### Events

Once we create a display and some widgets, and start up the application's message loop, where does the real work happen? It happens every time an event is read from the queue and dispatched to a widget. Most of the application logic is implemented as responses to user events.

The basic pattern is that you add a listener to some widget that you have created, and when the appropriate event occurs the listener code will be executed. This simple example is adapted from org.eclipse.swt.examples.helloworld.HelloWorld3:

       Display display = new Display ();
       Shell shell = new Shell (display);
       Label label = new Label (shell, SWT.CENTER);
       ...
       shell.addControlListener (new ControlAdapter () {
          public void controlResized (ControlEvent e) {
             label.setBounds (shell.getClientArea ());
          }
       });

For each type of listener, there is an interface that defines the listener (Xyz Listener), a class that provides event information (Xyz Event), and an API method to add the listener (add Xyz Listener). If there is more than one method defined in the listener interface then there is an adapter (Xyz Adapter) that implements the listener interface and provides empty methods. All of the events, listeners, and adapters are defined in the package org.eclipse.swt.events.

The following tables summarize the events that are available and the widgets that support each event. Events can be split into two general categories: high level events which represent a logical operation on a control, and low level events which describe more specific user interactions. High level events may be represented by multiple low level events which may differ per platform. Low level events should generally only be used for custom widget implementations.

#### High level events

Event Type | Description
--- | ---
Activate, Deactivate | Generated when a Control is activated or deactivated.
Arm | A MenuItem is armed (highlighted and ready to be selected).
Close | A Shell is about to close as requested by the window manager.
DefaultSelection | The user selects an item by invoking a default selection action. For example, by hitting Enter or double clicking on a row in a Table.
Dispose | A widget is about to be disposed, either programmatically or by user.
DragDetect | The user has initiated a possible drag operation.
EraseItem | A TableItem or TreeItem is about to have its background drawn.
Expand, Collapse | An item in a Tree is expanded or collapsed.
Help | The user has requested help for a widget. For example, this occurs when the F1 key is pressed under Windows.
Iconify, Deiconify | A Shell has been minimized, maximized, or restored.
MeasureItem | The size of a custom drawn TableItem or TreeItem is being requested.
MenuDetect | The user has requested a context menu.
Modify | The widget's text has been modified.
Move, Resize | A control has changed position or has been resized, either programmatically or by user.
PaintItem | A TableItem or TreeItem is about to have its foreground drawn.
Selection | The user selects an item in the control. For example, by single clicking on a row in a Table or by keyboard navigating through the items.
SetData | Data needs to be set on a TableItem when using a virtual table.
Settings | An operating system property, such as a system font or color, has been changed.
Show, Hide | A control's visibility has changed.
Traverse | The user is trying to traverse out of the control using a keystroke. For example, the escape or tab keys are used for traversal.
Verify | A widget's text is about to be modified. This event gives the application a chance to alter the text or prevent the modification.

#### Low level events

Event Type | Description
--- | ---
FocusIn, FocusOut | A control has gained or lost focus.
KeyDown, KeyUp | The user has pressed or released a keyboard key when the control has keyboard focus.
MouseDown, MouseUp, MouseDoubleClick | The user has pressed, released, or double clicked the mouse over the control.
MouseMove | The user has moved the mouse above the control.
MouseEnter, MouseExit, MouseHover | The mouse has entered, exited, or hovered over the control.
MouseWheel | The mouse wheel has been rotated.
Paint | The control has been damaged and requires repainting.

#### Untyped events

In addition to the typed event system described above, SWT supports a low level, untyped widget event mechanism. The untyped mechanism relies on a constant to identify the event type and defines a generic listener that is supplied with this constant. This allows the listener to implement a "case style" listener. In the following snippet, we define a generic event handler and add several listeners to a shell.

       Shell shell = new Shell ();
       Listener listener = new Listener () {
          public void handleEvent (Event e) {
             switch (e.type) {
                case SWT.Resize:
                   System.out.println ("Resize received");
                   break;
                case SWT.Paint:
                   System.out.println ("Paint received");
                   break;
                default:
                   System.out.println ("Unknown event received");
             }
          }
       };
       shell.addListener (SWT.Resize, listener);
       shell.addListener (SWT.Paint, listener);