Question

我正在寻找Swift中等效的以下Obj-C代码：

- newInstanceOf:(id)classRef {
    return [classRef new];
}

然后使用它：

id instance = [whatever newInstanceOf:NSArray.class]
[instance isKindOfClass:NSArray.class] == YES

我尝试过使用Swift模板：

func newSomething<T>(classRef:T.Type) -> T {
    return classRef()
}

我收到错误：error: 'T' cannot be constructed because it has no accessible initializers

Answer 1

您可以创建一个协议，作为可由void-argument初始化程序初始化的对象的类型约束，然后将您选择的类型扩展到此协议。

protocol SimplyInitializable {
    init()
}

extension Int : SimplyInitializable { }
extension Double : SimplyInitializable { }
extension String : SimplyInitializable { }

struct MyStruct {
    var myInt : Int
    init() {
        myInt = 0
    }
}
extension MyStruct : SimplyInitializable { }

func newSomething<T: SimplyInitializable>(classRef: T.Type) -> T {
    return classRef.init()
}

/* Examples */
var a = newSomething(Int)
var b = newSomething(Double)
var c = newSomething("".dynamicType)
var d = newSomething(MyStruct)
var e = newSomething(a.dynamicType)

print(a.dynamicType) // Int
print(b.dynamicType) // Double
print(c.dynamicType) // String
print(d.dynamicType) // MyStruct
print(e.dynamicType) // Int

Answer 2

实际上在Swift中，并非所有类都保证有init（）初始化器。它可以与NSObject类一起使用，因为NSObject确实有这个要求。

func newInstanceOf<T:NSObject>(aClass:T.Type) -> NSObject
{ return aClass.init() }

let string = newInstanceOf(NSString)   // ""
let date   = newInstanceOf(NSDate)     // 2016-01-15 05:27:29 +0000

Swift：编写一个分配（任意）类实例的函数

2 个答案: