Dependency Injection Pattern

1. Introduction

The Dependency Injection (DI) pattern is a software design pattern that implements Inversion of Control (IoC) for resolving dependencies. It allows a class to receive its dependencies from an external source rather than creating them itself, promoting loose coupling and enhancing testability.

2. Key Concepts

• Inversion of Control (IoC): A principle that transfers the control of object creation and binding to a container or framework.
• Loose Coupling: Reducing the dependencies between components to enhance modularity and reduce impact on changes.
• Service Locator vs. Dependency Injection: DI provides the dependencies directly, while Service Locator retrieves them from a registry.

3. Types of Dependency Injection

1. Constructor Injection: Dependencies are provided through a class constructor.
2. Setter Injection: Dependencies are provided through setter methods.
3. Interface Injection: The dependency provides an injector method that will inject the dependency into any client that passes itself to the injector.

4. Implementation Steps

            flowchart TD
                A[Client] -->|uses| B[Service]
                B -->|depends on| C[Dependency]
                C -->|inject| D[Constructor/Setter]
        

4.1 Example Code: Constructor Injection

class Service {
    constructor(private dependency: Dependency) {}
    execute() {
        this.dependency.performAction();
    }
}

class Dependency {
    performAction() {
        console.log("Action performed!");
    }
}

// Dependency Injection
const dependency = new Dependency();
const service = new Service(dependency);
service.execute();
            

5. Best Practices

• Favor constructor injection for mandatory dependencies.
• Use setter injection for optional dependencies.
• Keep constructors clean and focused on dependency injection.
• Utilize interfaces to define contracts for dependencies.
• Document dependencies clearly for maintainability.

6. FAQ