Provided by: tk8.6-doc_8.6.13-2_all 
NAME
Tk_CanvasTkwin, Tk_CanvasGetCoord, Tk_CanvasDrawableCoords, Tk_CanvasSetStippleOrigin,
Tk_CanvasWindowCoords, Tk_CanvasEventuallyRedraw, Tk_CanvasTagsOption - utility procedures
for canvas type managers
SYNOPSIS
#include <tk.h>
Tk_Window
Tk_CanvasTkwin(canvas)
int
Tk_CanvasGetCoord(interp, canvas, string, doublePtr)
Tk_CanvasDrawableCoords(canvas, x, y, drawableXPtr, drawableYPtr)
Tk_CanvasSetStippleOrigin(canvas, gc)
Tk_CanvasWindowCoords(canvas, x, y, screenXPtr, screenYPtr)
Tk_CanvasEventuallyRedraw(canvas, x1, y1, x2, y2)
Tk_OptionParseProc *Tk_CanvasTagsParseProc;
Tk_OptionPrintProc *Tk_CanvasTagsPrintProc;
ARGUMENTS
Tk_Canvas canvas (in) A token that identifies a canvas widget.
Tcl_Interp *interp (in/out) Interpreter to use for error reporting.
const char *string (in) Textual description of a canvas coordinate.
double *doublePtr (out) Points to place to store a converted coordinate.
double x (in) An x coordinate in the space of the canvas.
double y (in) A y coordinate in the space of the canvas.
short *drawableXPtr (out) Pointer to a location in which to store an x
coordinate in the space of the drawable currently
being used to redisplay the canvas.
short *drawableYPtr (out) Pointer to a location in which to store a y
coordinate in the space of the drawable currently
being used to redisplay the canvas.
GC gc (out) Graphics context to modify.
short *screenXPtr (out) Points to a location in which to store the screen
coordinate in the canvas window that corresponds
to x.
short *screenYPtr (out) Points to a location in which to store the screen
coordinate in the canvas window that corresponds
to y.
int x1 (in) Left edge of the region that needs redisplay.
Only pixels at or to the right of this coordinate
need to be redisplayed.
int y1 (in) Top edge of the region that needs redisplay. Only
pixels at or below this coordinate need to be
redisplayed.
int x2 (in) Right edge of the region that needs redisplay.
Only pixels to the left of this coordinate need to
be redisplayed.
int y2 (in) Bottom edge of the region that needs redisplay.
Only pixels above this coordinate need to be
redisplayed.
_________________________________________________________________________________________________
DESCRIPTION
These procedures are called by canvas type managers to perform various utility functions.
Tk_CanvasTkwin returns the Tk_Window associated with a particular canvas.
Tk_CanvasGetCoord translates a string specification of a coordinate (such as 2p or 1.6c)
into a double-precision canvas coordinate. If string is a valid coordinate description
then Tk_CanvasGetCoord stores the corresponding canvas coordinate at *doublePtr and
returns TCL_OK. Otherwise it stores an error message in the interpreter result and
returns TCL_ERROR.
Tk_CanvasDrawableCoords is called by type managers during redisplay to compute where to
draw things. Given x and y coordinates in the space of the canvas,
Tk_CanvasDrawableCoords computes the corresponding pixel in the drawable that is currently
being used for redisplay; it returns those coordinates in *drawableXPtr and *drawableYPtr.
This procedure should not be invoked except during redisplay.
Tk_CanvasSetStippleOrigin is also used during redisplay. It sets the stipple origin in gc
so that stipples drawn with gc in the current offscreen pixmap will line up with stipples
drawn with origin (0,0) in the canvas's actual window. Tk_CanvasSetStippleOrigin is
needed in order to guarantee that stipple patterns line up properly when the canvas is
redisplayed in small pieces. Redisplays are carried out in double-buffered fashion where
a piece of the canvas is redrawn in an offscreen pixmap and then copied back onto the
screen. In this approach the stipple origins in graphics contexts need to be adjusted
during each redisplay to compensate for the position of the off-screen pixmap relative to
the window. If an item is being drawn with stipples, its type manager typically calls
Tk_CanvasSetStippleOrigin just before using gc to draw something; after it is finished
drawing, the type manager calls XSetTSOrigin to restore the origin in gc back to (0,0)
(the restore is needed because graphics contexts are shared, so they cannot be modified
permanently).
Tk_CanvasWindowCoords is similar to Tk_CanvasDrawableCoords except that it returns
coordinates in the canvas's window on the screen, instead of coordinates in an off-screen
pixmap.
Tk_CanvasEventuallyRedraw may be invoked by a type manager to inform Tk that a portion of
a canvas needs to be redrawn. The x1, y1, x2, and y2 arguments specify the region that
needs to be redrawn, in canvas coordinates. Type managers rarely need to invoke
Tk_CanvasEventuallyRedraw, since Tk can normally figure out when an item has changed and
make the redisplay request on its behalf (this happens, for example whenever Tk calls a
configureProc or scaleProc). The only time that a type manager needs to call
Tk_CanvasEventuallyRedraw is if an item has changed on its own without being invoked
through one of the procedures in its Tk_ItemType; this could happen, for example, in an
image item if the image is modified using image commands.
Tk_CanvasTagsParseProc and Tk_CanvasTagsPrintProc are procedures that handle the -tags
option for canvas items. The code of a canvas type manager will not call these procedures
directly, but will use their addresses to create a Tk_CustomOption structure for the -tags
option. The code typically looks like this:
static const Tk_CustomOption tagsOption = {Tk_CanvasTagsParseProc,
Tk_CanvasTagsPrintProc, NULL
};
static const Tk_ConfigSpec configSpecs[] = {
...
{TK_CONFIG_CUSTOM, "-tags", NULL, NULL,
NULL, 0, TK_CONFIG_NULL_OK, &tagsOption},
...
};
KEYWORDS
canvas, focus, item type, redisplay, selection, type manager