Strategy Design Pattern in Kotlin

1. Definition

The Strategy Design Pattern defines a family of algorithms, encapsulates each one, and makes them interchangeable. It lets the algorithm vary independently from clients that use it.

2. Problem Statement

Suppose you're designing a navigation app that provides different modes of transportation: driving, walking, or taking public transit. If you try to implement all algorithms (routes) within the app, it will become cluttered and challenging to manage.

3. Solution

The Strategy Pattern suggests that you take a class that does something specific in a lot of different ways and extract all these algorithms into separate classes called strategies. The original class, called context, must maintain a reference to a strategy object and delegate the work to this strategy, instead of implementing multiple versions of the algorithm.

4. Real-World Use Cases

1. Navigation apps providing different routes based on travel mode: driving, biking, walking.

2. Image compression applications where users can choose different compression algorithms.

3. Payment gateways in e-commerce applications allow multiple payment strategies.

5. Implementation Steps

1. Define a strategy interface common to all supported algorithms.

2. Implement concrete strategies for the various algorithms.

3. Define a context class to maintain a reference to a strategy object and switch between strategies.

6. Implementation in Kotlin Programming

// Step 1: Strategy Interface
interface RouteStrategy {
    fun buildRoute(start: String, destination: String): String
}
// Step 2: Concrete Strategies
class DrivingStrategy : RouteStrategy {
    override fun buildRoute(start: String, destination: String): String {
        return "Driving route from $start to $destination"
    }
}
class WalkingStrategy : RouteStrategy {
    override fun buildRoute(start: String, destination: String): String {
        return "Walking route from $start to $destination"
    }
}
// Step 3: Context Class
class Navigator(private var strategy: RouteStrategy) {
    fun setStrategy(newStrategy: RouteStrategy) {
        strategy = newStrategy
    }
    fun buildRoute(start: String, destination: String): String {
        return strategy.buildRoute(start, destination)
    }
}
// Client Code
fun main() {
    val navigator = Navigator(DrivingStrategy())
    println(navigator.buildRoute("Point A", "Point B"))
    navigator.setStrategy(WalkingStrategy())
    println(navigator.buildRoute("Point A", "Point B"))
}

Output:

Driving route from Point A to Point B
Walking route from Point A to Point B

Explanation:

1. RouteStrategy defines a contract that all strategies (algorithms) must follow.

2. Concrete strategies (DrivingStrategy, WalkingStrategy) encapsulate the specific algorithms.

3. Navigator, the context class, maintains a reference to a strategy object and can switch between strategies using setStrategy.

4. The client can use the Navigator class and easily switch between different routing strategies without changing the navigator's implementation.

7. When to use?

Use the Strategy pattern when:

1. Many related classes differ only in their behavior.

2. You need different variants of an algorithm.

3. The algorithm has clients that should be unaware of its data structures.

4. A class defines multiple behaviors, and these appear as multiple conditional statements. They can be moved to their strategy classes.