Listeners

Swift
- In swift you use targets and tell them what method to call and when you'd like it called, for example with a button
```
  button.addTarget(self, action: #selector(self.buttonClicked), for: .touchUpInside)
  @objc private func buttonClicked() {
      // called when button tapped
  }
```
- A lot of things you would use a listener for in Java you would add a delegate for in swift, i.e if you wanted to know when a view changed size, you would call delegate methods to let the delegate know you are going to change the size.
- You can also add a willSet and didSet properties to a variable, for example :
```
  var myProperty: Int = 0 {
      didSet {
          print("The value of myProperty became \(myProperty)")
      }
  }
```
Java
- In java you use lamdas / anonymous inner class for event listeners
```
  button.addActionListener(e -> {
      // called when button tapped
  });
```
- You can also add these listeners to all sorts of things, like if a width / height changes.
  
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