Escaping Closures(@escaping)

A closure is said to escape a function when the closure is passed as an argument to the function, but is called after the function returns.

When you declare a function that takes a closure as one of its parameters, you can write @escaping before the parameter’s type to indicate that the closure is allowed to escape.

When you make the closure outlive the funciton scope and don't mark with @escaping, you will get a compiler error.

var completionHandlers: [() -> Void] = []
func someFunctionWithEscapingClosure(completionHandler: @escaping () -> Void) {
    completionHandlers.append(completionHandler)
}

References

• Escaping Closures