A
Canvas
is the interface to Java's AWT Canvas Component.
Yoix programs normally interact with a
Canvas
using event handlers and by reading or writing the following fields:
| background |
The
Color
that is used to paint the background of the canvas.
Reading returns a snapshot of the current color.
Writing immediately changes the background to the new color.
Storing
NULL
in
background
is special and means use the background of the nearest component
that contains the canvas and was assigned a background color other than
NULL;
if no component qualifies the canvas uses
VM.screen.background.
| | backgroundhints |
An
int
that controls how
backgroundimage
is displayed in the canvas.
The value should be one of
SCALE_AREA,
SCALE_DEFAULT,
SCALE_FAST,
SCALE_NONE,
SCALE_REPLICATE,
SCALE_SMOOTH,
or
SCALE_TILE,
which are all defined in
yoix.image.
SCALE_NONE
places the unscaled image in the upper left corner of the canvas,
while
SCALE_TILE
(the default) tiles the entire canvas with the unscaled image.
The other values select the algorithm used to scale
backgroundimage
so it fills the entire canvas.
Reading returns the current hints.
Writing immediately repaints the canvas using the new hints to
display the background image.
| | backgroundimage |
An
Object
that should be an
Image
or
String
that identifies an image that is automatically displayed as part of the
canvas's background in a way that is controlled by the value assigned to
backgroundhints.
A
NULL
value, which is the default, means there is no image.
A
backgroundimage
that is a
String
should name a local a file or URL that contains a GIF or JPEG image.
Reading returns the current image.
Writing immediately repaints the canvas using the new image.
| | border |
An
Object,
usually an
Insets
or
Number,
that sets the extent of the highlighted border that is drawn around
the canvas.
A
border
that is a number is an easy way to describe a uniform border.
Reading returns a snapshot of the current border.
Writing immediately sets the border to the new value.
| | bordercolor |
The base
Color
that is used to pick the shadow colors for the highlighted border.
The canvas's background is used when bordercolor is
NULL.
Reading returns a snapshot of the current color.
Writing immediately changes the bordercolor to the new color.
| | cursor |
An
Object
that should be an
int,
Image,
or
String
that selects the cursor shown when the pointer is over the canvas.
A
cursor
that is an
int
should be one of the cursors defined in the
yoix.awt.Cursor
dictionary.
A
cursor
that is an
Image
can describe the cursor using its
size
and
hotspot
fields and often draws it using its
paint
function.
A
cursor
that is a
String
should be the name of a cursor that is already defined in
yoix.awt.Cursor
or the name a local a file or URL that contains a GIF or JPEG image
that will be used as the cursor.
Reading returns the current cursor.
Writing immediately changes the canvas's cursor to the new value.
Storing
STANDARD_CURSOR
(the default) or
NULL
in
cursor
is special and means use the cursor assigned to the nearest component
that contains the canvas and was assigned a cursor other than
STANDARD_CURSOR;
if no component qualifies the canvas uses
DEFAULT_CURSOR.
| | enabled |
An
int
that is
1
when the canvas can respond to user input, and
0
otherwise.
Reading returns the current state.
Writing immediately sets the canvas's state to the new value.
| | focusowner |
A read-only
int
that is non-zero when the canvas has the focus.
| | font |
The
Font,
or font name if it is a
String,
that is used as the canvas's font.
Reading returns a snapshot of the current font.
Writing immediately sets the canvas's font to the new value.
| | foreground |
The
Color
that is used as the foreground of the canvas.
Reading returns a snapshot of the current color.
Writing immediately sets the canvas's foreground to the new color.
Storing
NULL
in
foreground
is special and means use the foreground of the nearest component
that contains the canvas and was assigned a foreground color other than
NULL;
if no component qualifies the canvas uses
VM.screen.foreground.
| | graphics |
A
Graphics
object that defines properties and built-ins that are used to apply
graphics operations to this canvas.
Writing after a canvas has been created is not allowed and will result in an
invalidaccess
error.
| | location |
A
Point
that determines the location of the canvas
in a coordinate system that has its origin at the upper left corner
of the container closest to the canvas (in the component hierarchy)
that actually contains it, positive x to the right, positive y down,
and a resolution of 72 dots per inch.
Reading returns a snapshot of the current location.
Writing is allowed, but layout managers usually get the final say, so setting
location
should be viewed as a request that may not be honored.
| | paint([Rectangle rect]) |
A
Function
that is called, if it is not
NULL,
whenever the canvas needs to be painted.
The optional
rect
argument describes the rectangle that needs repainting in the
coordinate system specified by
graphics.CTM,
which by default has its origin at the canvas's upper left corner,
positive x to the right, positive y down, and a resolution of 72 dots per inch.
| | popup |
A
PopupMenu
that is associated with the canvas.
Reading returns the current popup menu.
Writing immediately shows the popup menu at the point in the canvas's coordinate
system specified by the popup menu's
location
field, assuming of course that the canvas is showing on the screen.
