Differences between Proxy and Decorator Pattern

Question

The real difference is not ownership (composition versus aggregation), but rather type-information.

A Decorator is always passed its delegatee. A Proxy might create it himself, or he might have it injected.

But a Proxy always knows the (more) specific type of the delegatee. In other words, the Proxy and its delegatee will have the same base type, but the Proxy points to some derived type. A Decorator points to its own base type. Thus, the difference is in compile-time information about the type of the delegatee.

In a dynamic language, if the delegatee is injected and happens to have the same interface, then there is no difference.

The answer to your question is "Yes".

Decorator Pattern focuses on dynamically adding functions to an object, while Proxy Pattern focuses on controlling access to an object.

EDIT:-

Relationship between a Proxy and the real subject is typically set at compile time, Proxy instantiates it in some way, whereas Decorator is assigned to the subject at runtime, knowing only subject's interface.

Decorator get reference for decorated object (usually through constructor) while Proxy responsible to do that by himself.

Proxy may not instantiate wrapping object at all (like this do ORMs to prevent unnecessary access to DB if object fields/getters are not used) while Decorator always hold link to actual wrapped instance.

Proxy usually used by frameworks to add security or caching/lazing and constructed by framework (not by regular developer itself).

Decorator usually used to add new behavior to old or legacy classes by developer itself based on interface rather then actual class (so it work on wide range of interface instances, Proxy is around concrete class).

Here is the direct quote from the GoF (page 216).

Although decorators can have similar implementations as proxies, decorators have a different purpose. A decorator adds one or more responsibilities to an object, whereas a proxy controls access to an object.

Proxies vary in the degree to which they are implemented like a decorator. A protection proxy might be implemented exactly like a decorator. On the other hand, a remote proxy will not contain a direct reference to its real subject but only an indirect reference, such as "host ID and local address on host." A virtual proxy will start off with an indirect reference such as a file name but will eventually obtain and use a direct reference.

Popular answers indicate that a Proxy knows the concrete type of its delegate. From this quote we can see that is not always true.

The difference between Proxy and Decorator according to the GoF is that Proxy restricts the client. Decorator does not. Proxy may restrict what a client does by controlling access to functionality; or it may restrict what a client knows by performing actions that are invisible and unknown to the client. Decorator does the opposite: it enhances what its delegate does in a way that is visible to clients.

We might say that Proxy is a black box while Decorator is a white box.

The composition relationship between wrapper and delegate is the wrong relationship to focus on when contrasting Proxy with Decorator, because composition is the feature these two patterns have in common. The relationship between wrapper and client is what differentiates these two patterns.

• Decorator informs and empowers its client.
• Proxy restricts and disempowers its client.