Tk_CreateItemType(3) Tk (4.0) Tk_CreateItemType(3)
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NAME
Tk_CreateItemType, Tk_GetItemTypes - define new kind of
canvas item
SYNOPSIS
#include <tk.h>
Tk_CreateItemType(typePtr)
Tk_ItemType *
Tk_GetItemTypes()
ARGUMENTS
Tk_ItemType *typePtr (in) Structure that defines
the new type of canvas
item.
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INTRODUCTION
Tk_CreateItemType is invoked to define a new kind of canvas
item described by the typePtr argument. An item type
corresponds to a particular value of the type argument to
the create widget command for canvases, and the code that
implements a canvas item type is called a type manager. Tk
defines several built-in item types, such as rectangle and
text and image, but Tk_CreateItemType allows additional item
types to be defined. Once Tk_CreateItemType returns, the
new item type may be used in new or existing canvas widgets
just like the built-in item types.
Tk_GetItemTypes returns a pointer to the first in the list
of all item types currently defined for canvases. The
entries in the list are linked together through their
nextPtr fields, with the end of the list marked by a NULL
nextPtr.
You may find it easier to understand the rest of this manual
entry by looking at the code for an existing canvas item
type such as bitmap (file tkCanvBmap.c) or text
(tkCanvText.c). The easiest way to create a new type
manager is to copy the code for an existing type and modify
it for the new type.
Tk provides a number of utility procedures for the use of
canvas type managers, such as Tk_CanvasCoords and
Tk_CanvasPsColor; these are described in separate manual
entries.
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Tk_CreateItemType(3) Tk (4.0) Tk_CreateItemType(3)
DATA STRUCTURES
A type manager consists of a collection of procedures that
provide a standard set of operations on items of that type.
The type manager deals with three kinds of data structures.
The first data structure is a Tk_ItemType; it contains
information such as the name of the type and pointers to the
standard procedures implemented by the type manager:
typedef struct Tk_ItemType {
char *name;
int itemSize;
Tk_ItemCreateProc *createProc;
Tk_ConfigSpec *configSpecs;
Tk_ItemConfigureProc *configProc;
Tk_ItemCoordProc *coordProc;
Tk_ItemDeleteProc *deleteProc;
Tk_ItemDisplayProc *displayProc;
int alwaysRedraw;
Tk_ItemPointProc *pointProc;
Tk_ItemAreaProc *areaProc;
Tk_ItemPostscriptProc *postscriptProc;
Tk_ItemScaleProc *scaleProc;
Tk_ItemTranslateProc *translateProc;
Tk_ItemIndexProc *indexProc;
Tk_ItemCursorProc *icursorProc;
Tk_ItemSelectionProc *selectionProc;
Tk_ItemInsertProc *insertProc;
Tk_ItemDCharsProc *dCharsProc;
Tk_ItemType *nextPtr;
} Tk_ItemType;
The fields of a Tk_ItemType structure are described in more
detail later in this manual entry. When Tk_CreateItemType
is called, its typePtr argument must point to a structure
with all of the fields initialized except nextPtr, which Tk
sets to link all the types together into a list. The
structure must be in permanent memory (either statically
allocated or dynamically allocated but never freed); Tk
retains a pointer to this structure.
The second data structure manipulated by a type manager is
an item record. For each item in a canvas there exists one
item record. All of the items of a given type generally
have item records with the same structure, but different
types usually have different formats for their item records.
The first part of each item record is a header with a
standard structure defined by Tk via the type Tk_Item; the
rest of the item record is defined by the type manager. A
type manager must define its item records with a Tk_Item as
the first field. For example, the item record for bitmap
items is defined as follows:
typedef struct BitmapItem {
Tk_Item header;
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Tk_CreateItemType(3) Tk (4.0) Tk_CreateItemType(3)
double x, y;
Tk_Anchor anchor;
Pixmap bitmap;
XColor *fgColor;
XColor *bgColor;
GC gc;
} BitmapItem;
The header substructure contains information used by Tk to
manage the item, such as its identifier, its tags, its type,
and its bounding box. The fields starting with x belong to
the type manager: Tk will never read or write them. The
type manager should not need to read or write any of the
fields in the header except for four fields whose names are
x1, y1, x2, and y2. These fields give a bounding box for
the items using integer canvas coordinates: the item should
not cover any pixels with x-coordinate lower than x1 or y-
coordinate lower than y1, nor should it cover any pixels
with x-coordinate greater than or equal to x2 or y-
coordinate greater than or equal to y2. It is up to the
type manager to keep the bounding box up to date as the item
is moved and reconfigured.
