind of functionality
class MyManager<T = NSObject> {}

// So I can create my manager like so and it inserts the default type as NSObject
MyManager() //Or MyManager<>()

在Swift中有可能吗?

Is this possible in Swift?

推荐答案

不支持默认的通用参数，但是您可以通过在类型受限的扩展上定义默认的init()来伪造它，在这种情况下，编译器将足够聪明，可以使用这种类型.例如:

There's no support for default generic arguments, but you can fake it by defining the default init() on a type-constrained extension, in which case the compiler will be smart enough to use that type. E.g.:

class MyManager<T> {
    let instance: T

    init(instance: T) {
        self.instance = instance
    }
}

extension MyManager where T == NSObject {
    convenience init() {
        self.init(instance: NSObject())
    }
}

现在您可以不使用任何参数来初始化类型，它将默认为MyManager<NSObject>:

And now you can initialize the type with no argument and it will default to MyManager<NSObject>:

let mm1 = MyManager(instance: "Foo") // MyManager<String>
let mm2 = MyManager(instance: 1) // MyManager<Int>
let mm3 = MyManager() // MyManager<NSObject>

SwiftUI经常使用这种技术.

SwiftUI uses this technique quite a lot.

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