如何使用实例方法作为只接受func或文字闭包的函数的回调

Question

在"ViewController.swift"中我创建了这个回调:

func callback(cf:CFNotificationCenter!, 
    ump:UnsafeMutablePointer<Void>, 
    cfs:CFString!, 
    up:UnsafePointer<Void>, 
    cfd:CFDictionary!) -> Void {

}

使用此观察者:

CFNotificationCenterAddObserver(CFNotificationCenterGetDarwinNotifyCenter(), 
    nil, 
    self.callback, 
    "myMESSage", 
    nil, 
    CFNotificationSuspensionBehavior.DeliverImmediately)

导致此编译器错误:

"AC函数指针只能通过对'func'或文字闭包的引用形成"

Answer 1

回调是指向C函数的指针,在Swift中,您只能传递全局函数或闭包(不捕获任何状态),但不能传递实例方法.

所以这确实有效:

CFNotificationCenterAddObserver(CFNotificationCenterGetDarwinNotifyCenter(),
        nil,
        { (_, observer, name, _, _) in
            print("received notification: \(name)")
        },
        "myMessage",
        nil,
        .DeliverImmediately)

但由于闭包无法捕获上下文,因此您无法直接引用self其属性和实例方法.例如,您无法添加

self.label.stringValue = "got it"
// error: a C function pointer cannot be formed from a closure that captures context

在闭包内部,在通知到达时更新UI.

有一个解决方案,但由于Swift严格的类型系统,它有点复杂.与Swift 2 - UnsafeMutablePointer <Void>中的对象类似,您可以将指针转换 self为void指针,将其作为observer参数传递给注册,并将其转换回回调中的对象指针.

class YourClass { 

    func callback(name : String) {
        print("received notification: \(name)")
    }

    func registerObserver() {

        // Void pointer to `self`:
        let observer = UnsafePointer<Void>(Unmanaged.passUnretained(self).toOpaque())

        CFNotificationCenterAddObserver(CFNotificationCenterGetDarwinNotifyCenter(),
            observer,
            { (_, observer, name, _, _) -> Void in

                // Extract pointer to `self` from void pointer:
                let mySelf = Unmanaged<YourClass>.fromOpaque(
                        COpaquePointer(observer)).takeUnretainedValue()
                // Call instance method:
                mySelf.callback(name as String)
            },
            "myMessage",
            nil,
            .DeliverImmediately)
    }

    // ...
}

闭包充当实例方法的"蹦床".

指针是未保留的引用,因此必须确保在取消分配对象之前删除观察者.

Swift 3更新:

class YourClass {

    func callback(_ name : String) {
        print("received notification: \(name)")
    }

    func registerObserver() {

        // Void pointer to `self`:
        let observer = UnsafeRawPointer(Unmanaged.passUnretained(self).toOpaque())

        CFNotificationCenterAddObserver(CFNotificationCenterGetDarwinNotifyCenter(),
            observer,
            { (_, observer, name, _, _) -> Void in
                if let observer = observer, let name = name {

                    // Extract pointer to `self` from void pointer:
                    let mySelf = Unmanaged<YourClass>.fromOpaque(observer).takeUnretainedValue()
                    // Call instance method:
                    mySelf.callback(name.rawValue as String)
                }
            },
            "myMessage" as CFString,
            nil,
            .deliverImmediately)
    }

    // ...
}

有关对象指针和C指针之间"桥接"的更多信息,另请参见如何将self转换为UnsafeMutablePointer <Void>键入swift.