Whenever Tk calls a procedure in a type manager it passes in
a pointer to an item record. The argument is always passed
as a pointer to a Tk_Item; the type manager will typically
cast this into a pointer to its own specific type, such as
BitmapItem.
The third data structure used by type managers has type
Tk_Canvas; it serves as an opaque handle for the canvas
widget as a whole. Type managers need not know anything
about the contents of this structure. A Tk_Canvas handle is
typically passed in to the procedures of a type manager, and
the type manager can pass the handle back to library
procedures such as Tk_CanvasTkwin to fetch information about
the canvas.
NAME
This section and the ones that follow describe each of the
fields in a Tk_ItemType structure in detail. The name field
provides a string name for the item type. Once
Tk_CreateImageType returns, this name may be used in create
widget commands to create items of the new type. If there
already existed an item type by this name then the new item
type replaces the old one.
ITEMSIZE
typePtr->itemSize gives the size in bytes of item records of
this type, including the Tk_Item header. Tk uses this size
to allocate memory space for items of the type. All of the
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Tk_CreateItemType(3) Tk (4.0) Tk_CreateItemType(3)
item records for a given type must have the same size. If
variable length fields are needed for an item (such as a
list of points for a polygon), the type manager can allocate
a separate object of variable length and keep a pointer to
it in the item record.
CREATEPROC
typePtr->createProc points to a procedure for Tk to call
whenever a new item of this type is created. typePtr-
>createProc must match the following prototype:
typedef int Tk_ItemCreateProc(
Tcl_Interp *interp,
Tk_Canvas canvas,
Tk_Item *itemPtr,
int argc,
char **argv);
The interp argument is the interpreter in which the canvas's
create widget command was invoked, and canvas is a handle
for the canvas widget. itemPtr is a pointer to a newly-
allocated item of size typePtr->itemSize. Tk has already
initialized the item's header (the first sizeof(Tk_ItemType)
bytes). The argc and argv arguments describe all of the
arguments to the create command after the type argument.
For example, in the widget command
.c create rectangle 10 20 50 50 -fill black
argc will be 6 and argv[0] will contain the string 10.
createProc should use argc and argv to initialize the type-
specific parts of the item record and set an initial value
for the bounding box in the item's header. It should return
a standard Tcl completion code and leave an error message in
interp->result if an error occurs. If an error occurs Tk
will free the item record, so createProc must be sure to
leave the item record in a clean state if it returns an
error (e.g., it must free any additional memory that it
allocated for the item).
CONFIGSPECS
Each type manager must provide a standard table describing
its configuration options, in a form suitable for use with
Tk_ConfigureWidget. This table will normally be used by
typePtr->createProc and typePtr->configProc, but Tk also
uses it directly to retrieve option information in the
itemcget and itemconfigure widget commands. typePtr-
>configSpecs must point to the configuration table for this
type. Note: Tk provides a custom option type
tk_CanvasTagsOption for implementing the -tags option; see
an existing type manager for an example of how to use it in
configSpecs.
