Provide a surrogate or placeholder for another object to control access to it.
为其他对象提供一种代理以控制对这个对象的访问。

The classes and objects participating in this pattern are:

• maintains a reference that lets the proxy access the real subject. Proxy may refer to a Subject if the RealSubject and Subject interfaces are the same.
• provides an interface identical to Subject's so that a proxy can be substituted for for the real subject.
• controls access to the real subject and may be responsible for creating and deleting it.
• other responsibilites depend on the kind of proxy:
  • remote proxies are responsible for encoding a request and its arguments and for sending the encoded request to the real subject in a different address space.
  • virtual proxies may cache additional information about the real subject so that they can postpone accessing it. For example, the ImageProxy from the Motivation caches the real images's extent.
  • protection proxies check that the caller has the access permissions required to perform a request.

• defines the common interface for RealSubject and Proxy so that a Proxy can be used anywhere a RealSubject is expected.

• defines the real object that the proxy represents.