©2008 Felleisen, Proulx, et. al.

13  User Interactions


In this lab you will learn a little about programming user interactions using the Model-View-Control pattern for organizing the responsibilities.

The JPT library allows you to concentrate on the key concepts and avoid the pitfalls of multitude of details, typically associated with GUI programming.

The Model and the View

The diagram below (on the next page) describes the classes already included in this application:

        | DisplayPanel |
               / \   
   | BalloonControl            |
+--| BalloonInputView bView    |
|  | Balloon b                 |---------+
|  | BufferredPanel window     |---+     |
|  | JPTCanvas theCanvas       |-+ |     |
|  | SimpleAction paintAction  | | |     v
|  | SimpleAction newAction    | | |  +------------+
|  | SimpleAction cancelAction | | |  | Balloon    |
|  +---------------------------+ | |  +------------+
|  | void paintAction()        | | |  | int x      |
|  | void paintAction()        | | |  | int y      |
|  | void cancelAction()       | | |  | int radius |
|  +---------------------------+ | |  | Color c    |
|                      +---------+ |  +------------+
|                      v           v
|    +-----------------------+   +---------------+
|    | JPTCanvas             |   | BufferedPanel |
|    +-----------------------+   +---------------+
|    | BufferredPanel window |
|    +-----------------------+
|  +-------------------------------------+
|  | interface: BalloonInput             |
|  +-------------------------------------+
|  | Balloon demandBalloon()             |
|  | Balloon requestBalloon()            |
|  |           throws CancelledException |
|  +-------------------------------------+
|                  / \
|                   |
|          +- - - - - - - - - - - - - - - - -+
|          |                                 |
|  +-----------------------+     +---------------------+
|  | GUIBalloonInput       |     | ConsoleBalloonInput |
|  +-----------------------+     +---------------------+
|  | BalloonInputView bGUI |--+  +---------------------+
|  +-----------------------+  |      +---------+
|                             |      | Display |
|                             |      +---------+
+---------------------+       |           / \ 
                      |       |           ---
                      v       v            |
                   | BalloonInputView           |
                   | TextFieldView   xTFV       |
                   | TextFieldView   yTFV       |
                   | SliderView   rSlider       |
                   | ColorView      cVieW       |
                   | TablePanel createDisplay() |

Here is a brief description of the role these files play in the application.

The model

The program deals with balloons (for now just three of them).

The views

We can view the information about a particular Balloon object in several different ways. The BalloonInput interface provides two methods for getting the data needed to construct an instance of a new Balloon.

To display the information about a Balloon object, we can print a String that represents the Balloon object in the console, or paint it in the given window, or display the values of its fields in a GUI.

To get the data from the user that is needed to instantiate a new Balloon we can read from the console, or from a GUI.

The control

Run the code, and note the behavior in response to the various buttons.

Getting Familiar with the Environment

  1. The model

    Read the code for the class Balloon. Add the method eraseBalloon which will paint the balloon in a white color (Color.white). Make sure you have the examples and tests for this method.

  2. The console input

    Read the code for the method runConsoleInput in the class Interactions. Describe to your partner what the method does. Look at the ConsoleBalloonInput class and see how the methods demandBalloon and requestBalloon are implemented. Run the code and see what happens if you type in a wrong data, or when you do not provide any input.

  3. The actions

    Find the code for the action for the New button. Currently, it only sets the value of the Balloon instance variables. Add to this action a call to the method which paints the balloon, from the class Balloon. Make sure it works.

  4. Text input from a GUI

    Find all places where the xTFV is defined or used. It is constructed in the class BalloonInputView. This class also defines the methods demandBalloon and requestBalloon, each of them produces a new instance of a Balloon from the user inputs.

    In the class BalloonControl user input to the BalloonView initializes the value of a Balloon object that represents our model. We could add to our model a list of tied balloons and a list of floating balloons, and more - for example a child holding the balloon.

    DO IT Using a similar technique, define a new TextFieldView named rTFV, to represent the numerical value of the Balloon radius.

