Question

I am reading “A Definite Guide to SWT and JFace” and I am trying to understand the following code:

public class MultipleListenersExample implements HelpListener, VerifyListener, 
  ModifyListener{

    // Constants used for conversions
    private static final double FIVE_NINTHS = 5.0 / 9.0;
    private static final double NINE_FIFTHS = 9.0 / 5.0;

    // Widgets used in the window
    private Text fahrenheit;
    private Text celsius;
    private Label help;

    /**
     * Runs the application
     */
    public void run() {
        Display display = new Display();
        Shell shell = new Shell(display);
        shell.setText("Temperatures");
        createContents(shell);
        shell.pack();
        shell.open();
        while (!shell.isDisposed()) {
            if (!display.readAndDispatch()) {
                display.sleep();
            }
        }
        display.dispose();
    }

    /**
     * Create the main window's contents
     * @param shell the main window
     */
    private void createContents(Shell shell) {
        shell.setLayout(new GridLayout(3, true));

        // Create the label and input box for Fahrenheit
        new Label(shell, SWT.LEFT).setText("Fahrenheit:");
        fahrenheit = new Text(shell, SWT.BORDER);
        GridData data = new GridData(GridData.FILL_HORIZONTAL);
        data.horizontalSpan = 2;
        fahrenheit.setLayoutData(data);

        // Set the context-sensitive help
        fahrenheit.setData("Type a temperature in Fahrenheit");

        // Add the listeners
        fahrenheit.addHelpListener(this);
        fahrenheit.addVerifyListener(this);
        fahrenheit.addModifyListener(this);

        // Create the label and input box for Celsius
        new Label(shell, SWT.LEFT).setText("Celsius:");
        celsius = new Text(shell, SWT.BORDER);
        data = new GridData(GridData.FILL_HORIZONTAL);
        data.horizontalSpan = 2;
        celsius.setLayoutData(data);

        // Set the context-sensitive help
        celsius.setData("Type a temperature in Celsius");

        // Add the listeners
        celsius.addHelpListener(this);
        celsius.addVerifyListener(this);
        celsius.addModifyListener(this);

        // Create the area for help
        help = new Label(shell, SWT.LEFT | SWT.BORDER);
        data = new GridData(GridData.FILL_HORIZONTAL);
        data.horizontalSpan = 3;
        help.setLayoutData(data);
    }

    /**
     * Called when user requests help
     */
    public void helpRequested(HelpEvent event) {
        // Get the help text from the widget and set it into the help label
        help.setText((String) event.widget.getData());
    }

    /**
     * Called when the user types into a text box, but before the text box gets
     * what the user typed
     */
    public void verifyText(VerifyEvent event) {     
        // Assume you don't allow it
        event.doit = false;

        // Get the character typed
        char myChar = event.character;
        String text = ((Text) event.widget).getText();
        System.out.println(text);

        // Allow '-' if first character
        if (myChar == '-' && text.length() == 0) event.doit = true;

        // Allow zero to nine
        if (Character.isDigit(myChar)) event.doit = true;

        // Allow backspace
        if (myChar == '\b') event.doit = true;
    }

    /**
     * Called when the user modifies the text in a text box
     */
    public void modifyText(ModifyEvent event) {     
        // Remove all the listeners, so you don't enter any infinite loops
        celsius.removeVerifyListener(this);
        celsius.removeModifyListener(this);
        fahrenheit.removeVerifyListener(this);
        fahrenheit.removeModifyListener(this);

        // Get the widget whose text was modified
        Text text = (Text) event.widget;

        try {
            // Get the modified text
            int temp = Integer.parseInt(text.getText());

            // If they modified Fahrenheit, convert to Celsius
            if (text == fahrenheit) {
                celsius.setText(String.valueOf((int) (FIVE_NINTHS * (temp - 32))));
            } else {
            // Convert to Fahrenheit
                fahrenheit.setText(String.valueOf((int) (NINE_FIFTHS * temp + 32)));
            }
        } catch (NumberFormatException e) { /* Ignore */ }

        // Add the listeners back
        celsius.addVerifyListener(this);
        celsius.addModifyListener(this);
        fahrenheit.addVerifyListener(this);
        fahrenheit.addModifyListener(this);
    }

    /**
     * The application entry point
     * @param args the command line arguments
     */
    public static void main(String[] args) {
        new MultipleListenersExample().run();
    }

}

In “modifyText” method it removes all the listeners (VerifyListener and ModifyListener) and I don’t understand why? Why an infinite loop appears?

Answer 1

Because you’re actually modifying text string (value) in modifyText() method of Text GUI element, this change of text will trigger ModifyEvent which calls modifyText() method again. So you have to remove those listeners, change text to appropriate value and add those listeners back, otherwise infinite loop occurs.

EDIT (based on comments)

You’re right. First you type a key in Text field and verifyText() is called. When this method finish and typed text is approved, modifyText() method is called. And as I said previous, this method changes text string of Text field by setText() method. So before the text string could be actually changed verifyText() is called and approves the new text string, then goes to modifyText() method which should actually change the text inside Text GUI element, but it tries to call setText() of the element and starts the circle again.

So, you type some number into Text element (like '5'), verifyText() is called and modifyText() follows, it calls setText() which calls verifyText() and modifyText() again, it calls setText(), and so on.. Oh yeah, infinite loop is here..