RE-EVOLVING DESIGN PATTERNS

Chris Hance

Where do patterns come from? Canonical Answer

Design Patterns: Elements of Reusable Object-Oriented Software, “Gang Of Four”

Patterns of Enterprise Application Architecture(PoEAA), Martin Fowler

Real Answer “Lots of people do it this way.”

Lots of People are Wrong

Anti-pattern Common practice that’s (usually)

inefficient or incorrect.

Get used to “It Depends”

www.c2.com/cgi/wiki “Portland Pattern Repository's Wiki”

The Application

Record Student Arrivals / Departures (Check In / Check Out)

Enforce rules on student custody, etc.

Integrate with a legacy VB6 system

Intent: Partial Rewrite

First stab at MVC/MVP/… Isolate UI from Model COM-compatible Standardized Control behavior

Some (COM/VB6) Limitations No constructor parameters No overloading Events "disappear" in interfaces

Attempt #1

UI UberDLL

ModelDALDALStub

Controls"Controller"Form

How many DALs in one DLL?

Attempt #2

UI UberDLL

ModelIDAL

Controls"Controller"Form

DAL

DAL

DALStub

DALStub

CircularDependency

New Rule:Model doesn't talk to the DAL.

Attempt #3

Factory

Factory

UI

Controls"Controller"Form

Model

ModelIDAL

DAL(Stub)

DAL(Stub)

Next: Add Interfaces.

Attempt #4: With Interfaces

Factory

Factory

UI

Controls"Controller"Form

Model

ModelDTO

DAL(Stub)

DAL(Stub)

Interfaces

ModelInterfaceDTOInterfaceFactoryInterfaceDALInterface

Still Awake?

Figure 17. Obligatory Useless Diagram

Low Coupling!What is it good for?

Reusable Code?

Manageable Code Won't somebody think of the

maintenance programmers?

Back to the Application

Concepts

Check-In/Check-Out Student Contact School Teacher User

Multiple Factories

UI

Controls"Controller"Form

CheckInOutInterfaces

CheckInOutModelCheckInOutFactory

CheckInOutDAL

ContactInterfaces

ContactModelContactFactory

ContactDAL

Separate the concepts for maintenance programmer sanity, and some reusability. Do I need School,

Teacher, Student, etc?

Tour the Model

That means open Visual Studio.Yes, now.

So About the UI

MVC MVP (“retired” per Fowler)

Passive View Supervising Controller

MVVM Presentation Model

Supervising Controller

Figure π¾. Supervising Controller Sequence Diagram http://martinfowler.com/eaaDev/SupervisingPresenter.html

How to test the Controller?

View

FormControl(s)

ViewInterfacesIFormIControl(s)

Controller

Controller

ViewStub

FormStubControlStub(s)

Pick a View at RuntimePick a View at Runtime

IControl(s)?

Yes, define ITextbox, ICheckbox, ad nauseam.

Need control wrappers that implement ITextbox, etc. = Adapter pattern.

Also useful for standardized control behavior. = Decorator pattern.

And a concession to VB6

VB6 can’t define events in interfaces

Events only work withDim WithEvents txt As Textbox

They don’t fire forDim WithEvents itxt As ITextbox

Alternate Event System

WeakReference to objects String event / method name

Marshal.IsComObject() TypeLibInfo .InvokeSub() for COM

objects MemberInfo.Invoke() for CLR objects

Parameters are ugly.

Tour the UI

Or what I have of it. (Work in Progress)We can always fall back to VB6 sample

code.

The Whole… Thing

View

FormControl(s)

ViewInterfacesIFormIControl(s)

Controller

Controller

CheckInOutInterfaces

CheckInOutModelCheckInOutFactory

CheckInOutDAL

ContactInterfaces

ContactModelContactFactory

ContactDAL

Well, the Event library is omitted.

TODO

Replace custom DAL with NHibernate or similar (in lieu of manual caching).

Templates or other codegen for UI repetitiveness.

ObservableCollection?