Use vs. Inheritance. Learning Object-Oriented Programming and Design with TDD. Stéphane Ducasse.

Transcription

1 Learning Object-Oriented Programming and Design with TDD Use vs. Inheritance Stéphane Ducasse W6S06

2 W6S06 2 / 20 Outline An exercise Some criteria Solutions Comparing solutions

3 Exercise Set Up Imagine the TextEditor class and several algorithms formatwithtex (t TextEditor) formatfastcoloring (t TextEditor) formatslowbutprecisecoloring (t TextEditor) How can we create an editor that will format differently texts? W6S06 3 / 20

4 W6S06 4 / 20 Next step Propose one solution Propose different solutions Define some criteria Compare the approaches using such criteria

5 Solution 1: Inheritance W6S06 5 / 20 TextEditor subclass: #SlowFormatingTextEditor SlowFormatingTextEditor >> format self formatslowbutprecisecoloring: text TextEditor subclass: #FastFormatingTextEditor SlowFormatingTextEditor >> format self formatfastcoloring: text TextEditor subclass: #NullFormatingTextEditor NullFormatingTextEditor >> format ^ self "do nothing"

6 Inheritance W6S06 6 / 20 TextEditor text format NullFormatingTextEditor format FastFormatingTextEditor format SlowFormatingTextEditor format

7 Solution 2: With conditionals W6S06 7 / 20 TextEditor text formatslowbutprecise: t formatfastcoloring: t formatwithtex: t TextEditor >> format currentselection = #slow iftrue: [ self formatslowbutprecisecoloring: text] iffalse: [ currentselection = #fast iftrue: [self formatfastcoloring: text]...]

8 Alternate with Registry and Meta Programming W6S06 8 / 20 Object subclass: #TextEditor currentselection formatters text TextEditor class >> initialize self formatters at: #slow put: #slowformat: ; at: #fast put: #fastformat: ; at: #null put: #nullformat: ; at: #tex put: #texformat: TextEditor >> format self perform: (formatters at: currentselection) with: text What are your criterias to compare these and other solutions?

9 Criteria W6S06 9 / 20 Yes what are they?

10 W6S06 10 / 20 Criteria Adding a new formatting algo what is the cost to define a new formatting algorithm Dynamically use a formatter can I switch dynamically to a new formatting algorithm Packaging can I deploy a new formatting algorithm separately from others

11 W6S06 11 / 20 Analysing Solution 1: Inheritance? Adding a new formatting algo: we can add a new formatter Packaging: we can package a new formatter Not the best solution since: you have to create objects of the right class it is difficult to change the policy at run-time. we do not want to have and reopen the texteditor

12 W6S06 12 / 20 Analysing Solution 1: Inheritance? You can get an explosion of classes bloated with functionalities we do not want a hierarchy for each text editor features to be multiplied with previous ones TextEditor API can get large: no clear identification of responsibility NullFormatingTextEditor format MultipaneNull... TextEditor text format SlowFormatingTextEditor format FastFormatingTextEditor format MultipaneFast... SinglepaneFast... SinglepaneNull... SinglepaneSlow... MultipaneSlow... SpellMultipaneSlow... ExtraSpellMultipaneSlow...

13 Analysing Solution 2: Conditionals W6S06 13 / 20 Dynamic use: we can use a different formatter dynamically Adding a new formatting algo: adding a version requires to edit and recompile the conditionals Packaging: we cannot package a new algorithm separately

14 Another solution... W6S06 14 / 20 Delegating to a formatter Sketch the solution

15 Delegating to a formatter W6S06 15 / 20 Editor Client formatter: formatter text format(text) Formatter format: (Text) format: text self formatter format: text FastFormatter format: t #formatfastcoloring: t SlowFormatter format: t #formatslowbutpreci secoloring: t format: text NullFormatter format: t self formatfastcoloring: text myeditor formatter: FastFormatter new. myeditor format. myeditor formatter: SlowFormatter new.

16 Delegating to a formatter W6S06 16 / 20 Dynamic use: we can use a different formatter dynamically. Just create a new instance and set it. Adding a new formatting algo: adding a version is just adding a new class Packaging: we package a new algorithm separately

17 W6S06 17 / 20 Strategy Design Pattern Uniformize the communication (API) between the Editor and the Formatter all formatters should understand format: Modular Incremental

18 W6S06 18 / 20 But there is nothing like a free lunch The formatter should access the state (i.e. the text, positions... contained in the text editor) Information should flow between the texteditor and the formatter API of texteditor should be opened to support it

19 W6S06 19 / 20 Conclusion Inheritance is about incremental static definition It can lead of static design It can help build dynamic solutions structure abstractions It supports late binding Delegation (Use) can bring runtime flexibility can be combined with inheritance

20 A course by Stéphane Ducasse Reusing some parts of the Pharo Mooc by Damien Cassou, Stéphane Ducasse, Luc Fabresse Except where otherwise noted, this work is licensed under CC BY-NC-ND 3.0 France