Foundations of Programming - Karl Seguin

Chapter 9 - Back to Basics: Proxy This and Proxy That
Proxy Domain Pattern
A proxy is something acting as something else. In legal terms, a proxy is someone given authority to vote
or act on behalf of someone else. Such a proxy has the same rights and behaves pretty much li ke the
person being proxied. In the hardware world, a proxy server sits between you and a server you're
accessing. The proxy server transparently behaves just like the actual server, but with additional
functionality - be it caching, logging or filtering. In software, the proxy design pattern is a class that
behaves like another class. For example, if we were building a task tracking system, we might decide to
use a proxy to transparently apply authorization on top of a task object:
public class Task
{
public static Task FindById(int id)
{
return TaskRepository.Create().FindById(id);
}
public virtual void Delete()
{
TaskRepository.Create().Delete(this);
}
}
public class TaskProxy : Task
{
public override void Delete()
{
if (User.Current.CanDeleteTask())
{
base.Delete();
}
else
{
throw new PermissionException(...);
}
}
}
Thanks to polymorphism, FindById can return either a Task or a TaskProxy. The calling client
doesn't have to know which was returned - it doesn't even have to know that a TaskProxy exists. It just
programs against the Task's public API.
Since a proxy is just a subclass that implements additional behavior, you might be wondering if a Dog is
a proxy to a Pet. Proxies tend to implement more technical system functions (logging, caching,
authorization, remoting, etc) in a transparent way. In other words, you wouldn't declare a variable as
TaskProxy - but you'd likely declare a Dog variable. Because of this, a proxy wouldn't add members
(since you aren't programming against its API), whereas a Dog might add a Bark method.
Foundations of Programming Copyright © Karl Seguin www.codebetter.com
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