LECTURE 39:WHEN TO USE A PROXY

Computer Science 313 – Advanced Programming Topics

Why Do We Use Proxy Pattern? Discussed word proxy in

Wednesday's lecture

When and why would we ever use a proxy?

Real-World Uses of Proxy

Real-World Uses of Proxy

Real-World Uses of Proxy

Real-World Uses of Proxy

Why We Use Proxy

Add level of indirection before real subject Continue actions, but protect subject

from harm Create plausible deniability – "But it

wasn't me!" Get tasks done, but eliminate need to be

local Improve performance by using saved

results Same design needs met using Proxy

Pattern In all cases, proxy appears identical to

real subject Proxy acts first then lets subject do the

real work Real subject stays hidden – client does

not know

Proxy versus… Adapter

Proxy Pattern Adapter Pattern

Proxy versus… Adapter

Proxy Pattern Adapter Pattern Client only knows of

Proxy Uses interface client

needs Subject does all work

Interface identical to the real subjects

Does not change each calls' parameters & return value

Cannot access real subject

Client only knows of Adapter

Uses interface client needs

Adaptee(s) does all work

Interface differs with that of adaptee(s)

Modifies, adds or removes parameters & return value

Can access real subject

Proxy versus… Composite

Proxy Pattern Composite Pattern

Proxy versus… Composite

Proxy Pattern Composite Pattern Structural pattern Common interface

defined One of GoF's patterns

Only one subject used

Proxy acts as a gatekeeper

Structural pattern Common interface

defined One of GoF's patterns

Many leaves can be used

Composite collects leaves

Proxy versus… Decorator

Proxy Pattern Decorator Pattern

Proxy versus… Decorator

Proxy Pattern Decorator Pattern Client thinks it has

subject Hides subject from

others Proxy can intercept

call

One proxy per pattern Does not add or

modify basic functionality

Client thinks has concept

Hides concept from others

Decorator can intercept call

Many decorators per pattern

Adds or modifies concept's basic functionality

Proxy versus… Observer

Proxy Pattern Observer Pattern

Proxy versus… Observer

Proxy Pattern Observer Pattern

For Next Class

Last lab due “Friday”; submit until final exam time Start now == more committing acts involving

carrots Finish reading on our last design pattern

Lots of interesting code on Wednesday Will be presenting many cool & useful hacks Follow the white rabbit – they have tricks &

carrots

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