Computer Science 313 – Advanced Programming Topics

Why Do We Use Proxy Pattern?  Discussed word proxy in Friday's lecture  When and why would we ever use a 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 PatternAdapter Pattern

Proxy versus… Adapter Proxy PatternAdapter Pattern  Client thinks it does work  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 thinks it does work  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 PatternComposite Pattern

Proxy versus… Composite Proxy PatternComposite 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 PatternDecorator Pattern

Proxy versus… Decorator Proxy PatternDecorator 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 PatternObserver Pattern

Proxy versus… Observer Proxy PatternObserver Pattern

For Next Class  Last lab due Friday at end of lab time  Start now == more committing acts against man & beast  Finish reading on our last design pattern  Lots of interesting code on Wednesday  Will be presenting many cool & useful hacks  Follow the white rabbit – going through the looking glass -----censored 'till tenure-----