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Tk_CreateItemType(3) Tk (4.0) Tk_CreateItemType(3)
CONFIGPROC
typePtr->configProc is called by Tk whenever the
itemconfigure widget command is invoked to change the
configuration options for a canvas item. This procedure
must match the following prototype:
typedef int Tk_ItemConfigureProc(
Tcl_Interp *interp,
Tk_Canvas canvas,
Tk_Item *itemPtr,
int argc,
char **argv,
int flags);
The interp argument identifies the interpreter in which the
widget command was invoked, canvas is a handle for the
canvas widget, and itemPtr is a pointer to the item being
configured. argc and argv contain the configuration
options. For example, if the following command is invoked:
.c itemconfigure 2 -fill red -outline black
argc is 4 and argv contains the strings -fill through black.
argc will always be an even value. The flags argument
contains flags to pass to Tk_ConfigureWidget; currently this
value is always TK_CONFIG_ARGV_ONLY when Tk invokes
typePtr->configProc, but the type manager's createProc
procedure will usually invoke configProc with different flag
values.
typePtr->configProc returns a standard Tcl completion code
and leaves an error message in interp->result if an error
occurs. It must update the item's bounding box to reflect
the new configuration options.
COORDPROC
typePtr->coordProc is invoked by Tk to implement the coords
widget command for an item. It must match the following
prototype:
typedef int Tk_ItemCoordProc(
Tcl_Interp *interp,
Tk_Canvas canvas,
Tk_Item *itemPtr,
int argc,
char **argv);
The arguments interp, canvas, and itemPtr all have the
standard meanings, and argc and argv describe the coordinate
arguments. For example, if the following widget command is
invoked:
.c coords 2 30 90
argc will be 2 and argv will contain the string values 30
and 90.
The coordProc procedure should process the new coordinates,
update the item appropriately (e.g., it must reset the
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Tk_CreateItemType(3) Tk (4.0) Tk_CreateItemType(3)
bounding box in the item's header), and return a standard
Tcl completion code. If an error occurs, coordProc must
leave an error message in interp->result.
DELETEPROC
typePtr->deleteProc is invoked by Tk to delete an item and
free any resources allocated to it. It must match the
following prototype:
typedef void Tk_ItemDeleteProc(
Tk_Canvas canvas,
Tk_Item *itemPtr,
Display *display);
The canvas and itemPtr arguments have the usual
interpretations, and display identifies the X display
containing the canvas. deleteProc must free up any
resources allocated for the item, so that Tk can free the
item record. deleteProc should not actually free the item
record; this will be done by Tk when deleteProc returns.
DISPLAYPROC AND ALWAYSREDRAW
typePtr->displayProc is invoked by Tk to redraw an item on
the screen. It must match the following prototype:
typedef void Tk_ItemDisplayProc(
Tk_Canvas canvas,
Tk_Item *itemPtr,
Display *display,
Drawable dst,
int x,
int y,
int width,
int height);
The canvas and itemPtr arguments have the usual meaning.
display identifies the display containing the canvas, and
dst specifies a drawable in which the item should be
rendered; typically this is an off-screen pixmap, which Tk
will copy into the canvas's window once all relevant items
have been drawn. x, y, width, and height specify a
rectangular region in canvas coordinates, which is the area
to be redrawn; only information that overlaps this area
needs to be redrawn. Tk will not call displayProc unless
the item's bounding box overlaps the redraw area, but the
type manager may wish to use the redraw area to optimize the
redisplay of the item.
Because of scrolling and the use of off-screen pixmaps for
double-buffered redisplay, the item's coordinates in dst
will not necessarily be the same as those in the canvas.
displayProc should call Tk_CanvasDrawableCoords to transform
coordinates from those of the canvas to those of dst.
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Tk_CreateItemType(3) Tk (4.0) Tk_CreateItemType(3)
Normally an item's displayProc is only invoked if the item
overlaps the area being displayed. However, if typePtr-
>alwaysRedraw has a non-zero value, then displayProc is
invoked during every redisplay operation, even if the item
doesn't overlap the area of redisplay. alwaysRedraw should
normally be set to 0; it is only set to 1 in special cases
such as window items that need to be unmapped when they are
off-screen.
POINTPROC
typePtr->pointProc is invoked by Tk to find out how close a
given point is to a canvas item. Tk uses this procedure for
purposes such as locating the item under the mouse or
finding the closest item to a given point. The procedure
must match the following prototype:
typedef double Tk_ItemPointProc(
Tk_Canvas canvas,
Tk_Item *itemPtr,
double *pointPtr);
canvas and itemPtr have the usual meaning. pointPtr points
to an array of two numbers giving the x and y coordinates of
a point. pointProc must return a real value giving the
distance from the point to the item, or 0 if the point lies
inside the item.
