What are delegates?

A delegate is an object that acts on behalf of, or in coordination with, another object when that object encounters an event in a program. In object-oriented programming, delegates are often used to implement the observer pattern, which allows objects to observe events happening in other objects and respond to them.

In the context of Swift, a delegate is often used to enable communication between classes. For example, a class might have a delegate property that refers to another object, and that object conforms to a specific protocol. The class can then send messages or information to the delegate object through methods defined in the protocol.

Here's an example to illustrate how this works:

// First, we define the protocol. A protocol can require specific
// properties, methods, and other requirements to be implemented.
protocol MessageDelegate: class {
  func didReceiveMessage(message: String)
}

// This is a simple class that sends messages to its delegate.
// The class has a property called "delegate" which should be an
// object that conforms to the MessageDelegate protocol.
class MessageSender {
  weak var delegate: MessageDelegate?

  func sendMessage(message: String) {
    // When the message is sent, we inform the delegate.
    delegate?.didReceiveMessage(message: message)
  }
}

// This is a simple class that receives messages from a MessageSender.
// The class conforms to the MessageDelegate protocol by implementing
// the required didReceiveMessage(message:) method.
class MessageReceiver: MessageDelegate {
  func didReceiveMessage(message: String) {
    print("Received message: \(message)")
  }
}

// Now we can create an instance of each class and have them communicate.
let sender = MessageSender()
let receiver = MessageReceiver()

// We set the receiver as the delegate of the sender.
sender.delegate = receiver

// Now when we call the sendMessage(message:) method on the sender,
// the receiver's didReceiveMessage(message:) method will be called.
sender.sendMessage(message: "Hello world!")

In this example, the MessageSender class has a delegate property of type MessageDelegate. This means that the delegate object must conform to the MessageDelegate protocol. The MessageReceiver class conforms to the MessageDelegate protocol, so we can set it as the delegate of the sender.

When we call the sendMessage(message:) method on the sender, it sends the message to its delegate by calling the didReceiveMessage(message:) method on the delegate object. The MessageReceiver class provides an implementation of this method, so it is called and the message is printed to the console.

In this way, the MessageSender class communicates with its delegate using the methods defined in the MessageDelegate protocol, and the delegate object provides the actual implementation of those methods.