10.6: Events and Callbacks

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Roassal allows any visible component in a visualization, including the view itself, to emit and react to events. There are two kinds of events defined in Roassal. The first kind of event is low level and represents user actions, which includes clicking or moving the mouse or pressing a key. The second kind of event includes those triggered by the view itself, which typically includes movements of the camera, applying a layout, or refreshing the view. All events inherit from the ROEvent class.

To see how events work, we will show an example of a visualization that reacts to mouse clicks, translating an element to where the click was made. There are several event classes to deal with mouse events: ROMouseClick, ROMouseMove, ROMouseEnter and ROMouseLeave, among others; and to deal with key pressing, the ROKeyDown class.

We will make the visualization react to mouse left click using the ROLeftMouseClick event. The reaction will create an animation to translate the element to the event’s position.

We use the on:do: message to set a Roassal object to react to an event, as shown in the following code. The first parameter must be the class of the expected event and the second one a block, that defines the action to be executed when the event is received.

view := ROView new.
el := ROElement sprite.
view add: el.
view
    on: ROMouseLeftClick
    do: [ :event | ROLinearMove new for: el to: event position ].
view open.

ROLinearMove is one of the Roassal interactions. As its name suggests, it creates an animation for an element to be translated in a linear move. More about interactions is explained in the following section.