Swift选择器和闭包讨论

Question

I searched a lot and haven't found anything that really helped to solve my problem...

I'm trying to build a simple UIControl from scratch, like a UIButton. It can't be subclass of UIControl for particular reasons.

I need this control to do something like this:

myControl.addTarget(target: AnyObject?, action: Selector, forControlEvents: UIControlEvents)

The problem is that the implementation of the method that execute this selector when the button is touched needs the "performSelector:" method.

Swift doesn't have a "performSelector:". So I taught it could be implemented using closures.

I couldn't figure out how to capture the objects I want to modify inside the closure. And I'm not sure how I would deal with reference cycles and other things like that.

I don't even know if I am on the right path to succeed this. I'm sure you guys can put me on the right track!

I'm from Brazil, sorry for my poor English! Thanks! :D

Here is what a have so far...

struct ClosureForEvent {
    var closure:(control:MyControl!)->()
    var event:UIControlEvents 
}

class MyControl {
     private var closures:[ClosureForEvent]?

     init() {}

     func addClosureFor(event:UIControlEvents, closure:(control:MyControl!)->()) {

          if closures == nil {
              closures = [ClosureForEvent(closure: closure, event: event)]
          }
          else {
              closures!.append(ClosureForEvent(closure: closure, event: event))
          }
      }


      func executeClosuresOf(event:UIControlEvents) {
          if closures != nil {
              for closure in closures! {
                  if closure.event == event {
                      closure.closure(control: control)
                  }
              }
          }
      }
  }

  class Test {
       var testProperty = "Default String"

       init() {
            let control = MyControl()
            control.addClosureFor(UIControlEvents.TouchUpInside, closure: { (control) -> () in
            self.testProperty = "This is making a reference cycle?"
            })
       }
  }

Answer 1

In your case at the moment, it seems that no cyclic-reference is formed. But it's possible to accidentally be formed in the future. Like the followings for the instance.

// In the `Test` class.
let control = MyControl()
init() {
    control.addClosureFor(UIControlEvents.TouchUpInside, closure: { (control) -> () in
        self.testProperty = "This is making a reference cycle?"
    })
}

In this code, self has a reference to the control as its field and the control has a reference to self through the closure added by addClosureFor . This forms a cyclic reference so these objects in the cycle are never released.

closure capture list helps this issue.

control.addClosureFor(UIControlEvents.TouchUpInside) { [weak self] /* <-- */ control -> () in
    // Here, `self` is visible as `Optional<Test>` so the `?` operator is required.
    self?.testProperty = "This is making a reference cycle?"
    return
}

In the block in this code above, self is weakly captured so that an only one unidirectional reference is formed from self to the control . Note that, now in the closure, the access to self isn't assured because self is weakly-referenced. You have to consider self an optional value.

https://developer.apple.com/library/prerelease/ios/documentation/Swift/Conceptual/Swift_Programming_Language/AutomaticReferenceCounting.html