Aspect-
Oriented
Programming

by examples

@AkimBoyko

A well-written program is a program where the cost of implementing a feature is constant throughout the program's lifetime

Itay Maman

Aspect: appearance, visible form, the action of looking at appearance to the eye or mind

Advice: an opinion or a suggestion about what somebody should do in a particular situation

Weaver: a person whose job is weaving cloth

Composition: the combining of distinct parts or elements to form a whole

Intercept: to stop, deflect, or interrupt the progress or intended course o

Scattering: the dispersal of a beam of particles or of radiation into a range of directions as a result of physical interactions; placed irregularly and far apart

Tangling: To mix together or intertwine in a confused mass

When to apply?

Code Tangling
Code Scattering
Code Coupling
Boilerplate code

Cross-Cutting concerns

Logging and Monitoring
Performance analysis
Debugging and Tracing
Undo Functionality
Validation of inputs and outputs
Morphing the behavior of existing objects
Object Filters
Security Implementation
Managing transactions

Demo #0: prototype → tangled → aspects

AOP w/o AOP

Is it possible at all? Déjà vu

Proxy and Decorator
- nHiberante: Lazy Loading
- DI containers: Interceptors
ASP.NET MVC Action Filter
- IAuthorizationFilter: OnAuthorization()
- IActionFilter: OnActionExecuted(), OnActionExecuting()
- IExceptionFilter: OnException()
Code Contract
Demo #1

Poor man's AOP

Decorator: extend an instance without changing class
Proxy: substitute initial instance
Demo #2

Single responsibility
Open/closed
Liskov substitution
~~Interface segregation~~
Dependency inversion
Don't Repeat Yourself

AOP components

Component language
Aspect language
Aspect weaver
Aspect life time
Demo #3: Aspect, IL and weaving and Dynaminc Interceptor

Describe the transformation.
Apply the transformation to the business code.
Because there are two phases we separate all the aspect code from the business code. Get nice separation of concerns and principally nice business code.

Gael Fraiteur

AOP Examples

Demo #4

Authorization/Authentication
Parameters Validation
Auditing/Affiliation
Transaction handling
Inject dependencies into aspect
Architecture Validation
Modify external libraries calls

Other examples

Declarative threading
Circuit breaker
Deadlocks detection
Error handling
… and a lot more

Post-compile vs. Runtime

PostSharp vs. Castle Dynamic Proxy

Run-time performance
Interception
Interface/virtual functions
No need to recompile
Post-compilation
WCF services and WPF forms, no way to hide behind proxy
Lack of information during compile-time

AOP and testing

Testing of separate aspects in isolation
Testing of aspect composition and configuration
Post-compilation: Need to manually create classes to apply aspects

AOP as anti-patter and alternatives

Black magic or silver bullet? Neither!
AOP vs. Dependency Injection
AOP Weaver vs. Interfaces Decoration
Aspects vs. Convension over Configuration
B/L in AOP
CI server integration