Closures are self-contained blocks of functionality that can be passed around and used in your code. Closures in Swift are similar to blocks in C and Objective-C and to lambdas in other programming languages.
By giving you a simple example that we can not change the corner radius from the design view. We have to change it run time. If you want to make possible it with design time then you can use the @IBDesignable and @IBInspectable.