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Behavorial Pattern
Observer Pattern
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Observer Pattern
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Define
Observer pattern
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One object changes state, all of
its dependents are updated automatically.
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Where
to use & benefits
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One change affects one or many
objects.
Many object behavior depends on one object state.
Need broadcast communication.
AKA Publish-Subscribe.
Maintain consistency between objects
keep classes from becoming too tightly coupled, which would hamper
reusability.
Related patterns include
- Singleton, which is used to make observable object unique
and accessible globally.
- Mediator, which is used to encapsulate updated objects.
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Example
of Observer pattern
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Observer pattern is often used
in GUI application. For example, defining a one-to-many dependency
between objects so that when one object changes state, all its
dependents are notified and updated automatically, like stock change
affecting many data or diagram updated accordingly.
Java API provides a built-in interface Observer and class Observable
for use.
To show how observer pattern works, two windows have been created. One
is for user input; another is for display. When the data has been
entered in the textfield, another window gets the message and display
it with a dialog. The private inner classes have been used in this
example.
import javax.swing.*;
import java.awt.event.*;
import java.util.*;
class DisplayForm extends JFrame {
InputFormObserver input = new InputFormObserver();
InputForm inputForm ;
Observable obsInput;
JTextField display;
//...
public DisplayForm()
{
addWindowListener(new
WindowAdapter() {
public void windowClosing(WindowEvent e) {
System.exit(0);
}});
inputForm = new InputForm();
obsInput =
inputForm.getInputInfo();
obsInput.addObserver(input);
display = new JTextField(10);
display.addActionListener(new ActionListener() {
public void actionPerformed(ActionEvent e) {
}
});
getContentPane().add(display);
setTitle("Observer form");
setSize(200,100);
setLocation(200,100);
setVisible(true);
}
private class InputFormObserver implements Observer {
public void
update(Observable ob, Object o) {
doSomeUpdate();
if (obsInput.countObservers()>0)
obsInput.deleteObservers();
obsInput = inputForm.getInputInfo();
obsInput.addObserver(input);
}
}
public void doSomeUpdate() {
display.setText(inputForm.getText());
JOptionPane.showMessageDialog(DisplayForm.this,
"This form has been updated");
}
public static void main(String args[]) {
DisplayForm df = new
DisplayForm();
}
}
class InputForm extends JFrame {
public InformDisplay inform = new InformDisplay();
//...
JTextField input= new JTextField(10);
public InputForm() {
JPanel panel= new JPanel();
input.addActionListener(new
ActionListener() {
public void actionPerformed(ActionEvent e) {
inform.notifyObservers();
}
});
panel.add(new JLabel("Enter:
"));
panel.add(input);
addWindowListener(new
WindowAdapter() {
public void windowClosing(WindowEvent e) {
System.exit(0);
}});
getContentPane().add(panel);
setTitle("Observable form");
setSize(200,100);
setVisible(true);
}
public Observable getInputInfo() {
return inform;
}
public String getText() {
return input.getText();
}
private class InformDisplay extends Observable {
public void
notifyObservers() {
setChanged();
super.notifyObservers();
}
public String getChange() {
return input.getText();
}
}
//...
}
java DisplayForm
Two windows will come up:
one for user input, the other for observing user input.
Enter data in the input window, a dialog box from other window shows up
and the input data has been shown in the other window.
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