泛型类型集合

基本上答案是：正确，您不能这样做。Swift需要在编译时而不是在运行时确定类型参数的具体类型。这在很多小的情况下都会出现。例如，您不能构造通用闭包并将其存储在没有类型指定的变量中。

如果我们将其简化为最小的测试用例，则可能会更清晰一些

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在这种情况下，应为哪种类型instantiate？这很容易，应该是Object。这是一个具体的类型，所以一切都很好。

在第2行，Swift不能创建任何具体类型T。它所具有的只是Creatable一个抽象类型（通过代码检查我们知道的实际值type，但是Swift并不考虑该值，而只是类型）。可以接受和返回协议，但是不能将它们变成类型参数。今天只是不合法的Swift。

Swift编程语言：通用参数和参数中暗示了这一点：

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。这些类型参数充当占位符，当创建泛型类型的实例或调用泛型函数或初始化程序时，这些类型参数将由实际的具体类型参数替换。（强调我的）

在Swift中，您将需要做的另一种尝试。

作为一项有趣的奖励，尝试明确要求不可能的事情：

let thing = instantiate(Creatable.self)

而且...迅速崩溃。

从您的进一步评论中，我认为闭包确实可以满足您的需求。您已使协议要求构造简单（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，并且完全不需要这种类型的参数化。这不是解决方法；我相信这是实现您描述的模式的更好方法。考虑以下内容（一些小的更改，以使示例更像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)
}