Datascope Systems Ltd.

How to write meaningful code

Bullet Points

SOLID• Single Responsibility Principle• Open/Closed Principle• Liskov Substitution Principle• Inversion Of control• Dependency Inversion Principle

POLA Demeter Law Lowering Principle Live project example

Single Responsibility principle

The class has only one reason to change! Repository – is that a good example? What if

persistence mechanism for saving/retrieving business entities will be changed separately? CQRS!

Facade pattern for interacting with printing subsystem.

RFID Facade example

Class has more than 1 responsibility, but only one reason to change.

Open-Closedprinciple

Open-Closedprinciple

Modules should be open for extension but closed to modification

Barbara-Liskov substitution principle

x(O) = y(O) if Y is type of T, and T is derived from X a subtype of X

Interface Segregation Principle

Interface Segregation Principle

Let’s imagine we have an interface which is pretty much the same as a sub-repository

This is how saving/loading supposed to look like

And Boooooom, at some point business requires only read-only settings

Interface Segregation Principle

Load probably will be similarly to that then …

And start from here we have breach ISP. Client should not depend on any method he is not using

Interface Segregation Principle

Solution: split into two interfaces

Interface Segregation Principle

Dependency InversionPrinciple

Dependency InversionPrinciple

Example of inversion of control in DB is a many to many relation. Order has no idea of OrderDetails existence. As long as you have a magic word

1) using i.e C# 2) import i.e. Java 3) include i.e C,C++

You have an arrow pointing from you (you are depend on smth…)

Examples

Time to go Real…

Let’s be interactive

DeliveryCollectionHelper vs POLA and SRP

ФабричноЛоггеровыйКартинкоДоставочноСервисныйХелпер

Factory OR DI?

ЗависимостиИнжектируемаяСинглтоноФабрика

Let’s apply some refactoringGet rid of unnecessary dependencies and apply SRP. Lowering Law, Internal Coupling,Descriptors and Views (Logical diff), Refactoring stages

OCP

Unnecessary coupling (Class contains two services), horizontal dependencies, try to avoid that

Strategy and Factory

Strategy and Factory

Strategy is ready, but still something is wrong??? Proper Factory

Usage of Factory and Strategies example

Test is pretty damn simple

What`s wrong here?

Class usage example:

Class definition (object will goes down to service layer):

Demeter law in action

• Each unit should have only limited knowledge about other units: only units "closely" related to the current unit.

• Each unit should only talk to its friends; don't talk to strangers.• Only talk to your immediate friends.

Demeter law in action

What about up/down casting through explicit operator? Hide SP’s - Sort of ACL

Right way of doing it (this is service layer)

Testability problem. Action chain solution

Action chain usage

ISP in action

ISP in action

DI in action

DI in action (diff between DIP andDependency Injection)

DI in action (more complicated example)

What if you need sort of magnet objects like reports, user roles, email generators etc.So, how to invert arrows ? How to get rid of using, import etc in target class

DI in action (report example)

Dependency diagram you want to achieve:

DI in action (report example)Reports side effect (two reports 99% equals)

This is how it looks like in practice:

Report application layer

Report Collector in action

My LSP mistake in Quilt project

Software Development is amazing!