Notifier Design Pattern

Say you want to develop an application which will have many viewers/pages and all these have to be refreshed with the latest data as soon as the data changes in the engine.

A very simple example is - Weather Reports. When the data changes in the engine, the weather report page has to be refreshed to show the latest data.

Let's follow the below steps:

1. Develop your report dialog - which will show the weather information.
2. Implement the ChangeListener in the dialog report- This will act as a listener
3. Implement your report data class, which contains the weather information. This will act a notifier and also as a registry.
4. Register your listener with the data class so that the viewer will get a notification if the data changes.

Here is the sample code for it:

import java.util.List;
import javax.swing.event.ChangeEvent;

import javax.swing.event.ChangeListener;

import org.eclipse.jface.viewers.TableViewer;

import org.eclipse.swt.SWT;

import org.eclipse.swt.layout.GridData;

import org.eclipse.swt.layout.GridLayout;

import org.eclipse.swt.widgets.Composite;

import org.eclipse.swt.widgets.Display;

import org.eclipse.swt.widgets.Group;

import org.eclipse.swt.widgets.Shell;

import org.eclipse.swt.widgets.Table;

import org.eclipse.swt.widgets.TableColumn;

/**

 *

 * Weather report dialog.

 *

 */

public class WeatherDetailsDialog implements ChangeListener {

  

 private TableViewer weatherViewer;

  

 protected WeatherDetailsDialog() {  

  //your specific data passed through constructor 

 }

 protected void createReport(Composite topComposite) {

   

  Group dependantsViewerGroup = new Group(topComposite, SWT.NONE);

  dependantsViewerGroup.setLayout(new GridLayout());

  GridData gridData3 = new GridData(SWT.FILL, SWT.FILL, true, false);

  dependantsViewerGroup.setLayoutData(gridData3);

  dependantsViewerGroup.setText("Weather Report"); //$NON-NLS-1$

   

  weatherViewer = new TableViewer(

    dependantsViewerGroup, SWT.FULL_SELECTION | SWT.H_SCROLL | SWT.BORDER);

  final Table reportTable = weatherViewer.getTable();

  reportTable.setLinesVisible(false);

  reportTable.setHeaderVisible(true);

  reportTable.setLayoutData(gridData3);

  GridData gridData = new GridData(SWT.FILL, SWT.TOP, true, false);

  gridData.heightHint = 140;

  reportTable.setLayoutData(gridData);

  TableColumn cityName = new TableColumn(reportTable, SWT.LEFT);

  cityName.setText("City"); //$NON-NLS-1$

  cityName.pack();

  cityName.setWidth(100);

   

  TableColumn valueColumn = new TableColumn(reportTable, SWT.LEFT);

  valueColumn.setText("Value"); //$NON-NLS-1$

  valueColumn.pack();

  valueColumn.setWidth(100);

   

  TableColumn descriptionColumn = new TableColumn(reportTable, SWT.LEFT);

  descriptionColumn.setText("Description"); //$NON-NLS-1$

  descriptionColumn.pack();

  descriptionColumn.setWidth(120);

   

  weatherViewer.setContentProvider(new WeatherContentProvider()); 

  weatherViewer.setLabelProvider(new WeatherLabelProvider());

  //Register viewer with the weathe report data.

   

  List<IWeather> reportData = computingEngine.getReportData();

  for (IWeather data : reportData) {

   ((Weather)data).addChangeListener(this);

  }

  weatherViewer.setInput(reportData);

 }

/**

     * Invoked when the target of the listener has changed its state.

     *

     * @param e  a ChangeEvent object

     */

   

 @Override

 public void stateChanged(ChangeEvent arg0) {

  Display.getDefault().asyncExec(new Runnable() {

   @Override

   public void run() {

    weatherViewer.refresh();

   }

  });

 }

}

This will notfiy as soon the data changes.

import java.util.ArrayList;

import java.util.List;

import javax.swing.event.ChangeListener;

public class Weather {

 private List<ChangeListener> registeredListners = new ArrayList<ChangeListener>();
 private ComputingEngine ComputingData;
 private Float newValue;
 private String description;
 
 public Weather(ComputingEngine ComputingData) {

  this.ComputingData = ComputingData;

 }

  

 /**

  * Updating the status change. This will notify to all the registered listeners

  * Ex: It will notify to the viewer/UI about the weather change report

  *   */

  

 public void setValue(Float newValue) {

  this.newValue = newValue;

  notifyListeners();

 }

  

 public ComputingEngine getComputingData() {

  return ComputingData;

 }

  

 public void setDescription(String description) {

  this.description = description;

 }

  

 public String getDescription() {

  return description;

 }

  

 public Float getNewValue() {

  return newValue;

 }

 //Add your listener here

 public void addChangeListener(ChangeListener listener) {

  if (!registeredListners.contains(listener))

   registeredListners.add(listener);

 }

 //removing the listener

 public void removeChangeListener(ChangeListener listener) {

  if (registeredListners.contains(listener))

   registeredListners.remove(listener);

 }

 //This will be triggered when the state changes

 public void notifyListeners() {

  for (ChangeListener listener : registeredListners) {

   listener.stateChanged(null);

  }

 }

}