1. Proxy

2. PBA WEB – BEWP 2 The Proxy pattern What is a Proxy? A Proxy is a ”placeholder” for a different object, to which we will not allow direct access However, the ”user” (i.e programmer) should not be aware of this

3. PBA WEB – BEWP 3 The Proxy pattern Why would we want to use a Proxy…? Several possible reasons: –Security –Less costly computations –Bookkeeping –Caching –Remoting Using a Proxy must not change the functionality as such

4. PBA WEB – BEWP 4 Constructing a Proxy The object we wish to create a Proxy for has the class Subject, and implements the interface SubjectInterface The Proxy class must therefore also implement the interface SubjectInterface User only accesses the functionality through the interface!

5. PBA WEB – BEWP 5 Constructing a Proxy SubjectInterface Method1 Method2 Method3 … SubjectProxy Method1 Method2 Method3 … Subject Method1 Method2 Method3 …

6. PBA WEB – BEWP 6 User code public void UserMethod(SubjectInterface si) { si.Method1();... } User does not know if the real type of si is a Subject or SubjectProxy Setting up the Proxy is done elsewhere

7. PBA WEB – BEWP 7 User cannot tell the difference! The Proxy pattern Our code Subject Interface SubjectProxySubject

8. PBA WEB – BEWP 8 Inside the Proxy The actual implementation of the Proxy will of course vary with the purpose Usually, the Proxy will hold a reference to the real object, and (sometimes) make calls to the real object

9. PBA WEB – BEWP 9 Inside the Proxy public class SubjectProxy : SubjectInterface { private Subject theSubject;... public SubjectProxy() { theSubject = new Subject(); } public void Method1() { theSubject.Method1(); }... } Not a very useful Proxy…

10. PBA WEB – BEWP 10 Inside the Proxy The previous implemen- tation of a Proxy was not very useful – does the same as direct access How can we change the implementation to better suit certain scenarios?

11. PBA WEB – BEWP 11 Inside the Proxy Scenario 1: Subject is very expensive to create Low probability that subject is ever called during a run of the program Solution: Delay creation of subject

12. PBA WEB – BEWP 12 Inside the Proxy public class SubjectProxy : SubjectInterface { private Subject theSubject; public SubjectProxy() { // Postpone creation of Subject } private void CreateSubject() { if (theSubject == null) theSubject = new Subject(); }

13. PBA WEB – BEWP 13 Inside the Proxy public class SubjectProxy : SubjectInterface { // (continued…) public void Method1() { CreateSubject(); // Make sure Subject is ready theSubject.Method1(); } public void Method2() { CreateSubject(); // Make sure Subject is ready theSubject.Method2(); }... }

14. PBA WEB – BEWP 14 Inside the Proxy Scenario 2: Method calls to the subject are very expensive Same input always gives same output Solution: Make a cache of results, and check the cache before calling subject

15. PBA WEB – BEWP 15 Inside the Proxy public class SubjectProxy : SubjectInterface { private Subject theSubject; Dictionary cache; public Result ExpensiveMethod(Input in) { if (cache.ContainsKey(in)) return cache[in]; else { Result res = theSubject.ExpensiveMethod(in); cache.Add(in, res); return res; }

16. PBA WEB – BEWP 16 Inside the Proxy Scenario 3: Some methods can only be called by users with certain user rights Solution: Obtain and check user rights, before forwarding calls to subject

17. PBA WEB – BEWP 17 Inside the Proxy public class SubjectProxy : SubjectInterface {... public Result Method1() { UserRights theUserRights = GetUserRights(); if (CheckUserRights(theUserRights, ”Method1”)) { return theSubject.Method1(); else { // E.g. throw exception... }