Optional trailing closures in Swift

Swift language has a handy feature called trailing closures. It is a clean way of passing a closure into a function.

func makeTea(doNext: () -> ()) {
  // making tea ...
  doNext()
}

We can then call the function this way.

makeTea { drinkTea() }

Here we call makeTea function and pass a closure expression drinkTea() into it. Notice that we did not write parentheses () when calling makeTea, which is allowed if there are no other parameters.

Making trailing closure optional

But what if we don’t always need to pass a closure into our function? It is achieved by

1. making parameter an optional type (...)?,
2. using nil for its default value ` = nil` and
3. calling parameter using optional chaining ?().

func makeTea(doNext: (() -> ())? = nil) {
  // making tea ...
  doNext?()
}

Now we can call makeTea function without supplying the closure.

makeTea()