Store a closure as a variable in Swift

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)