Function pointer in swift

Question

I'm following this tutorial, especially I have problems to convert this function in Swift language:

- (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;
}

of this, in the init function is context variable to give me problems.

In the code above, context is a variable of type : CFRunLoopSourceContext and the initialization of this object in the apple documentation is like this

so, I in my initialization, used the following code, concentrating on the schedule parameter :

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)

The function RunLoopSourceScheduleRoutine is like so :

    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)
    
}

but the compiler gives me the following error message : a c function pointer can only be formed from a reference to a 'func' or a literal closure

even if I make the following closure :

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)
        
    }

and i put it like this :

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)

gives me the same error. What is the problem ? any tips

Martin R · Accepted Answer · 2016-12-23 15:05:37Z

If you use func RunLoopSourceScheduleRoutine() as the callback then it needs to be a global function, not an instance method.

If you define the callback as a closure then it needs to be marked as a pure C callback:

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

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

Alternatively, pass a closure expression to that the compiler infers the type as a C callback:

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)

Note also that you have to call the method on obj, not on self. I would recommend GCD instead of performSelector(), this allows the compiler to check that the method is called with correct arguments:

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

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