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Lecture

Lecture. Model View Controller s/w architecture AND general tips on structuring classes. Structuring your classes. When you create your various classes in an application, you want to: Promote reusability (e.g. panels for OK/Cancel buttons might be used in several screens)

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Lecture

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  1. Lecture Model View Controller s/w architecture AND general tips on structuring classes

  2. Structuring your classes.. • When you create your various classes in an application, you want to: • Promote reusability (e.g. panels for OK/Cancel buttons might be used in several screens) • Minimise code maintenance when something changes (e.g. dB structure, new GUI device ) • Maintain as much flexibility as possible

  3. Model View controller • MVC is a common s/w architecture pattern used for interactive applications (i.e. applications that have a Graphical User i/f) • Model = classes that deal with storing and using data • View = classes that deal with the GUI part • Controller = (non graphical) classes that bridge the View and the Model

  4. Model • Model = classes that deal with storing and using data • If data structure(s) or rules change.. Just change these classes • Can have multiple different GUIs using the same model • E.g. calculator functions add/substract etc • Different GUIs (standard web, accessible web etc) Aside: Data may be persistent or not persistent – how to deal with persistent?

  5. View • View = classes that deal with the GUI part • If the GUI changes, don’t (necessarily) need to change the model or control classes • E.g. change borders of buttons

  6. Controller • Controller = (non graphical) classes that bridge the View and the Model • Kept separate • E.g. control (driver) class with ‘main’ method to run an application • Listener classes that trigger behaviour • Generally.. Consists of everything that is not the GUI or not directly storing/using data

  7. Example • Counter – this screen displays a number in them middle which you can move up by 1 or down by 1 using the buttons Frame A simple Counter Title 25 Up Down Buttons Label

  8. MVC - Counter example Classes needed? For View bit.. • Frame? With some buttons “on” it? Label? Title? For Model bit.. - What “data” is involved? What can be done to it? Is there a dB involved? For Controller bit.. What’s needed apart from GUI and doing the model part? At LEAST 3 classes…

  9. Counter Example For View bit.. • How many classes? Are any “bits” reusable? If not, put in a single class. Otherwise separate out.. For Model bit.. - Need to be able to add, subtract, and store the “data” .. A class that has methods to do this For Controller bit.. - Need a control class to run the counter.. What else?

  10. Counter Example – possible class structure that is MVC compliant For View bit.. • Class for the counterPanel • Class for the counter Frame For Model bit.. - class that has methods to do the increment (up) , decrement () etc. i.e. that “does” the data part. For Controller bit.. - class for the listener (that extends actionListener0 - class to run the overall application = 5 classes in total • Q: How will they link together? E.g. how do I get the GUI class(es) to “call” the model class methods to add/decrement the counters?

  11. To use MVC in your own GUI applications Often easiest to start with the View first… • What class(es) deal just with the visual part (GUI)? = View classes • What classes are then needed to deal with manipulating/storing data? = Model • What’s left? What control classes are need to respond to things that happen in the view?

  12. Another example • A Calculator • What’s the View part? • What’s the model part? • What’s the control part?

  13. Other things.. • Giant all purpose class for everything – wrong! • Hard to maintain • Nothing reusable • Decide which “parts” might be reusable • E.g. button panels, listener behaviour, • This gives a clue as to how to break out classes • Pick out the definable “things” in your app e.g. circle, square – suggests separate classes • Keep MVC as a guiding principle but…

  14. Finally • As with any design • There is no single “right” answer to how you break your app into classes • But you should design in advance, rather than just letting your code wander along in a single class or two… • MVC is one s/w architecture • Some techniques we’ve seen contradict it • E.g. implementing listeners directly in the GUI code • Anonymous listeners – embedded against the GUI component – has its own advantages.. • These approaches have their own merits.. • Be aware of the trade offs.

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