5.2 The class hierarchy for DUIM-Sheets

5.2.3 Subclasses of <device-event>

Table 5.3 shows the subclasses of the <device-event> class that are exposed by the DUIM-Sheets library. Device events, broadly speaking, describe any event that can occur on a device connected to the computer.

Table 5.3 Subclasses of the `<device-event>` class
<device-event>
	<pointer-event>
		<pointer-button-event>
			<button-press-event>
			<button-release-event>
			<button-click-event>
			<double-click-event>
			<pointer-drag-event>
		<pointer-motion-event>
			<pointer-drag-event>
			<pointer-boundary-event>
	<keyboard-event>			<pointer-exit-event>
		<key-press-event>		<pointer-enter-event
		<key-release-event>

Note: The <pointer-drag-event> class is a subclass of both <pointer-button-event> and <pointer-motion-event>.

Device events fall into two distinct categories:

Keyboard events that occur on the keyboard attached to the computer: subclasses of <keyboard-event>
Pointer events that occur on the pointing device attached to the computer: subclasses of <pointer-event>

There are two classes of keyboard event. The classes <key-press-event> and <key-release-event> describe the events that occur when any key on the keyboard is pressed or released, respectively.

There are three classes of pointer event, some of which provide a number of subclasses. Note that there are another two classes of pointer event that are immediate subclasses of <object>. These are described in Section 5.2.1 on page 200.

<pointer-button-event>
: These events occur whenever there is any activity on one of the buttons on the pointing device. Several subclasses of this class are provided.
<pointer-exit-event>
: This is an event that occurs when the pointer leaves a specified area such as a sheet.
<pointer-motion-event>
: This class of events occur when the pointer is in motion. There is one subclass provided, <pointer-boundary-event>, for the specific case when the motion of the pointer causes the boundary of a sheet to be crossed.
: Note: Unlike <pointer-drag-event>, no button needs to be pressed on the attached pointing device.

The subclasses provided for <pointer-button-event> are as follows:

<button-press-event>
: This event occurs when any button on the pointing device is pressed down by the user. Note that this is distinct from <button-click-event>, described below.
<button-release-event>
: This event occurs when any previously pressed button on the pointing device is released by the user.
<button-click-event>
: This event occurs when any button on the pointing device is pressed down by the user and then released again within a certain time frame. An instance of this class is created if the creation of an instance of <button-press-event> is closely followed by the creation of an instance of <button-release-event>. The necessary time frame is dictated by the configuration of your computer. In Windows, for example, this time can be set using the Control Panel.
<double-click-event>
: This event occurs when a button is clicked twice within a certain time frame. An instance of this class is created if the creation of an instance of <button-click-event> is closely followed by the creation of another instance of <button-click-event>. The necessary time frame is dictated by the configuration of your computer.