Understanding @Injectable and @Inject Decorators in Angular

In Angular, decorators are used to attach metadata to classes, methods, properties, and parameters. Two commonly used decorators are @Injectable and @Inject. While they might seem similar, they serve different purposes. Let’s dive into their differences and see how they are used with examples.

@Injectable Decorator

The @Injectable decorator is used to mark a class as available for dependency injection. It tells Angular's dependency injection system that the class can be injected as a dependency into other classes.

Visitor Design Pattern in C#

The Visitor Pattern is a behavioral design pattern that allows you to add further operations to objects without having to modify them. It is particularly useful when you have a structure of objects and you want to perform operations on these objects that can be defined outside of their classes. This pattern follows the Open/Closed Principle by allowing new functionality to be added without altering the existing code.

The Visitor Design Pattern should be used when you have distinct and unrelated operations to perform across a structure of objects (element objects). That means the Visitor Design is used to create and perform new operations on a set of objects without changing the object structure or classes.

Components of Visitor Pattern

1. Visitor Interface: Declares a Visit method for each type of ConcreteElement in the object structure.
2. ConcreteVisitor: Implements the Visitor interface and defines the actions for each type of ConcreteElement.
3. Element Interface: Declares an Accept method that takes a Visitor as an argument.
4. ConcreteElement: Implements the Element interface and defines the Accept method to call the Visitor's method.
5. Object Structure: Can be a collection or a composite of elements which can be iterated to apply the Visitor.

State Design Pattern in C#

The State Design Pattern is a behavioral design pattern that allows an object to alter its behavior when its internal state changes. This pattern is particularly useful for implementing state machines and ensuring that the object’s behavior remains manageable and modular.

This pattern is useful when an object needs to go through several states, and its behavior differs for each state. Instead of having conditional statements throughout a class to handle state-specific behaviors, the State Design Pattern delegates this responsibility to individual state classes.

Components of State Desgin Pattern

1. Context: This is the class that contains an instance of a state and delegates state-specific behavior to the current state object.
2. State: An interface that encapsulates the behavior associated with a particular state of the Context.
3. Concrete States: Classes that implement the State interface, each representing a specific state and defining its behavior.

Chain of Responsibility Pattern in C#

The Chain of Responsibility pattern is a behavioral design pattern that allows a group of objects to handle a request sequentially, each potentially handling the request or passing it on to the next object in the chain until the request is handled or reaches the end of the chain.

In simple words, we can say that the chain of responsibility design pattern creates a chain of receiver objects for a given request. In this design pattern, normally, each receiver contains a reference to the next receiver. If one receiver cannot handle the request, it passes the same request to the next receiver, and so on. In this case, one receiver can handle the request in the chain, or one or more receivers can handle the request.

Components of Chain of Resposibility Pattern

1. Handler Interface (or Abstract Class): This defines a common interface for all handlers. It typically includes a method for handling requests and a reference to the next handler in the chain.
2. Concrete Handlers: These are the actual handlers in the chain. Each handler implements the Handler interface and contains logic to handle requests. If it can't handle a request, it passes it to the next handler in the chain.
3. Client: This initiates the request and sends it to the first handler in the chain.

Mediator Design Pattern in C#

The Mediator Design Pattern is a behavioral design pattern that promotes loose coupling between objects by encapsulating how they interact. It centralizes complex communication logic between multiple objects into a mediator object, thus reducing direct dependencies between them. This promotes easier maintenance and scalability of the system.

The Mediator Design Pattern restricts direct communications between the objects and forces them to collaborate only via a mediator object. This pattern is used to centralize complex communications and control between related objects in a system. The Mediator object acts as the communication center for all objects. That means when an object needs to communicate with another object, it does not call the other object directly. Instead, it calls the mediator object, and it is the responsibility of the mediator object to route the message to the destination object.

Components of Mediator Design Pattern

1. Mediator: Defines an interface for communication between colleague objects.
2. Colleague: It is an abstract class, and Concrete Colleague classes will implement this abstract class.
3. ConcreteMediator: Implements the mediator interface, coordinating communication between colleague objects.
4. ConcreteColleague: Implements the colleague interface and communicates with other colleagues through the mediator.