Design Patterns in Swift: Mastering the State Pattern

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Design Patterns in Swift: Mastering the State Pattern

When developing software applications, design patterns are essential for creating maintainable and extensible code. In this article, we’ll discuss the state pattern and how it can be implemented in Swift.

The state pattern is a behavior pattern that allows an object to change its behavior based on its internal state. It is used when an object needs to have different behaviors depending on its state. The state pattern is an object-oriented version of the classic if-else or switch statement.

The state pattern is composed of three parts: the context, the state interface, and concrete states. The context is an object that encapsulates the current state of the system. It maintains an instance of one of the concrete states, which defines the behavior for the context. The state interface defines the contract for concrete states to implement.

To illustrate the state pattern, let’s consider an example of an order processing application. An order can have one of three states: pending, processing, or completed. Depending on the state, different actions can be taken on the order. For example, if an order is pending, it can be cancelled or shipped. If an order is processing, it can be canceled or marked as completed.

Let’s create a simple Swift class to model the order and its states. We’ll start by defining the Order class. This class will contain the state of the order and a method to transition to a new state.

class Order {
    var state: OrderState
    
    init(state: OrderState) {
        self.state = state
    }
    
    func transitionToState(state: OrderState) {
        self.state = state
    }
}

Next, we need to define the OrderState protocol. This protocol defines the contract for concrete states to implement. For this example, we’ll define three methods: canCancel(), canShip(), and canComplete().

protocol OrderState {
    func canCancel(order: Order) -> Bool
    func canShip(order: Order) -> Bool
    func canComplete(order: Order) -> Bool
}

Now, we can define the concrete states for our order. Each state will implement the OrderState protocol and provide an implementation for the three methods.

class PendingState: OrderState {
    func canCancel(order: Order) -> Bool {
        return true
    }
    
    func canShip(order: Order) -> Bool {
        return true
    }
    
    func canComplete(order: Order) -> Bool {
        return false
    }
}

class ProcessingState: OrderState {
    func canCancel(order: Order) -> Bool {
        return true
    }
    
    func canShip(order: Order) -> Bool {
        return false
    }
    
    func canComplete(order: Order) -> Bool {
        return true
    }
}

class CompletedState: OrderState {
    func canCancel(order: Order) -> Bool {
        return false
    }
    
    func canShip(order: Order) -> Bool {
        return false
    }
    
    func canComplete(order: Order) -> Bool {
        return false
    }
}

Now that we have defined our order states, we can create an instance of an order and transition between states. 

let order = Order(state: PendingState())

print(order.state.canCancel(order: order)) // prints "true"
print(order.state.canShip(order: order)) // prints "true"
print(order.state.canComplete(order: order)) // prints "false"

order.transitionToState(state: ProcessingState())

print(order.state.canCancel(order: order)) // prints "true"
print(order.state.canShip(order: order)) // prints "false"
print(order.state.canComplete(order: order)) // prints "true"

The state pattern is a powerful way to encapsulate complex state-based behavior. It helps to reduce the number of conditionals in your code and makes it easier to add new states. By using the state pattern, you can keep your code clean and maintainable.

In this article, we discussed the state pattern and how it can be implemented in Swift. We created a simple order processing application to illustrate the pattern. We discussed how the state pattern helps to reduce the number of conditionals in your code and make it easier to add new states. Finally, we saw how the state pattern can help keep your code clean and maintainable.

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