GWT event listeners

GWT defines event listeners that you can attach to your widgets to monitor browser events. Event Listeners are just interfaces that you can implement in your widget. If you want to trap mouse click events, then your widget should implement the ClickListener interface and define the onClick(Widget sender) method. The code inside this method will be fired every time the user clicks on your widget.

You can create a new anonymous ClickListener every time you want to assign it to a widget as given in the Hello gwt example.

button.addClickListener(new ClickListener() {
 public void onClick(Widget sender) {
  processData(sender.getText());
  reportData(sender.getText());
  logResults();
 }
});

But this results in localized code and lesser code reusability. Let us say we want to add another widget (a HTML link), which has to run the same code as in the onClick() method. We would be using the addClickListener method for this link and would be rewriting the same code again separately. And if we need to add another method in the onClick() method code between processData() and reportData() we will have to change each instance of onClick().

Alternatively, we can implement the listeners in the container widget to handle the events of contained widgets as shown below, making the code more reusable.

public class MainPanel extends Composite 
 implements ClickListener{

 private Button button=new Button("Button");
 public MainPanel(){
 
  //...
  button.addClickListener(this);
 }

 public void onClick(Widget sender){
  if(sender==button){
   processData(sender.getText());
   reportData(sender.getText());
   logResults();
  }
 }
}

With this approach, if we want to add a HTML widget link that implements the same function, we would just have to add the line

link.addClickListener(this);

in MainPanel() and modify one line in the onClick(Widget sender) method

if(sender==button||sender==link)

Given Below is a listing of GWT Event listeners for handling various events:

Listener	Event notifications	Methods Defined
HistoryListener	Changes to Browser History	onHistoryChanged(String historyToken)
WindowCloseListener	Window Closure Events	onWindowClosed() onWindowClosing()
WindowResizeListener	Window Resizing Events	onWindowResized(int width, int height)
ChangeListener	fires when a widget changes	onChange(Widget sender)
ClickListener	fires when a widget receives a 'click'	onClick(Widget sender)
FormHandler	fires on receiving Form events from the FormPanel to which it is attached	onSubmit(FormSubmitEvent event) onSubmitComplete( FormSubmitCompleteEvent event)
FocusListener	Focus Events	onFocus(Widget sender) onLostFocus(Widget sender)
KeyBoardListener	Keyboard Events	onKeyDown(Widget sender, char keycode, int modifiers) onKeyPress(Widget sender, char keycode, int modifiers) onKeyUp(Widget sender, char keycode, int modifiers)
LoadListener	listens to 'load' event of a widget	onLoad(Widget Sender) onError(Widget Sender)
MouseListener	listens to mouse events of a widget	onMouseEnter(Widget sender) onMouseLeave(Widget sender) onMouseDown(Widget sender, int x, int y) onMouseUp(Widget sender, int x, int y) onMouseMove(Widget sender, int x, int y)
PopupListener	listens to the events of a PopupPanel widget	onPopupClosed(PopupPanel sender, boolean autoClose)
ScrollListener	listens to Scroll events	onScroll(Widget widget, int scrollLeft, int scrollTop)
TableListener	listens to events pertaining to a Table widget	onCellClicked( SourcesTableEvents sources, int row, int cell)
TabListener	listens to the events of a Tab Widget	onBeforeTabSelected( SourcesTabEvents sender, int tabIndex) onTabSelected( SourcesTabEvents sender, int tabIndex)
TreeListener	listens to the events of a Tree Widget	onTreeItemSelected( TreeItem item) onTreeItemChanged( TreeItem item)

The Button widget defines the addClickListener method which attaches a ClickListener object to it. Similarly, the Tree widget defines the addTreeListener method. Not all event listeners are available for all widgets - for example, the addMouseListener method is not defined for a DockPanel Widget. One way to add the MouseListener functionality to this DockPanel is to create a composite that includes a FocusPanel widget (which supports addMouseListener) and wrap the DockPanel widget within it.