Storing
TRUE
in the popup menu's
visible
field, which was added in release 1.2.0, is an easy way to show the
popup menu that currently belongs to the canvas.
| | preferredsize |
A
Dimension
that is used by layout managers when they need to know the canvas's
preferred size in units of 72 dots per inch.
A
NULL
value means the canvas has no preference.
A non-positive height or width is allowed and simply means the canvas
has no preference for that dimension.
Reading returns the current preferred size.
Writing changes the preferred size and immediately notifies
root.layoutmanager,
which means the components contained in
root
may be repositioned and resized.
| | repaint() |
A
Builtin
that tells the canvas to completely repaint itself,
which means the background is regenerated and then the canvas's
paint
function is called.
Obviously
repaint
should not be called, either directly or indirectly, from the canvas's
paint
function, however
erasedrawable
is safe because it does not trigger a
paint
call.
| | requestfocus |
An
int
that can be used to request or transfer the keyboard focus.
Storing a non-zero value in
requestfocus
tries to get the focus.
Storing
0
tries to transfer the focus.
Reading
requestfocus
does not currently return any useful information.
| | root |
An
Object
that is automatically updated by the interpreter's layout machinery
so it is always the top-level object that contains the canvas.
For example, put a canvas in a panel and
root
will be set to that panel;
add the panel to a frame and the canvas's
root
field will be set to that frame.
A canvas's event handlers can use
root
when they need to interact with the other components in the container.
| | showing |
A read-only
int
that is non-zero when the canvas is showing on the screen.
| | size |
A
Dimension
that determines the size of the canvas
in units of 72 dots per inch.
Reading returns a snapshot of the current size.
Writing is allowed, but layout managers usually get the final say, so setting
size
should be viewed as a request that may not be honored.
| | state |
An
int
that controls how the border of the canvas, if there is one,
is drawn.
When state is
0
the left and top parts of the border are drawn in a brighter
color than the right and bottom parts, which makes it look like
the interior of the canvas is raised.
The border colors are reversed when state is
1,
which makes it look like the interior of the canvas is depressed.
| | tag |
A
String
used to identify the canvas that is either supplied when
the canvas is declared, or automatically generated otherwise.
Add a canvas to a container, like a
Frame
or
Panel,
and the interpreter's layout machinery updates the
root
field so it points at the top-level container and then adds the canvas, as
tag,
to the
root.components
dictionary.
| | visible |
An
int
that is
1
when the canvas is visible, and
0
otherwise.
Reading returns the current visibility.
Writing immediately sets the canvas's visibility to the new state.
|
Several permanent fields have not been documented and should not be
used in Yoix applications.
Event handlers are functions that must be added to a canvas when it is
declared.
The handlers that work with canvases are listed below;
the names should be familiar if you have done some Java programming.
| |
| Event Handlers: |
actionPerformed,
componentHidden,
componentMoved,
componentResized,
componentShown,
dragDropEnd,
dragEnter,
dragExit,
dragGestureRecognized,
dragMouseMoved,
dragOver,
drop,
dropActionChanged,
focusGained,
focusLost,
invocationRun,
itemStateChanged,
keyPressed,
keyReleased,
keyTyped,
mouseClicked,
mouseDragged,
mouseEntered,
mouseExited,
mouseMoved,
mousePressed,
mouseReleased,
mouseWheelMoved
| | |
| Example: |
The program,
import yoix.*.*;
Frame f = {
FlowLayout layoutmanager = {
int hgap = 72/4;
int vgap = 72;
};
Array layout = {
new Canvas {
Color background = {double red = .5;};
int border = 72/12;
int state = FALSE;
Dimension size = {
int height = 72;
int width = 2*72;
};
},
new Canvas {
Color background = {double green = .5;};
int border = 72/12;
int state = TRUE;
Dimension size = {
int height = 72;
int width = 72;
};
},
new Canvas {
Color background = Color.blue;
Dimension preferredsize = {
int height = 36;
int width = 72;
};
},
};
};
f.visible = TRUE;
sleep(5);
f.visible = FALSE;
shows three canvases in a frame, but only for a few seconds.
The next program,
import yoix.*.*;
final double SHADOW = 72.0/36;
Image att = {
String source = "http://www.yoix.org/imgs/ATTlogo.gif";
};
Canvas c = {
double border = SHADOW;
Color background = Color.gray;
Image backgroundimage = att;
int backgroundhints = SCALE_DEFAULT;
int state = FALSE;
Dimension size = {
double width = att.size.width + 2*SHADOW;
double height = att.size.height + 2*SHADOW;
};
actionPerformed(ActionEvent e) {
printf("URL=%s\n", att.source);
}
};
Frame f = {
FlowLayout layoutmanager = {
int vgap = 72;
};
Array layout = {c};
};
f.visible = TRUE;
sleep(5);
exit(0);
shows how to build a custom button out of a canvas that displays
an image rather than a text string.
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| See Also: |
Button,
Checkbox,
Choice,
Dialog,
FileDialog,
Frame,
Graphics,
Image,
Label,
List,
Panel,
postEvent,
ScrollPane,
Scrollbar,
TableColumn,
TableManager,
TextArea,
TextCanvas,
TextField,
TextTerm,
Window
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