如何测试泛型变量是否为 AnyObject 类型 [英] How to test whether generic variable is of type AnyObject

问题描述

在 Swift 3 中，我不再能够检查泛型变量类型是否为类 (AnyObject).即使特定类型 T 和传递的值是结构而不是类，以下代码为 isObject 返回 true.在 Swift 2.3 和 2.2 中，它按预期工作，isObject 为 false.

In Swift 3 I am no longer able to check whether generic variable type is class (AnyObject) or not. Following code returns true for isObject even though specific type T and passed value is struct and not class. In Swift 2.3 and 2.2 it works as expected and isObject is false.

struct Foo<T>
{
    var value: Any?
    var isObject: Bool = false

    init (val: T?)
    {
        if val != nil
        {
            // following line shows warnings in Swift 3
            // conditional cast from 'T?' to 'AnyObject' always succeeds
            // 'is' cast is always true
            isObject = val is AnyObject

            self.value = val
        }
    }
}

struct Bar
{
    var bar = 0
}

let b = Foo<Bar>(val: Bar())

print(b.isObject) // -> true

如何让它在 Swift 3 中正常工作?

How can I make it work properly in Swift 3?

推荐答案

在 Swift 3 中，由于 _SwiftValue(参见 this Q&A 了解更多信息)，它可以将任何不能直接桥接到 Objective-C 的东西包装成不透明的Objective-C 兼容的盒子.

In Swift 3, everything is bridgeable to AnyObject due to the introduction of _SwiftValue (see this Q&A for more info), that can wrap anything that isn't directly bridgeable to Objective-C in an opaque Objective-C compatible box.

因此 is AnyObject 将始终为真，因为任何东西都可以通过包装在 _SwiftValue 中表示为 AnyObject.

Therefore is AnyObject will always be true, as anything can be represented as an AnyObject via wrapping in a _SwiftValue.

一种检查值是否为引用类型的方法(如本问答中所示;A) 是根据 AnyObject、AnyClass(又名 AnyObject.Type)的元类型对值的类型进行类型检查).

One way to check whether a value is a reference type (as shown in this Q&A) is to type-check the type of the value against the metatype of AnyObject, AnyClass (aka AnyObject.Type).

对于泛型，如果要检查T的静态类型是否为引用类型，可以这样做:

For generics, if you want to check whether the static type of T is a reference type, you can do:

isObject = T.self is AnyClass

如果你想检查一个类型为 T 的值的 dynamic type 是否是一个引用类型(比如你的例子中的 val)，您可以对解包的值使用 type(of:) 函数，正如前面提到的问答所建议的:

If you want to check whether the dynamic type of a value typed as T is a reference type (such as val in your example), you can use the type(of:) function on the unwrapped value, as the aforementioned Q&A suggests:

if let val = val {
    isObject = type(of: val) is AnyClass

    // ...
}

这两种方法的区别在于，当 T 是 Any 类型(或非 AnyObject 抽象类型)时，T.self is AnyClass 将返回 false(如果你想要一个值可以是引用或值类型的框，这可能很有用) – type(of: val)is AnyClass 然而，将返回 val 本身是否是一个引用类型.

The difference between these two approaches is that when T is of type Any (or a non AnyObject abstract type), T.self is AnyClass will return false (which could be useful if you want a box where the value could be a reference or value type) – type(of: val) is AnyClass however, will return whether val itself is a reference type.

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