AREAPROC
typePtr->areaProc is invoked by Tk to find out the
relationship between an item and a rectangular area. It
must match the following prototype:
typedef int Tk_ItemAreaProc(
Tk_Canvas canvas,
Tk_Item *itemPtr,
double *rectPtr);
canvas and itemPtr have the usual meaning. rectPtr points
to an array of four real numbers; the first two give the x
and y coordinates of the upper left corner of a rectangle,
and the second two give the x and y coordinates of the lower
right corner. areaProc must return -1 if the item lies
entirely outside the given area, 0 if it lies partially
inside and partially outside the area, and 1 if it lies
entirely inside the area.
POSTSCRIPTPROC
typePtr->postscriptProc is invoked by Tk to generate
Postcript for an item during the postscript widget command.
If the type manager is not capable of generating Postscript
then typePtr->postscriptProc should be NULL. The procedure
must match the following prototype:
typedef int Tk_ItemPostscriptProc(
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Tk_CreateItemType(3) Tk (4.0) Tk_CreateItemType(3)
Tcl_Interp *interp,
Tk_Canvas canvas,
Tk_Item *itemPtr,
int prepass);
The interp, canvas, and itemPtr arguments all have standard
meanings; prepass will be described below. If
postscriptProc completes successfully, it should append
Postscript for the item to the information in interp->result
(e.g. by calling Tcl_AppendResult, not Tcl_SetResult) and
return TCL_OK. If an error occurs, postscriptProc should
clear the result and replace its contents with an error
message; then it should return TCL_ERROR.
Tk provides a collection of utility procedures to simplify
postscriptProc. For example, Tk_CanvasPsColor will generate
Postscript to set the current color to a given Tk color and
Tk_CanvasPsFont will set up font information. When
generating Postscript, the type manager is free to change
the graphics state of the Postscript interpreter, since Tk
places gsave and grestore commands around the Postscript for
the item. The type manager can use canvas x coordinates
directly in its Postscript, but it must call Tk_CanvasPsY to
convert y coordinates from the space of the canvas (where
the origin is at the upper left) to the space of Postscript
(where the origin is at the lower left).
In order to generate Postscript that complies with the Adobe
Document Structuring Conventions, Tk actually generates
Postscript in two passes. It calls each item's
postscriptProc in each pass. The only purpose of the first
pass is to collect font information (which is done by
Tk_CanvPsFont); the actual Postscript is discarded. Tk
sets the prepass argument to postscriptProc to 1 during the
first pass; the type manager can use prepass to skip all
Postscript generation except for calls to Tk_CanvasPsFont.
During the second pass prepass will be 0, so the type
manager must generate complete Postscript.
SCALEPROC
typePtr->scaleProc is invoked by Tk to rescale a canvas item
during the scale widget command. The procedure must match
the following prototype:
typedef void Tk_ItemScaleProc(
Tk_Canvas canvas,
Tk_Item *itemPtr,
double originX,
double originY,
double scaleX,
double scaleY);
The canvas and itemPtr arguments have the usual meaning.
originX and originY specify an origin relative to which the
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item is to be scaled, and scaleX and scaleY give the x and y
scale factors. The item should adjust its coordinates so
that a point in the item that used to have coordinates x and
y will have new coordinates x' and y', where
x' = originX + scaleX*(x-originX)
y' = originY + scaleY*(y-originY)
scaleProc must also update the bounding box in the item's
header.
TRANSLATEPROC
typePtr->translateProc is invoked by Tk to translate a
canvas item during the move widget command. The procedure
must match the following prototype:
typedef void Tk_ItemTranslateProc(
Tk_Canvas canvas,
Tk_Item *itemPtr,
double deltaX,
double deltaY);
The canvas and itemPtr arguments have the usual meaning, and
deltaX and deltaY give the amounts that should be added to
each x and y coordinate within the item. The type manager
should adjust the item's coordinates and update the bounding
box in the item's header.
