Swift: What does "completionHandler: ((Bool) -> Void)" mean? [duplicate]

Question

Can someone explain what completionHandler: ((Bool) -> Void) means?

For instance it appears when requesting camera access:

AVCaptureDevice.requestAccess(for: AVMediaType.depthData, completionHandler: (<#T##(Bool) -> Void#>))

I usually do this to check if access was granted or not:

func requestCamera() {
        AVCaptureDevice.requestAccess(for: AVMediaType.video) { (response) in
            if response {
                print("true")
            } else {
                print("denied")
            }
        }

    }

Obviously I do stuff there, but that doesn't matter here. I just want to understand what ((Bool) -> Void) means and why I have to use the completion handler here. With other functions I can just set the handler to nil, but in this case it expects a response in some way.

So what does this mean?

Answer 1

Read Closures section of the Swift documentation.

It is a completion closure that gets executed when the AVCaptureDevice.requestAccess is finished with requesting access from the user. It has one Bool parameter that is true/false according to whether the user granted the access or not. It is not optional, so you have to provide some closure - that was a decision of the AVCaptureDevice.requestAccess author, and it makes sense - if you are requesting the access, you are requesting it because you want to use AVCaptureDevice. Therefore the author expects you to react to completing the requestAccess in some way.

Answer 2

Closure expression syntax has the following general form:

{ (parameters) -> return type in
    statements
}

The parameters in closure expression syntax can be in-out parameters, but they can’t have a default value. Variadic parameters can be used if you name the variadic parameter. Tuples can also be used as parameter types and return types.

completionHandler: ((Bool) -> Void) this means that you'll get a Boolean value in your closure and it will return nothing(Void). Just like a method.

You can find this more about in Apple's documentation about closures