  5. Connecting slider with a text field

    Test the behavior of the slider. Does it have any effect on the balloon? Does it have any effect on the value displayed in the rTFV field? Change the value of the rTFV field. Does it affect the slider? Does it affect the balloon?

    The two views represent the same value and so should be designed to mimic each other. The slider has to act by changing its position whenever a new value is typed into the text field. The value in the text field has to change when the slider is moved, so it reflects its current position.

    Define two new SimpleActions and the corresponding methods — an rTFVaction and a SliderAction. It does not matter what you choose for the label, because we are not going to use the actions with a button.

    The first one void rTFVaction will be invoked when the value in the field rTFV changes. It should then set the value of the balloon radius and the value of the rSlider to the value displayed in the rTFV. To set the state of the rSlider use the method

    rSlider.setViewState("" + b.radius);

    The second method void rSliderAction() will be invoked every time the location of the slider (and the value it represents) changes. It must then change the radius of the balloon and set the view state of the rTFV calling the method setViewState in a manner similar to the above. If you run the program now, you may be surprised to see that these changes have no effect. Can you think of the way to test that the methods work correctly?

  6. Listening to changes in the values

    Now you have to tell the rSlider and the rTFV to perform this action when their values change. The following two statements have to be added at the end of the method void createViews():


    The first one tells the rTFV to perform the rTFVaction whenever its value changes. The second one tells the rSlider to perform the sliderAction whenever the position of the slider (and thus the value it represents) changes.

    Test that this works.

  7. Reporting changes in the model to the view

    Now that you have seen the method setViewState, add such method to the class BalloonInputView. To see that is works, we need to modify some of the fields of a Balloon instance and invoke the method. Try it.

  8. Adding mouse actions

    In the last part you will control the balloon with the mouse. You need to define what should happen when the mouse is clicked (or dragged, or released, etc.). You need to specify which GUI component should listen to the mouse and the user mouse actions. You then need to connect the MouseListener with the action it should trigger.

    Build a separate frame

    The first thing you need to do is to change the manner in which the GUI is displayed. Look at the code in the class Interactions for the method runBalloonControl(). Replace the line which calls the method showOKDialog with the following:

    JPTFrame.createQuickJPTFrame("Balloon Control", bc);

    This places the BalloonControl into a window that runs in its own thread, i.e. independently of the rest of the application. That allows the rest of the application to watch out for the mouse movement and clicks inside of the graphics window.

    Define a mouse action The first mouse action you will build will increase the radius of the balloon by ten, every time you click the mouse. All of this is in the class BalloonControl. Start by defining the method
    protected click(MouseEvent mevt) which does the following:

    • Print into the console a message that the mouse was clicked.

    • Erase the balloon

    • Increase the balloon radius by 10

    • Set the view state of the BalloonInputView bView to the current values of the balloon. (Only the radius has changed, but it is easier to let the BalloonView do the whole job by invoking the method setViewState.

    • Finally, paint the changed balloon.         

  9. Defining and installing Mouse action adapter

    Install a MouseActionAdapter for the BufferedPanelas follows:

    • After the definition of the BufferedPanel, add the definition:

        public MouseActionAdapter mouseAdapter;

    • Inside of the constructor for the class BalloonControl first initialize the mouseAdapter as follows:

        mouseAdapter = window.getMouseActionAdapter();

    • Add the action to perform when the mouse is clicked as follows:

        // respond to mouse clicks
          new MouseAction() {
            public void mouseActionPerformed(MouseEvent mevt){

      At this point you should test that your program runs as you expected.

  10. Tracking the mouse movement

    Finally, you will make the balloon move when the mouse moves. Do all the steps you have done for the clicked action, but do not get a new mouseAdapter. The following code will add the action:

      // track mouse motions
        new MouseAction() {
          public void mouseActionPerformed(MouseEvent mevt){

    Inside of the track method get the coordinates of the mouse as follows:

      b.x = mevt.getX();
      b.y = mevt.getY();

    and see what your program does. (Probably nothing - you still have to erase the old balloon, before you make the changes, paint the new balloon, and as a courtesy, set the view state for the view.) Now you should have fun.

Last modified: Tuesday, April 8th, 2008 12:19:47am