Design Patterns with Swift: Observing the Observer Pattern
The Observer Pattern is one of the most popular design patterns used in software engineering. It is a behavioral pattern that defines a one-to-many dependency between objects so that when one object changes state, all its dependents are notified and updated automatically. This pattern is commonly used in many applications such as user interfaces, event handling systems, and other distributed systems.
In this article, we will explore how to implement the Observer Pattern in Swift. We will learn what the pattern is and why it is useful, as well as how to create an Observer class and use it in our code. Finally, we will look at a few examples of how the Observer Pattern can be used in a real-world application.
What is the Observer Pattern?
The Observer Pattern is a software design pattern that allows objects to observe and respond to changes in the state of another object. It defines a one-to-many relationship between objects so that when one object changes state, all its dependent objects are notified and updated automatically.
The Observer Pattern is used to create a loosely coupled system where the objects being observed do not need to know the details of the observers. This makes it easy to add or remove observers without having to make changes to the observed object.
Why Use the Observer Pattern?
The Observer Pattern is useful for a variety of reasons. It allows you to decouple objects, making them more modular and easier to maintain. It also allows you to create a system where different objects can be notified of changes in the state of another object without needing to know the details of the observers.
Additionally, the Observer Pattern can be used to create a system where objects can communicate with each other without needing to be tightly coupled. This makes it easy to add or remove observers without having to make changes to the observed object.
Creating an Observer Class in Swift
To create an Observer class in Swift, we need to define a protocol that conforms to the ObserverPattern protocol. This protocol will define the methods that an observer must implement in order to be notified of changes in the state of an observed object.
We will also need to define a Subject class that will keep track of the observers and notify them when the state of the observed object has changed. The Subject class should also have methods for adding and removing observers.
Example: Using the Observer Pattern with a Button
Let’s take a look at an example of how the Observer Pattern can be used with a button. We will create a Subject class called ButtonObserver that will keep track of an array of observers and notify them when the state of the button changes.
We will also create an Observer protocol called ButtonObserverProtocol that will define the methods that an observer must implement in order to be notified when the state of the button has changed.
Finally, we will create a Button class that will keep track of its state and notify the observers when the state of the button has changed.
ButtonObserver Class
First, let’s create the ButtonObserver class. This class will keep track of an array of observers and notify them when the state of the button has changed.
class ButtonObserver {
var observers: [ButtonObserverProtocol] = []
func addObserver(_ observer: ButtonObserverProtocol) {
observers.append(observer)
}
func removeObserver(_ observer: ButtonObserverProtocol) {
if let index = observers.firstIndex(where: { $0 === observer }) {
observers.remove(at: index)
}
}
func notifyObservers() {
for observer in observers {
observer.buttonDidChangeState()
}
}
}ButtonObserverProtocol Protocol
Next, let’s create the ButtonObserverProtocol protocol. This protocol will define the methods that an observer must implement in order to be notified when the state of the button has changed.
protocol ButtonObserverProtocol {
func buttonDidChangeState()
}Button Class
Finally, let’s create the Button class. This class will keep track of its state and notify the observers when the state of the button has changed.
class Button {
var buttonObserver: ButtonObserver
var isSelected: Bool = false {
didSet {
buttonObserver.notifyObservers()
}
}
init(buttonObserver: ButtonObserver) {
self.buttonObserver = buttonObserver
}
func toggleState() {
isSelected = !isSelected
}
}Conclusion
In this article, we explored the Observer Pattern and how it can be used in Swift. We looked at how to create an Observer class and use it in our code, as well as a few examples of how the Observer Pattern can be used in a real-world application.
The Observer Pattern is a powerful tool for decoupling objects and creating a system where objects can communicate with each other without needing to be tightly coupled. It is a great tool to have in your software engineering toolbox and can be used in a variety of situations.