ios - 如何在 Swift 中的 AnyClass 上使用 performSelector() 调用类方法？

Question

在 ObjC 中，您可以使用 NSObject 中的类方法简单地调用类方法。

[Machine performSelector:@selector(calculate:) withObject:num];

但是在 Swift 2.2 中如何做到这一点？

@objc(Machine) // put it here, so you can simply copy/paste into Playground
class Machine: NSObject {
    static func calculate(param: NSNumber) -> String {
        if param.integerValue > 5 {
            return "42"
        }
        return "42" // there is only 1 answer to all the questions :D
    }
}

if let aClass = NSClassFromString("Machine") {
    let sel = #selector(Machine.calculate(_:))
    let num = NSNumber(integer: 1337)
    let answer = aClass.performSelector(sel, withObject: num) // compiler error
    // let answer = aClass.calculate(num)                     // <-- this works
    print(answer)
}

使用这段代码，我得到以下编译器错误:

error: cannot invoke 'performSelector' with an argument list of type '(Selector, withObject: NSNumber)'

我在这里错过了什么？

Answer 1

AnyClass 不符合开箱即用的 NSObjectProtocol。我必须将 aClass 转换为 NSObjectProtocol 才能使用 performSelector (performSelector:withObject: 作为方法桥接到 Swift在 NSObjectProtocol 上:

swift 3:

if let aClass = NSClassFromString("Machine") {
    let sel = #selector(Machine.calculate(param:))
    let num = NSNumber(value: 1337)

    if let myClass = aClass as? NSObjectProtocol {
        if myClass.responds(to: sel) {
            let answer = myClass.perform(sel, with: num).takeRetainedValue() // this returns AnyObject, you may want to downcast to your desired type
            print(answer) // "42\n"
        }
    }
}

swift 2.x:

(aClass as! NSObjectProtocol).performSelector(sel, withObject: num) // Unmanaged<AnyObject>(_value: 42) 

更安全一点:

if let aClass = NSClassFromString("Machine") {
    let sel = #selector(Machine.calculate(_:))
    let num = NSNumber(integer: 1337)

    if let myClass = aClass as? NSObjectProtocol {
        if myClass.respondsToSelector(sel) {
            let answer = myClass.performSelector(sel, withObject: num).takeUnretainedValue()
            print(answer) // "42\n"
        }
    }
}

performSelector 返回一个 Unmanaged 对象，这就是为什么 takeUnretainedValue()(或者可选的 takeRetainedValue()，如果你想要转移内存所有权)是必需的。