First we need to create a Protocol object to define what kind of functions will exists in the delegate. Here is a template:

1
2
3
4
public protocol CustomDelegate: class {
  func callback();
  optional func optionalCallback();
}

Use optional if the function is not required to implement for this protocol. If you want to use optional, you will need to expose the protocol to Objective-C. Add @objc in front of the Protocol.

If you do not want to use @objc for the optional keyword, you can remove the optional and provide a default implementation for the protocol.

1
2
3
4
5
6
7
8
9
10
protocol CustomDelegate: class {
  func callback();
  func optionalCallback();
}

extension CustomDelegate {
  func optionalCallback() {
    return
  }
}

Note: If you are making a framework and want to expose this Protocol, you will need to mark both Protocol and Extension public.

Reference to Delegate

Assume you have an object hosting the delegate, like this:

1
2
3
open class Main {
  public weak var delegate: CustomDelegate?
}

Delegates will stored as optional variables inside a class. Remember to set it as WEAK. This is usually because your class has the ownership of the delegate.

To call the callback, use delegate?.callback(). Or delegate?.optionalCallback?() for optional functions.