Design Pattern - Proxy

Definition
Provide a surrogate or placeholder for another object to control access to it.

UML Class Diagram

Proxy: when and where use it
- In object-oriented languages objects do the work they advertise through their public interface. Clients of these objects expect this work to be done quickly and efficiently.

- However, there are situations where an object is severely constrained and cannot live up to its responsibility. Typically this occurs when there is a dependency on a remote resource (a call to another computer for example) or when an object takes a long time to load. In situations like these you apply the Proxy pattern and create a proxy object that stands in? for the original object. The Proxy forwards the request to a target object.  The interface of the Proxy object is the same as the original object and clients may not even be aware they are dealing with a proxy rather than the real object. 

- The proxy pattern is meant to provide a surrogate or placeholder for another object to control access to it. There are 3 different types of proxies:
    + Remote proxies are responsible for encoding a request and for forwarding the encoded request to a real object in a different address space (app domain,
process, or machine)
    + Virtual proxies may cache additional information about a real object so that they can postpone accessing it (this process is known by many names, such as, just-in-time loading, on-demand loading, or lazy loading)
    + Protection proxies check that the caller has the proper access permissions to perform the request.  

Sample
- Interface

- Object

- Proxy


Ref: Gang of Four tutorial