The Delegate Design Pattern
Tags: typescript, oop, javascript
Published: 2024-01-13 00:43:35

Delegation, is a software design pattern where a class will pass on the responsibility of carrying out certain tasks to another class. Instead of a situation where one class is responsible for handling everything related to its feature delegation involves passing some of the responsibilities of a class to another class. Imagine it like a manager delegating tasks to their team members, each with their expertise, to achieve a larger goal. When an object receives a request, it can assign responsibility for fulfilling it to another object, called a delegate. This delegate acts as a skilled assistant, equipped to handle specific subtasks crucially, the delegator object maintains the context of the request, ensuring a cohesive experience.

Programming languages often have built-in ways to facilitate delegation seamlessly. They achieve this by ensuring the delegate operates within the original object's context, even though it's a separate entity. Specific design patterns, like the Delegate Pattern, provide structured ways to implement delegation explicitly, ensuring clarity and maintainability in code. Delegation differs from forwarding, where a request is merely passed along without preserving the original context. Delegation involves a deeper level of collaboration and context-sharing between objects.

In today's post, we will talk about Delegation in software design and thus we are going to consider the following talking points.

- Understanding the concept of delegation
- Real-world examples
- Benefits and drawbacks
- Implementation strategies

## Understanding The Concept Of Delegation

Imagine a restaurant where a customer (request) asks for a specific dish, and the waiter (object) takes the order but doesn't cook it themselves. The Chef (delegate) is the helper object who prepares the dish. Here's how delegation works in this scenario: Customer places the order (sends the request). The waiter receives the request but doesn't cook instead the waiter delegates the task to the chef, giving them the order details (passing the original object). The Chef uses their expertise to cook the dish (handles the request). The Chef then passes the completed dish back to the waiter (returns the result).

The waiter delivers the dish to the customer (fulfills the request). The Delegate is a helper object: It has specific skills to handle a task. The original context is preserved: The waiter (object) remains the customer's point of contact, even though the chef (delegate) does the work.

## Real-world examples

We are going to see how we can implement the concept of delegation in software design;

```typescript
interface Printer {
  print(document: string): boolean;
}

interface Scanner {
  scan(): string;
}

class InkPrinter implements Printer {
  print(document: string): boolean {
    console.log(`Printing document: ${document}`);
    return true; // Simulate successful printing
  }
}

class ColorScanner implements Scanner {
  scan(): string {
    return `Scanned document at ${new Date().toLocaleTimeString()}`;
  }
}

class Device implements Printer, Scanner {
  private printer: Printer = new InkjetPrinter();
  private scanner: Scanner = new ColorScanner();

  // Delegate printing to the printer object
  print(document: string): boolean {
    return this.printer.print(document);
  }

  // Delegate scanning to the scanner object
  scan(): string {
    return this.scanner.scan();
  }
}

// Usage
const device = new MultifunctionDevice();
console.log(device.print("Hello, world!")); // Output: Printing document: Hello, world!
console.log(device.scan()); // Output: Scanned document at 14:35:22 (example time)

```

From the code snippet we have above, you can see that we have two interfaces, the `Printer` and `Scanner` interfaces which are collectively responsible for describing what a printer and a scanner can do, we then create a class from each of the interfaces. We then create another class `Device` which implements the `Printer` and `Scanner` interfaces, inside the `Device` class we create some properties. a printer which is an instance of a `Printer` and a scanner which is an instance of a `ColorScanner`. The `Device` class also implements two methods, a `print` method and a `scan` method, however, these methods are just `delegating` the responsibility of printing and scanning to the respective classes to handle while returning the result. Except you're running on a 5KB RAM then you will observe that this pattern fits what we described as delegation because the main class which is the `Device` just takes the order which might be `Printing` or `Scanning` however it does not process this request but instead the `Device` forwards the responsibility of printing or scanning to the respective class which handles them but you can see that the context of the `Device` is preserved by storing the printer and the scanner class as properties on the `Device` and we use them via the `this` keyword, this preserves the link between the original object and the delegate while maintaining the same context between the both of them.

## Benefits and drawbacks of using Delegation

Overall, delegation is a powerful technique in software design. When used in a Godly manner, it can significantly improve the modularity, flexibility, maintainability, and reusability of your software. However, it's essential to be aware of potential pitfalls and use delegation strategically to reap its benefits without incurring the wrath of your code. The decision to use delegation should be based on the specific needs of your project and the complexity of the tasks involved. 

### Benefits of Delegation

Let's look at the advantages of using delegation;

- Delegation breaks down complex functionalities into smaller, well-defined units, making code easier to understand, maintain, and test. Each object focuses on its specific responsibility, leading to clearer and more concise code.
- By delegating tasks to specialized objects, you can easily swap or modify them without affecting the core logic. This makes your software adaptable to changing requirements and easier to extend with new features.
- Delegating facilitates isolating and fixing bugs or modifying specific functionalities without impacting the entire system. This reduces the risk of cascading errors and makes debugging smoother.
- Delegate objects can be shared across different parts of the system, reducing code duplication and improving efficiency. This also encourages the creation of reusable libraries and components.
- Delegation enables each object to handle its specific responsibility, resulting in cleaner code separation and increased focus on individual object's functionalities. This reduces code complexity and enhances readability.

### Drawbacks of Delegation

Now let's look at the evil aspects of implementing delegation;

- Creating and managing delegate objects can introduce additional complexity, especially for small or simple tasks, it's a no-brainer for very small tasks. The trade-off between modularity and overhead needs to be considered.
- Adding another layer of indirection through delegation can slightly impact performance. Evaluate the potential performance bottleneck when using delegation for performance-critical tasks.
- Misusing delegation can introduce implicit dependencies between objects, making the code less modular and more difficult to test. Ensure clear separation of responsibilities and loose coupling between objects.
- While isolating issues within individual objects helps debugging, tracing problems across several delegated tasks can be more challenging. Utilize clear logging and error-handling mechanisms when employing delegation.

## Implementation strategies

Delegation offers numerous benefits for software design, promoting modularity, flexibility, and maintainability. By understanding and utilizing these strategies, you can leverage the power of delegation to create well-structured, maintainable, and adaptable software systems. Let's dive into the key strategies for achieving effective delegation.

### Design Pattern

Delegate Pattern: The Delegate Pattern is a classic design pattern that explicitly defines how an object delegates a task to another object, providing a structured approach for responsibility decoupling and flexible implementation. The example we just considered in this post follows this approach.

Strategy Pattern: You can also use the `Strategy Pattern` and this pattern uses delegation to switch between different algorithms for the same task at runtime, promoting flexibility and adaptability. It allows you to define and switch between different algorithms or behaviors and provides a common interface for interchangeable implementations (strategies) of an operation. It Allows you to choose the right strategy at runtime based on specific conditions.

Observer Pattern: The Observer Pattern pattern leverages delegation to notify interested objects about state changes, facilitating efficient event handling and decoupling. Objects (observers) register their interest in another object (subject) and are automatically notified when the subject's state changes. This pattern also avoids tight coupling with specific values or events and enables multiple objects to react to changes in a single source, observers can define how they react to changes but the subject dictates what changes occur Observers have no control over when or how the subject changes, they only react to notifications.

Explicitly assign responsibilities to delegator and delegate objects to avoid confusion and maintain a clean code structure. It will also help to minimize dependencies between objects to promote modularity and ease of testing and replacement. Design robust error handling mechanisms to ensure smooth operation even if delegate tasks fail. Also Clearly document the expected behavior and contracts of delegate objects to guide integration and future maintenance. Evaluate whether the benefits of delegation outweigh the potential overhead of creating and managing delegate objects for less complex tasks. You can also Inject dependencies like delegate objects at runtime to improve test isolation and flexibility.

Delegation in software design is a powerful technique for building modular, flexible, and maintainable systems. By entrusting specialized objects with specific tasks, you can significantly improve the clarity, adaptability, and efficiency of your code. The key lies in understanding the core principles of delegation, weighing its benefits against potential drawbacks, and implementing it strategically using design patterns and best practices.

That's going to be it for this piece, I hope you found this useful and learned something new, what are your thoughts on implementing delegation, do you think the benefits of this approach outweigh the drawbacks? Do you personally use Delegation and if yes, what is your best approach when implementing delegation? What are your thoughts on this post? Let me know all these and more using the comment section below.