Store a closure as a variable in Swift

Objective-C

@interface PopupView : UIView
@property (nonatomic, copy) void (^onHideComplete)();
@end

@interface PopupView ()

...

- (IBAction)hideButtonDidTouch:(id sender) {
    // Do something
    ...
    // Callback
    if (onHideComplete) onHideComplete ();
}

@end

PopupView * popupView = [[PopupView alloc] init]
popupView.onHideComplete = ^() {
    ...
}

Swift

class PopupView: UIView {
    var onHideComplete: (() -> Void)?

    @IBAction func hideButtonDidTouch(sender: AnyObject) {
        // Do something
        ....
        // Callback
        if let callback = self.onHideComplete {
            callback ()
        }
    }
}

var popupView = PopupView ()
popupView.onHideComplete = {
    () -> Void in 
    ...
}

The compiler complains on

var completionHandler: (Float)->Void = {}

because the right-hand side is not a closure of the appropriate signature, i.e. a closure taking a float argument. The following would assign a "do nothing" closure to the completion handler:

var completionHandler: (Float)->Void = {
    (arg: Float) -> Void in
}

and this can be shortened to

var completionHandler: (Float)->Void = { arg in }

due to the automatic type inference.

But what you probably want is that the completion handler is initialized to nil in the same way that an Objective-C instance variable is inititialized to nil. In Swift this can be realized with an optional:

var completionHandler: ((Float)->Void)?

Now the property is automatically initialized to nil ("no value"). In Swift you would use optional binding to check of a the completion handler has a value

if let handler = completionHandler {
    handler(result)
}

or optional chaining:

completionHandler?(result)

I've provide an example not sure if this is what you're after.

var completionHandler: (_ value: Float) -> ()

func printFloat(value: Float) {
    print(value)
}

completionHandler = printFloat

completionHandler(5)

It simply prints 5 using the completionHandler variable declared.