Java Reference
In-Depth Information
a displayable object within the application using the
addCommand()
method.
•
ItemCommandListener
is a listener type for receiving notification
of commands that have been invoked on
Item
objects. This provides
the mechanism for associating commands with specific
Form
items,
thus contextualizing user input and actions according to the current
active item on the form, making it more intuitive.
•
ItemStateListener
is used by applications that need to receive
events that indicate changes in the internal state of the interactive items
within a form; for example, a notification is sent to the application
when the set of selected values within a
ChoiceGroup
changes.
2.3.4 LCDUI Low-Level API:
Canvas
The low-level API allows developers to have total control of how the
user interface looks and how components are rendered on the screen.
Canvas
, the main base class for low-level UI programming, is used
to exercise such fine-grained control. An application should subclass
Canvas
to create a new displayable screen object. As it is displayable,
it can be used as the current display for an application just like the
high-level components. Therefore a MIDlet application can have a user
interface with, for example,
List
,
Form
and
Canvas
objects, which can
be displayed one at a time to provide the application functionality to the
users.
Canvas
is commonly used by game developers when creating sprite
animation and it also forms the basis of
GameCanvas
, which is part
of the Game API (see Section 2.3.6).
Canvas
can be used in normal
mode, which allows title and softkey labels to be displayed, and full-
screen mode, where
Canvas
takes up as much of the display as the
implementation allows. In either mode, the dimensions of the
Canvas
can be accessed using the
getWidth()
and
getHeight()
methods.
Graphics are drawn to the screen by implementing code in the abstract
paint()
method. This method must be present in the subclass and
is called as part of the event model. The event model provides a
series of user-input methods such as
keyPressed()
and
pointer-
Pressed()
, depending upon the device's data input implementation.
The
paint(Graphics g)
method passes in a
Graphics
object, which
is used to draw to the display.
The
Graphics
object provides a simple 2D, geometric-rendering
capability, which can be used to draw strings, characters, images, shapes,
etc. For more details, please check the MIDP documentation.
Such methods as
keyPressed()
and
pointerPressed()
repre-
sent the interface methods for the
CommandListener
. When a key is
pressed, it returns a key code to the command listener. These key codes are
