The Proxy Pattern provides a surrogate for another object to control access to it. The intent is to control access to a resource-intensive, remote, or secure object.

Components

• Proxy (Wrapper)
• Object

V.S Decorator

It can add Control access dynamically and wrap by several proxy (chaining), but these are not the emphasis. | Flexibility(Add dynamically + chaining) in augmenting behavior without changing original class |

Middleware in web servers often behaves like a hybrid of Proxy and Decorator.

SOLID principles

1. Single Responsibility Principle (SRP): Each proxy class has a single responsibility.
2. Open/Closed Principle (OCP): Proxy extends a subject function without modifying itself.
3. Liskov Substitution Principle (LSP): Proxy will call subject’s method the maintain the same purpose.
4. Dependency Inversion Principle (DIP): Proxy depends on subject’s abstraction.

Implementation

Inheritance

Since the Proxy pattern doesn’t emphasize runtime wrapping like the Decorator does, it can be implemented by inheritance.

// Subject
interface SubjectI {
  request(): void;
}
class Subject implements SubjectI {
  request(): void {
    console.log("Handling request.");
  }
}

// Proxy
class Proxy extends Subject implements SubjectI {
  request(): void {
    console.log("Proxy: Checking access before delegating...");
    super.request();
    console.log("Proxy: Logging after request.");
  }
}

// Client
function compute(subject: Subject) {
  subject.request();
}

compute(new Proxy());

Composition

// Subject
interface SubjectI {
  request(): void;
}
class Subject implements SubjectI {
  request(): void {
    console.log("Handling request.");
  }
}

// Proxy
class Proxy implements SubjectI {
  private subject: Subject;

  constructor(subject: Subject) {
    this.subject = subject;
  }

  request(): void {
    console.log("Proxy: Checking access before delegating...");
    this.subject.request();
    console.log("Proxy: Logging after request.");
  }
}

// Client
function compute(subject: Subject) {
  subject.request();
}

compute(new Proxy(new Subject()));

Go Composition (Embedding)

It allows Anonymous Embedding if: