swift中的函数指针答案

【问题标题】：Function pointer in swiftswift中的函数指针
【发布时间】：2016-12-23 14:04:07
【问题描述】：

我正在关注这个tutorial，尤其是我在用 Swift 语言转换这个函数时遇到了问题：

- (id)init
{
    CFRunLoopSourceContext    context = {0, self, NULL, NULL, NULL, NULL, NULL,
                                    &RunLoopSourceScheduleRoutine,
                                    RunLoopSourceCancelRoutine,
                                    RunLoopSourcePerformRoutine};

    runLoopSource = CFRunLoopSourceCreate(NULL, 0, &context);
    commands = [[NSMutableArray alloc] init];

    return self;
}

其中，在init函数中是context变量给我出问题。

在上面的代码中，context是一个类型为CFRunLoopSourceContext的变量，而苹果文档中这个对象的初始化是like this

所以，我在初始化时使用了以下代码，专注于schedule 参数：

var context = CFRunLoopSourceContext(version: 0, info: bridge(obj: self) ,
                                         retain: nil,
                                         release: nil,
                                         copyDescription: nil,
                                         equal: nil,
                                         hash: nil,
                                         schedule: RunLoopSourceScheduleRoutine,
                                         cancel: nil,
                                         perform: nil)

函数RunLoopSourceScheduleRoutine是这样的：

    func RunLoopSourceScheduleRoutine(info:UnsafeMutableRawPointer? ,rl:CFRunLoop? , mode:CFRunLoopMode?)  {
    
    let obj :  RunLoopSource = Unmanaged<RunLoopSource>.fromOpaque(info!).takeUnretainedValue()
    let theContext = RunLoopContext(withSource: obj, andLoop: rl!)
    performSelector(onMainThread: #selector(myMethod), with: theContext, waitUntilDone: false)
    
}

但是编译器给了我以下错误信息：a c function pointer can only be formed from a reference to a 'func' or a literal closure

即使我做了以下关闭：

let runLoopSourceScheduleRoutine = { (info:UnsafeMutableRawPointer? ,rl:CFRunLoop? , mode:CFRunLoopMode?)-> Void in return
        
        let obj :  RunLoopSource = Unmanaged<RunLoopSource>.fromOpaque(info!).takeUnretainedValue()
        let theContext = RunLoopContext(withSource: obj, andLoop: rl!)
        performSelector(onMainThread: #selector(myMethod), with: theContext, waitUntilDone: false)
        
    }

我是这样说的：

var context = CFRunLoopSourceContext(version: 0, info: bridge(obj: self) ,
                                         retain: nil,
                                         release: nil,
                                         copyDescription: nil,
                                         equal: nil,
                                         hash: nil,
                                         schedule: runLoopSourceScheduleRoutine,
                                         cancel: nil,
                                         perform: nil)

给了我同样的错误。问题是什么？任何提示

【问题讨论】：

标签： ios objective-c swift function pointers

【解决方案1】：

如果你使用func RunLoopSourceScheduleRoutine() 作为回调那么它需要是一个全局函数，而不是实例方法。

如果将回调定义为闭包，则需要对其进行标记作为纯 C 回调：

let runLoopSourceScheduleRoutine: @convention(c) (UnsafeMutableRawPointer?, CFRunLoop?, CFRunLoopMode?) -> Void =
    { (info, rl, mode) in

        let obj = Unmanaged<RunLoopSource>.fromOpaque(info!).takeUnretainedValue()
        // ...
}

或者，将闭包表达式传递给编译器将类型推断为 C 回调：

var context = CFRunLoopSourceContext(version: 0, info: UnsafeMutableRawPointer(Unmanaged.passUnretained(self).toOpaque()) ,
                 retain: nil,
                 release: nil,
                 copyDescription: nil,
                 equal: nil,
                 hash: nil,
                 schedule: { (info, rl , mode) in

                    let obj = Unmanaged<RunLoopSource>.fromOpaque(info!).takeUnretainedValue()
                    // ...
                 }, cancel: nil,
                 perform: nil)

还请注意，您必须在 obj 上调用该方法，而不是在 self 上。我会推荐 GCD 而不是 performSelector()，这允许编译器检查方法是否被正确调用论据：

let obj = Unmanaged<RunLoopSource>.fromOpaque(info!).takeUnretainedValue()
let theContext = RunLoopContext(withSource: obj, andLoop: rl!)
DispatchQueue.main.async {
    obj.myMethod(theContext)
}

【讨论】：