Example of using an event handler and a callback

In the following example, a small graphics application interface is created. The widget tree created by the code is illustrated on the right.

The code to create this tree is similar to that for the example on page Example code to create a widget tree, but in this example event and callback handler code (bold text) has been added. This code registers the event and callback handlers just after the widgets are created.

When the push-button widget is pressed (that is, when the user clicks mouse button 1 while the mouse pointer is over the push-button widget), the pressed:clientData:callData: method is run. When the mouse is moved in the drawing area with button 1 held down, the button1Move:clientData:event: method is run.

| shell main form drawArea button | shell := CwTopLevelShell createApplicationShell: 'shell' argBlock: [:w | w title: 'Graphics Example']. main := shell createMainWindow: 'main' argBlock: nil. main manageChild. form := main createForm: 'form' argBlock: nil. form manageChild.

drawArea := form

createDrawingArea: 'drawing'

argBlock: [:w |

w

width: 300;

height: 300;

borderWidth: 1].

"After the drawing area is created, an event handler is added to the widget

The method specifies that the application shall be notified of Button1Motion

events (button 1 is down and the mouse is moving) within the drawing area.

When mouse events occur, the button1Move:clientData:event message is sent

to self (the application)."

drawArea

addEventHandler: Button1MotionMask

receiver:self

selector: #button1Move:clientData:event:

clientData:nil.

drawArea manageChild.

button := form

createPushButton: '1'

argBlock: nil.

"Here a callback is added to the newly created push-button widget.

The method specifies that the application shall be notified of

activate callback (the button is pressed and released). When the

button is activated, the pressed:clientData:callData: message is

sent to self."

button

addCallback:XmNactivateCallback

receiver: self

selector: #pressed:clientData:callData:

clientData:nil.

button manageChild.

drawArea

setValuesBlock: [:w |

w

topAttachment: XmATTACHFORM;

topOffset: 2;

bottomAttachment: XmATTACHFORM;

bottomOffset: 2;

leftAttachment: XmATTACHWIDGET;

leftWidget: button;

leftOffset: 2;

rightAttachment: XmATTACHFORM;

rightOffset: 2].

button

setValuesBlock: [:w |

w

topAttachment: XmATTACHFORM;

topOffset: 2;

leftAttachment: XmATTACHFORM;

leftOffset: 2].

shell realizeWidget.