JavaDesign PatternsThe MVC patternThe ModelThe ControllerThe ViewCombining the Controller and ViewSeparation of concernsThe Bouncing Ball AppletThe Ball Applet: ModelSample CRC index cardModelModel IModel IIModel IIIModel IVModel (repeated)The Ball Applet: ViewViewView IView IIView (repeated)The Ball Applet: ControllerControllerController IController IIController IIIController IVController VController (repeated)The EndJavaModel-View-ControllerDesign 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 designThe 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 classesThe 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, robustnessThe 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 ControllerThe 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 happeningCombining the 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 readThe 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 lineThe 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 GUISample CRC index cardClass NameResponsibilities. . .. . .. . .Collaborators. . .. . .. . .ModelModelSet initial positionMove one stepNo collaborators.......but provide access methods to allow view to see what is going onModel Iclass Model { 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; } } // end of makeOneStep method} // end of Model classModel (repeated)ModelSet initial positionMove one stepNo collaborators.......but provide access methods to allow view to see what is going onThe 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 elseViewViewPaint the ballGet necessary info from ModelView Iclass View extends Canvas { 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); } // end paint methodView (repeated)ViewPaint the ballGet necessary info from ModelThe 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 ViewControllerControllerCreate ModelCreate ViewGiveView access to ModelTell Model to advanceTell View to repaintModelViewController Ipublic class Controller extends Applet { Panel buttonPanel = new Panel ( ); Button stepButton = new Button ("Step"); Model model = new Model (); View view = new View (); // more...import java.applet.*;import java.awt.*;import java.awt.event.*;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 (); view.repaint (); }}); // more...Controller IV // Tell the View about myself (Controller) and // about the Model view.model = model; 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 classController (repeated)ControllerCreate ModelCreate ViewGiveView access to ModelTell Model to advanceTell View to repaintModelViewThe
View Full Document
Unlocking...