Model View Controller Jan 14 2019 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 major topic in O O design 2 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 3 The Model Most programs are supposed to do work not just be another pretty face 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 there are some exceptions useful programs existed long before GUIs But it provides services methods for them to use Independence gives flexibility robustness 4 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 5 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 6 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 7 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 8 The Reverser program In this program we combine the Controller and the View indeed it s hard to separate them The Model which does the computation reversing the string we put in a separate class 9 ReverserGUI java A bunch of import statements then public class ReverserGUI extends JFrame ReverserModel model new ReverserModel JTextField text new JTextField 30 JButton button new JButton Reverse public static void main String args ReverserGUI gui new ReverserGUI gui create gui setDefaultCloseOperation JFrame EXIT ON CLOSE create 10 The create method private void create setLayout new GridLayout 2 1 getContentPane add text getContentPane add button button addActionListener new ActionListener public void actionPerformed ActionEvent arg0 String s text getText s model reverse s text setText s pack setVisible true 11 The model public class ReverserModel public String reverse String s StringBuilder builder new StringBuilder s builder reverse return builder toString 12 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 13 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 14 Observer and Observable java util provides an Observer interface and an Observable class An Observable is an object that can be observed An Observer is notified when an object that it is observing announces a change Here s an analogy An Observable is like a Button An Observer is like a Listener You have to attach a Listener to a Button Another analogy An Observable is like a bulletin board An Observer is like someone who reads the bulletin board 15 Observable An Observable is an object that can be observed An Observer is notified when an object that it is observing announces a change When an Observable wants the world to know about what it has done it executes setChanged notifyObservers or notifyObservers arg The arg can be any object The Observable doesn t know or care who is looking But you have attach an Observer to the Observable with myObservable addObserver myObserver This is best done in the controller class not in the model class 16 Observer Observer is an interface An Observer implements public void update Observable obs Object arg This method is invoked whenever an Observable that it is listening to does an addNotify or addNotify arg The obs argument is a reference to the observable object itself If the Observable did addNotify the arg is null 17 Sample CRC index card Class Name Responsibilities Collaborators 18 Model Model Set initial position Move one step No collaborators but provide access methods to allow view to see what is going on 19 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 20 Model II void makeOneStep xPosition xDelta if xPosition 0 xPosition 0 xDelta xDelta more 21 Model III if xPosition xLimit xPosition xLimit xDelta xDelta still more 22 Model IV yPosition yDelta if yPosition 0 yPosition yLimit yDelta yDelta yPosition yDelta setChanged notifyObservers end of makeOneStep method end of Model class 23 Model repeated Model Set initial position Move one step No collaborators but provide access methods to allow view to see what is going on 24 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 25 View View Paint the ball Get necessary info from Model 26 View I import java awt import java util class View extends Canvas implements Observer Controller controller Model model int stepNumber 0 View Model model this model model more 27 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 method 28 View III public void update Observable obs Object
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