Java Reference
In-Depth Information
The
RoundButton
shows all of the principles I discuss in the previous section, and it
uses double buffering to an offscreen
Image
instance to prevent the possibility of flicker-
ing when the component repaints. It's not a complex component, but it is a little long, so
let's take it field by field and method by method.
The
RoundButton
has a
label
field that contains the string labeling the button for
the user. It must also track its state, whether it's pressed or not, which it does using
the
protected pressed
field. It maintains an
Image
for offscreen drawing, aptly named
offscreen
, and two private constant variables: the preferred size of the button and the
default padding for the button label.
■
Tip
You may be wondering why I chose
protected
for the access protection level of these fields. I'm
assuming that this class will be part of a more sophisticated GUI framework, and thus might have subclasses
that customize the behavior of this button and need access to its fields.
The
RoundButton
constructor simply caches aside the label you provide and enables
mouse events for the component. The
RoundButton
must override the
invalidate
method
as part of the double buffering. The windowing environment calls
invalidate
as part of the
component's sizing and layout, which in turn requires the
RoundButton
to allocate a new
offscreen bitmap for drawing. For speed, the
invalidate
method only invalidates the cur-
rent offscreen bitmap; it leaves allocating the new offscreen bitmap to the
paint
method.
The
update
method—invoked by the windowing toolkit hierarchy as a result of a sys-
tem redraw—must, by definition, clear the component's background and completely
redraw the component. Because
paint
draws the entire component to an offscreen
bitmap and then copies that bitmap to the screen,
update
can simply call
paint
directly.
The
paint
method does the real work for the
RoundButton
. It begins by creating the
offscreen bitmap if one is required, and then it obtains a
Graphics
instance for the off-
screen bitmap. It then draws the button on the offscreen bitmap from back to front, with
the following operations:
1.
It computes the button background, which is slightly darker if the button is
being pressed.
2.
It draws a filled circle for the button using the color it computed in the
previous step.
3.
Once it has drawn the background, it draws the border using a shade darker
than the button's background.
4.
With the background and border drawn, it draws the button's label, which is
centered in the region defined by the button.
