Swift Closure Single Statement Issue

Question

Here I have a simple snippet to use simple animation with my defined UIView.

UIView.animateWithDuration(0.1) { [weak self] in

    self?.popOverView.center = gesture.locationInView(self?.view)
}

Here [weak self] is to avoid reference cycle,and I also use trailing closure to simply the code.Howerver,the compiler is unhappy with that and gives me the wrong message.

Cannot invoke 'animateWithDuration' with an argument list of type '(FloatLiteralConvertible, () -> () -> $T2)'

What does $T2 stands for ? And the strange thing is that when there is two or more statements in the closure body, it compiles correctly.

UIView.animateWithDuration(0.1) { [weak self] in
    println()
    self?.popOverView.center = gesture.locationInView(self?.view)
}

And I know that if there is only one statement in the closure body, it is automatically returned.

Antonio · Accepted Answer · 2014-11-24 11:07:05Z

5

Single statement body in a closure have implicit return, so what happens is that the compiler tries to set the result of this statement:

self?.popOverView.center = gesture.locationInView(self?.view)

as the return value. You can fix that by adding an explicit return

self?.popOverView.center = gesture.locationInView(self?.view)
return

This happens for single statements only, that's why it works correctly in your 2nd case

More info: Implicit Returns from Single-Expression Closures

answered Nov 24, 2014 at 11:07

Antonio

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