Java观察者/可观察的更新

Question

I've tried to apply the Observable/Observer pattern but there is something wrong with my code when I try to change a the textfield of a JTextPane .

I've got 3 classes, Play , Controller and SecondWindow here are a sample of their code.

public class Play() {
    Controller c = new Controller();
    SecondWindow sw = new SecondWindow();
    c.addObserver(sw)

    c.setText("blabla");
}

My class Controller :

public class Controller extends Observable(){

    private String text ="";

    private static Controller getInstance() {
        if (instance == null) {
            instance = new Controller();
        }
        return instance;
    }

    public void setText(String s) {
        text = s;
        setChanged();
        notifyObservers();
    }
}

and SecondWindow :

public class SecondWindow extends JFrame implements Observer{
    private JPanel contentPane;
    private Controller c;
    private JTextPane txt = new JTextPane();

    public SecondWindow () {
        EventQueue.invokeLater(new Runnable() {
            public void run() {
                try {
                    SecondWindow frame = new SecondWindow();
                    frame.setVisible(true);
                } catch (Exception e) {
                    e.printStackTrace();
                }
            }
        });
    }

    public SecondWindow() {
        initComponents();
        createEvents();
        c = Controller.getInstance();
    }

    public void initComponents() {

        setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        setBounds(1000, 0, 300,500);
        contentPane = new JPanel();
        contentPane.setBorder(new EmptyBorder(5, 5, 5, 5));
        setContentPane(contentPane);
        contentPane.setLayout(null);

        txt.setBounds(0, 0, 280, 460);
        txt.enable(false);
        contentPane.add(txt);
    }

    public void update(Observable arg0 , Object arg1){
        // Things to change here
    }

I can't manage to put the variable c in the textField (like a txt.setText(c.getText) instruction). I'm sure that it reads the method update, but I don't know how to make sure it works.

Answer 1

Hint: Per the Observerable API the notifyObservers method has an overload that accepts any object as a parameter:

public void notifyObservers(Object arg)

This can even be a String. And as per the Observer API , this object is then passed into the update method in the observer, and you can use it there.

 void update(Observable o, Object arg) 

arg - an argument passed to the notifyObservers method.

---

Separate side issue here:

contentPane.setLayout(null);

For most Swing aficionados, seeing this is like hearing nails on a chalkboard -- it's painful. While null layouts and setBounds() might seem to Swing newbies like the easiest and best way to create complex GUI's, the more Swing GUI'S you create the more serious difficulties you will run into when using them. They won't resize your components when the GUI resizes, they are a royal witch to enhance or maintain, they fail completely when placed in scrollpanes, they look gawd-awful when viewed on all platforms or screen resolutions that are different from the original one. Instead you will want to study and learn the layout managers and then nest JPanels, each using its own layout manager to create pleasing and complex GUI's that look good on all OS's.

---

Side issue number two: your code is not Swing thread safe, since the Swing GUI could very well be notified by the observable off of the Swing event dispatch thread or EDT. While it is not likely to cause frequent or serious problems with this simple program, in general it would be better to use a SwingPropertyChangeSupport and PropertyChangeListeners rather than Observer / Observable if you can.

---

Next Side Issue
This:

public class Controller extends Observable(){

isn't compilable / kosher Java. Same for the duplicate parameter-less constructors for the SecondWindow class. Yes, we know what you're trying to do, but it's hard enough trying to understand someone else's code, you really don't want to make it harder by posting kind-of sort-of uncompilable code, trust me.

---

For example, something simple could be implemented in Swing using PropertyChangeListeners, like so:

import java.util.concurrent.TimeUnit;

public class Play2 {
    public static void main(String[] args) {
        Model2 model2 = new Model2();
        View2 view2 = new View2();
        new Controller2(model2, view2);
        view2.show();
        for (int i = 0; i < 10; i++) {
            try {
                TimeUnit.SECONDS.sleep(1);
            } catch (InterruptedException e) {
                // one of the few times it's OK to ignore an exception
            }
            String text = String.format("Counter Value: %d", i);
            model2.setText(text);
        }
    }
}

import java.beans.PropertyChangeListener;
import javax.swing.event.SwingPropertyChangeSupport;

public class Model2 {
    private SwingPropertyChangeSupport pcSupport = new SwingPropertyChangeSupport(this);
    public static final String TEXT = "text"; // name of our "bound" property
    private String text = "";

    public String getText() {
        return text;
    }

    public void setText(String text) {
        String oldValue = this.text;
        String newValue = text;
        this.text = text;
        pcSupport.firePropertyChange(TEXT, oldValue, newValue);
    }

    public void addPropertyChangeListener(PropertyChangeListener listener) {
        pcSupport.addPropertyChangeListener(listener);
    }

    public void removePropertyChangeListener(PropertyChangeListener listener) {
        pcSupport.removePropertyChangeListener(listener);
    }

    public void addPropertyChangeListener(String name, PropertyChangeListener listener) {
        pcSupport.addPropertyChangeListener(name, listener);
    }

    public void removePropertyChangeListener(String name, PropertyChangeListener listener) {
        pcSupport.removePropertyChangeListener(name, listener);
    }
}

import javax.swing.*;

public class View2 {
    private JPanel mainPanel = new JPanel();
    private JTextField textField = new JTextField(10);

    public View2() {
        textField.setFocusable(false);
        mainPanel.add(new JLabel("Text:"));
        mainPanel.add(textField);
    }

    public JPanel getMainPanel() {
        return mainPanel;
    }

    public void setText(String text) {
        textField.setText(text);
    }

    public void show() {
        SwingUtilities.invokeLater(() -> {
            JFrame frame = new JFrame("View");
            frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
            frame.add(getMainPanel());
            frame.pack();
            frame.setLocationRelativeTo(null);
            frame.setVisible(true);
        });
    }
}

import java.beans.PropertyChangeEvent;
import java.beans.PropertyChangeListener;

public class Controller2 {
    private Model2 model2;
    private View2 view2;

    public Controller2(Model2 model2, View2 view2) {
        this.model2 = model2;
        this.view2 = view2;
        model2.addPropertyChangeListener(Model2.TEXT, new ModelListener());
    }

    private class ModelListener implements PropertyChangeListener {
        @Override
        public void propertyChange(PropertyChangeEvent pcEvt) {
            view2.setText((String) pcEvt.getNewValue());
        }
    }
}