Design Patterns: State

What is State?

State refers to the condition of a system or an object at a particular moment in time, especially in terms of its behavior or properties. This concept is fundamental not just in physical science, where we discuss states of matter (solid, liquid, gas), but also in programming, where state determines how an object behaves.

For example, consider water:

• As a solid (ice), it’s hard and maintains a fixed shape.
• As a liquid, it flows and takes the shape of its container.
• As a gas, it expands to fill any space available.

Similarly, in programming, an object’s state can drastically change its behavior. Take an online shopping order, for example:

• Is it a new order?
• Is it being processed?
• Has it been cancelled?
• Has it been shipped?

Each of these states represents a different stage in the lifecycle of an order, and the state determines what actions are permissible. For instance, a user can’t edit an order that has been cancelled, and a completed order cannot be cancelled.

What Does the State Design Pattern Address?

The State Design Pattern provides a structured approach to managing the changing states of an object, allowing the object to alter its behavior when its internal state changes. This pattern is particularly useful when:

• You want to avoid complex conditionals (e.g., if statements) based on the state.
• You need to manage state-specific behavior in a way that allows for easy extension and maintenance.

When writing code, if we don’t have a centralized place that manages the state of an object. This can lead to:

• Interdependent logic: Different parts of the codebase rely on specific states, creating tight coupling.
• Time-consuming state management: Tracking and updating state across various parts of an application can be error-prone and labor-intensive.
• Difficulty in extending functionality: Adding new states or behaviors often requires modifying existing code, which increases the risk of bugs.
• Challenges in debugging: Interdependencies make it harder to trace the source of issues.

The State Design Pattern minimizes conditional complexity, allowing you to manage state transitions cleanly and without relying on multiple boolean fields or nested conditionals. The benefits of using this pattern include:

• Modularity: State-specific behavior is encapsulated in individual classes, promoting a clear separation of concerns.
• Maintainability: Modifications to one state don’t affect others, making the code easier to update.
• Readability: The structure of the pattern makes the code easier to understand at a glance.
• Debuggability: Isolating behavior by state makes it easier to identify and resolve issues.

How Does the State Pattern Work?

The State Design Pattern encapsulates state-specific behaviors within separate state objects. The context class, which represents the object whose state is changing, delegates behavior execution to these state objects, switching between them as the state changes.

Elements of the State Pattern

1. Context
   • Maintains an instance of a concrete state as the current state.
   • Delegates state-specific behavior to the current state.
2. Abstract State
   • An abstract class or interface that defines the behavior all concrete states must implement.
3. Concrete State(s)
   • Subclasses of the abstract state.
   • :w Implement behaviors specific to a particular state of the context.

Here’s a visual representation of the pattern:

In this diagram, we see:

• The Context holds a reference to an abstract state, which could be any of the concrete states.
• ConcreteStateA andConcreteStateB implement the behavior defined in the AbstractState interface.
• The Context delegates its behavior to the state object it holds.
• This structure may seem simple, but it encourages a different approach to coding. By enforcing that an object can only be in one state at a time, the pattern ensures that state-specific logic is isolated and well-defined.

Example: Managing Order States in an E-Commerce System

Consider a system for buying clothes online. The possible states for an order could include:

• New Order
• Cancelled
• Pending
• Completed

The conditions for transitioning between these states might look like this:

As we mentioned earlier, the State Design Pattern “delegates the execution of state-specific behaviors to one state object at a time.” This means that the order is in only one state at any given time, and there’s no need to check for other states because they are not relevant in the current context.

Although this may seem like a limitation at first, it is this very restriction that makes the State Design Pattern so powerful. It allows developers to focus on what should happen within a state without worrying about how it will affect other states. As a result, the code is cleaner, more modular, and easier to maintain.

Conclusion

The State Design Pattern is a powerful tool in a developer’s toolkit for managing state-related complexity. By encapsulating state-specific behavior within distinct classes, it simplifies state management, making your code more modular, maintainable, and easier to understand. Whether you’re dealing with orders in an e-commerce system or any other scenario where objects undergo state transitions, the State Design Pattern can help you manage these changes elegantly and efficiently.