Model View Controller Design Patterns The hard problem in O O programming is deciding what objects to have and what their responsibilities are Design Patterns describe the higher level organization of solutions to common problems Design patterns are a current hot topic in O O design The MVC pattern MVC stands for Model View Controller The Model is the actual internal representation The View or a View is a way of looking at or displaying the model The Controller provides for user input and modification These three components are usually implemented as separate classes The Model Most programs are supposed to do work not just be another pretty face but there are some exceptions useful programs existed long before GUIs The Model is the part that does the work it models the actual problem being solved The Model should be independent of both the Controller and the View But it can provide services methods for them to use Independence gives flexibility robustness The Controller The Controller decides what the model is to do Often the user is put in control by means of a GUI in this case the GUI and the Controller are often the same The Controller and the Model can almost always be separated what to do versus how to do it The design of the Controller depends on the Model The Model should not depend on the Controller The View Typically the user has to be able to see or view what the program is doing The View shows what the Model is doing The View is a passive observer it should not affect the model The Model should be independent of the View but but it can provide access methods The View should not display what the Controller thinks is happening Combining Controller and View Sometimes the Controller and View are combined especially in small programs Combining the Controller and View is appropriate if they are very interdependent The Model should still be independent Never mix Model code with GUI code Separation of concerns As always you want code independence The Model should not be contaminated with control code or display code The View should represent the Model as it really is not some remembered status The Controller should talk to the Model and View not manipulate them The Controller can set variables that the Model and View can read The Bouncing Ball Applet Each click of the Step button advances the ball a small amount The step number and ball position are displayed in the status line The Ball Applet Model The Ball Applet shows a ball bouncing in a window The Model controls the motion of the ball In this example the Model must know the size of the window so it knows when the ball should be made to bounce The Model doesn t need to know anything else about the GUI Sample CRC index card Class Name Responsibilities Collaborators Model Model Set initial position Move one step No collaborators but provide access methods to allow view to see what is going on Model I import java util Observable class Model extends Observable final int BALL SIZE 20 int xPosition 0 int yPosition 0 int xLimit yLimit int xDelta 6 int yDelta 4 more Model II void makeOneStep xPosition xDelta if xPosition 0 xPosition 0 xDelta xDelta more Model III if xPosition xLimit xPosition xLimit xDelta xDelta still more Model IV yPosition yDelta if yPosition 0 yPosition yLimit yDelta yDelta yPosition yDelta setChanged notifyObservers end of makeOneStep method end of Model class Model repeated Model Set initial position Move one step No collaborators but provide access methods to allow view to see what is going on The Ball Applet View The View needs access to the ball s state in this case its x y location For a static drawing the View doesn t need to know anything else View View Paint the ball Get necessary info from Model View I import java awt import java util class View extends Canvas implements Observer Controller controller Model model int stepNumber 0 more View II public void paint Graphics g g setColor Color red g fillOval model xPosition model yPosition model BALL SIZE model BALL SIZE controller showStatus Step stepNumber x model xPosition y model yPosition View III public void update Observable obs Object arg repaint end class View repeated View Paint the ball Get necessary info from Model The Ball Applet Controller The Controller tells the Model what to do The Controller tells the View when it needs to refresh the display The Controller doesn t need to know the inner workings of the Model The Controller doesn t need to know the inner workings of the View Controller Controller Create Model Create View GiveView access to Model Tell Model to advance Tell View to repaint Model View Controller I import import import import java applet java awt java awt event java util public class Controller extends Applet Panel buttonPanel new Panel Button stepButton new Button Step Model model new Model View view new View more Controller II public void init Lay out components setLayout new BorderLayout buttonPanel add stepButton this add BorderLayout SOUTH buttonPanel this add BorderLayout CENTER view more Controller III Attach actions to components stepButton addActionListener new ActionListener public void actionPerformed ActionEvent event model makeOneStep more Controller IV Tell the View about myself Controller and about the Model model addObserver view view controller this end init method more Controller V public void start model xLimit view getSize width model BALL SIZE model yLimit view getSize height model BALL SIZE repaint end of start method end of Controller class Controller repeated Controller Create Model Create View GiveView access to Model Tell Model to advance Tell View to repaint Model View The End
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