It is possible for a closure to be called after the function it was passed into returns. This is called an escaping closure and is wrapped with the @escaping keyword. This is common when a function takes a closure, performs a long-running operation (e.g. a network request), and calls the closure after the function has returned. Closures also escape when they are assigned to a variable to be used later on. The closure may be called long after the calling function returns. Swift needs to hold a reference to the closure in this case which is accomplished with the @escaping keyword. Let’s take a look at an example:

struct Recipe {

  let ingredientAdder: (String) -> Void

  init(ingredientAdder: (String) -> Void) { //ERROR
    self.ingredientAdder = ingredientAdder
  }
}

Above we have a struct named Recipe that takes a closure in its initializer. Within the initializer, we assign it to a constant named ingredientAdder on the Recipe struct. This will not compile because we hold a reference to the closure via ingredientAdder which can be called at some point in the future. To fix this we wrap the closure with @escaping to tell the compiler to hold a reference to the closure for later use.

struct Recipe {

  let ingredientAdder: (String) -> Void

  init(ingredientAdder: @escaping (String) -> Void) { 
    self.ingredientAdder = ingredientAdder
  }
}