INDEXPROC
typePtr->indexProc is invoked by Tk to translate a string
index specification into a numerical index, for example
during the index widget command. It is only relevant for
item types that support indexable text; typePtr->indexProc
may be specified as NULL for non-textual item types. The
procedure must match the following prototype:
typedef int Tk_ItemIndexProc(
Tcl_Interp *interp,
Tk_Canvas canvas,
Tk_Item *itemPtr,
char indexString,
int *indexPtr);
The interp, canvas, and itemPtr arguments all have the usual
meaning. indexString contains a textual description of an
index, and indexPtr points to an integer value that should
be filled in with a numerical index. It is up to the type
manager to decide what forms of index are supported (e.g.,
numbers, insert, sel.first, end, etc.). indexProc should
return a Tcl completion code and set interp->result in the
event of an error.
ICURSORPROC
typePtr->icursorProc is invoked by Tk during the icursor
widget command to set the position of the insertion cursor
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Tk_CreateItemType(3) Tk (4.0) Tk_CreateItemType(3)
in a textual item. It is only relevant for item types that
support an insertion cursor; typePtr->icursorProc may be
specified as NULL for item types that don't support an
insertion cursor. The procedure must match the following
prototype:
typedef void Tk_ItemIndexProc(
Tk_Canvas canvas,
Tk_Item *itemPtr,
int index);
canvas and itemPtr have the usual meanings, and index is an
index into the item's text, as returned by a previous call
to typePtr->insertProc. The type manager should position
the insertion cursor in the item just before the character
given by index. Whether or not to actually display the
insertion cursor is determined by other information provided
by Tk_CanvasGetTextInfo.
SELECTIONPROC
typePtr->selectionProc is invoked by Tk during selection
retrievals; it must return part or all of the selected text
in the item (if any). It is only relevant for item types
that support text; typePtr->selectionProc may be specified
as NULL for non-textual item types. The procedure must
match the following prototype:
typedef int Tk_ItemSelectionProc(
Tk_Canvas canvas,
Tk_Item *itemPtr,
int offset,
char *buffer,
int maxBytes);
canvas and itemPtr have the usual meanings. offset is an
offset in bytes into the selection where 0 refers to the
first byte of the selection; it identifies the first
character that is to be returned in this call. buffer
points to an area of memory in which to store the requested
bytes, and maxBytes specifies the maximum number of bytes to
return. selectionProc should extract up to maxBytes
characters from the selection and copy them to maxBytes; it
should return a count of the number of bytes actually
copied, which may be less than maxBytes if there aren't
offset+maxBytes bytes in the selection.
INSERTPROC
typePtr->insertProc is invoked by Tk during the insert
widget command to insert new text into a canvas item. It is
only relevant for item types that support text; typePtr-
>insertProc may be specified as NULL for non-textual item
types. The procedure must match the following prototype:
typedef void Tk_ItemInsertProc(
Tk_Canvas canvas,
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Tk_CreateItemType(3) Tk (4.0) Tk_CreateItemType(3)
Tk_Item *itemPtr,
int index,
char *string);
canvas and itemPtr have the usual meanings. index is an
index into the item's text, as returned by a previous call
to typePtr->insertProc, and string contains new text to
insert just before the character given by index. The type
manager should insert the text and recompute the bounding
box in the item's header.
DCHARSPROC
typePtr->dCharsProc is invoked by Tk during the dchars
widget command to delete a range of text from a canvas item.
It is only relevant for item types that support text;
typePtr->dCharsProc may be specified as NULL for non-textual
item types. The procedure must match the following
prototype:
typedef void Tk_ItemDCharsProc(
Tk_Canvas canvas,
Tk_Item *itemPtr,
int first,
int last);
canvas and itemPtr have the usual meanings. first and last
give the indices of the first and last bytes to be deleted,
as returned by previous calls to typePtr->indexProc. The
type manager should delete the specified characters and
update the bounding box in the item's header.
SEE ALSO
Tk_CanvasPsY, Tk_CanvasTextInfo, Tk_CanvasTkwin
KEYWORDS
canvas, focus, item type, selection, type manager
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