泛型类型集合

Question

基于上一个已解决的问题，但它导致了另一个问题。如果协议/类类型存储在集合中，则检索并实例化它们会引发错误。下面是一个假设的例子。该范例基于“Program to Interface not an implementation”What does it mean to "program to an interface"?

instantiate from protocol.Type reference dynamically at runtime

public protocol ISpeakable {
    init()
    func speak()
}

class Cat : ISpeakable {
    required init() {}
    func speak() {
        println("Meow");
    }
}

class Dog : ISpeakable {
    required init() {}
    func speak() {
        println("Woof");
    }
}

//Test class is not aware of the specific implementations of ISpeakable at compile time
class Test {
    func instantiateAndCallSpeak<T: ISpeakable>(Animal:T.Type) {
        let animal = Animal()
        animal.speak()
    }
}

// Users of the Test class are aware of the specific implementations at compile/runtime

//works
let t = Test()
t.instantiateAndCallSpeak(Cat.self)
t.instantiateAndCallSpeak(Dog.self)

//doesn't work if types are retrieved from a collection
//Uncomment to show Error - IAnimal.Type is not convertible to T.Type
var animals: [ISpeakable.Type] = [Cat.self, Dog.self, Cat.self]

for animal in animals {
    //t.instantiateAndCallSpeak(animal) //throws error
}

for (index:Int, value:ISpeakable.Type) in enumerate(animals) {
    //t.instantiateAndCallSpeak(value) //throws error
}

编辑 - 我当前的解决方法是遍历集合，但当然它是有限的，因为 api 必须知道各种实现。另一个限制是这些类型的子类（例如 PersianCat、GermanShepherd）不会调用它们的覆盖函数，或者我去 Objective-C 进行救援（NSClassFromString 等）或等待 SWIFT 支持此功能。

注意（背景）：这些类型由实用程序的用户推送到数组中，并在通知时执行 for 循环

var animals: [ISpeakable.Type] = [Cat.self, Dog.self, Cat.self]

for Animal in animals {
    if Animal is Cat.Type {
        if let AnimalClass = Animal as? Cat.Type {
            var instance = AnimalClass()
            instance.speak()
        }
    } else if Animal is Dog.Type {
        if let AnimalClass = Animal as? Dog.Type {
            var instance = AnimalClass()
            instance.speak()
        }
    }
}

Answer 1

基本上答案是：正确，你不能那样做。 Swift 需要在编译时确定类型参数的具体类型，而不是在运行时。这出现在很多小角落案例中。例如，你不能构造一个泛型闭包并将其存储在一个变量中而不指定类型。

如果我们把它归结为一个最小的测试用例，这可能会更清楚一点

protocol Creatable { init() }

struct Object : Creatable { init() {} }

func instantiate<T: Creatable>(Thing: T.Type) -> T {
    return Thing()
}

// works. object is of type "Object"
let object = instantiate(Object.self)   // (1)

// 'Creatable.Type' is not convertible to 'T.Type'
let type: Creatable.Type = Object.self
let thing = instantiate(type)  // (2)

在第 1 行，编译器有一个问题：T 在这个 instantiate 的实例中应该是什么类型？这很简单，应该是Object。这是一个具体的类型，所以一切都很好。

在第 2 行，没有 Swift 可以生成的具体类型T。它只有Creatable，这是一个抽象类型（我们通过代码检查知道type 的实际值，但Swift 不考虑值，只考虑类型）。取回协议是可以的，但是把它们做成类型参数就不行了。今天只是不合法的 Swift。

Swift Programming Language: Generic Parameters and Arguments 中暗示了这一点：

在声明泛型类型、函数或初始化程序时，请指定泛型类型、函数或初始化程序可以使用的类型参数。当创建泛型类型的实例或调用泛型函数或初始化程序时，这些类型参数充当占位符，由 实际具体 类型参数替换。 （强调我的）

你需要在 Swift 中做任何你想做的事情。

作为一个有趣的奖励，尝试明确要求不可能：

let thing = instantiate(Creatable.self)

而且...迅速崩溃。

---

从您进一步的 cmets 来看，我认为闭包完全符合您的要求。您已经使您的协议需要简单的构造 (init())，但这是不必要的限制。您只需要调用者告诉函数如何构造对象。使用闭包很容易，并且完全不需要类型参数化。这不是解决方法。我相信这是实现您所描述的模式的更好方法。考虑以下内容（一些小改动使示例更像 Swift）：

// Removed init(). There's no need for it to be trivially creatable.
// Cocoa protocols that indicate a method generally end in "ing" 
// (NSCopying, NSCoding, NSLocking). They do not include "I"
public protocol Speaking {
    func speak()
}

// Converted these to structs since that's all that's required for
// this example, but it works as well for classes.
struct Cat : Speaking {
    func speak() {
        println("Meow");
    }
}

struct Dog : Speaking {
    func speak() {
        println("Woof");
    }
}

// Demonstrating a more complex object that is easy with closures,
// but hard with your original protocol
struct Person: Speaking {
    let name: String
    func speak() {
        println("My name is \(name)")
    }
}

// Removed Test class. There was no need for it in the example,
// but it works fine if you add it.
// You pass a closure that returns a Speaking. We don't care *how* it does
// that. It doesn't have to be by construction. It could return an existing one.
func instantiateAndCallSpeak(builder: () -> Speaking) {
    let animal = builder()
    animal.speak()
}

// Can call with an immediate form.
// Note that Cat and Dog are not created here. They are not created until builder()
// is called above. @autoclosure would avoid the braces, but I typically avoid it.
instantiateAndCallSpeak { Cat() }
instantiateAndCallSpeak { Dog() }

// Can put them in an array, though we do have to specify the type here. You could
// create a "typealias SpeakingBuilder = () -> Speaking" if that came up a lot.
// Again note that no Speaking objects are created here. These are closures that
// will generate objects when applied.
// Notice how easy it is to pass parameters here? These don't all have to have the
// same initializers.
let animalBuilders: [() -> Speaking] = [{ Cat() } , { Dog() }, { Person(name: "Rob") }]

for animal in animalBuilders {
    instantiateAndCallSpeak(animal)
}