通用类型集合

Question

在之前的问题得到解决的基础上,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.一个假设的例子如下.范例基于"程序到接口而非实现""编程到接口" 是什么意思？

在运行时动态地从protocol.Type引用实例化

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 - 我目前的解决方法是迭代集合,但当然它是有限的,因为api必须知道各种各样的实现.另一个限制是这些类型的子类(例如PersianCat,GermanShepherd)不会调用它们被覆盖的函数,或者我会去Objective-C进行救援(NSClassFromString等)或者等待SWIFT支持这个功能.

注意(背景):实用程序的用户将这些类型推送到数组中,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:正确的,你不能这样做.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.这出现在许多小角落案件中.例如,您无法构造通用闭包并将其存储在变量中,而不指定类型.

如果我们将其归结为最小的测试用例,这可能会更清楚一些

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在这个实例中应该是什么类型的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没有具体的类型T.它Creatable只是一个抽象类型(我们通过代码检查知道实际值type,但Swift不考虑值,只考虑类型).获取和返回协议是可以的,但是将它们变成类型参数是不可行的.今天,斯威夫特不合法.

这是Swift编程语言中的暗示:通用参数和参数:

声明泛型类型,函数或初始化程序时,可以指定泛型类型,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,由实际的具体类型参数替换.(强调我的)

你需要做任何你想在Swift中做另一种方式的事情.

作为一个有趣的奖金,尝试明确要求不可能:

let thing = instantiate(Creatable.self)

并且...迅速崩溃.

从您的进一步评论中,我认为闭包完全符合您的要求.你已经使你的协议需要简单的构造(init()),但这是一个不必要的限制.您只需要调用者告诉函数如何构造对象.使用闭包很容易,并且不需要以这种方式进行类型参数化.这不是一个解决方法; 我相信这是实现您所描述的模式的更好方法.请考虑以下(一些小的更改,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)
}