Tk_CreateItemType(3tk) Tk Library Procedures Tk_CreateItemType(3tk)
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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 de-
scribed by the typePtr argument. An item type corresponds to a partic-
ular value of the type argument to the create widget command for can-
vases, 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 (in
the file tkCanvBmap.c) or text (tkCanvText.c). The easiest way to cre-
ate 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 de-
scribed in separate manual entries.
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 {
const char *name;
int itemSize;
Tk_ItemCreateProc *createProc;
const 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 ini-
tialized except nextPtr, which Tk sets to link all the types together
into a list. The structure must be in permanent memory (either stati-
cally 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;
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 us-
ing 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 man-
agers 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 pro-
cedures such as Tk_CanvasTkwin to fetch information about the canvas.
TK_ITEMTYPE FIELDS
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.
FLAGS (IN ALWAYSREDRAW)
The typePtr->alwaysRedraw field (so named for historic reasons) con-
tains a collection of flag bits that modify how the canvas core inter-
acts with the item. The following bits are defined:
1 Indicates that the item should always be redrawn when any part
of the canvas is redrawn, rather than only when the bounding box
of the item overlaps the area being redrawn. This is used by
window items, for example, which need to unmap subwindows that
are not on the screen.
TK_CONFIG_OBJS
Indicates that operations which would otherwise take a string
(or array of strings) actually take a Tcl_Obj reference (or an
array of such references). The operations to which this applies
are the configProc, the coordProc, the createProc, the indexProc
and the insertProc.
TK_MOVABLE_POINTS
Indicates that the item supports the dCharsProc, indexProc and │
insertProc with the same semantics as Tk's built-in line and │
polygon types, and that hence individual coordinate points can │
be moved. Must not be set if any of the above methods is NULL.
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 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 al-
locate 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 fol-
lowing prototype:
typedef int Tk_ItemCreateProc(
Tcl_Interp *interp,
Tk_Canvas canvas,
Tk_Item *itemPtr,
int objc,
Tcl_Obj *const objv[]);
The interp argument is the interpreter in which the canvas's create
widget command was invoked, and canvas is a handle for the canvas wid-
get. itemPtr is a pointer to a newly-allocated item of size type-
Ptr->itemSize. Tk has already initialized the item's header (the first
sizeof(Tk_ItemType) bytes). The objc and objv arguments describe all
of the arguments to the create command after the type argument. Note
that if TK_CONFIG_OBJS is not set in the typePtr->alwaysRedraw field,
the objv parameter will actually contain a pointer to an array of con-
stant strings. For example, in the widget command:
.c create rectangle 10 20 50 50 -fill black
objc will be 6 and objv[0] will contain the integer object 10.
createProc should use objc and objv 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 the interpreter result if an error oc-
curs. 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 configu-
ration options, in a form suitable for use with Tk_ConfigureWidget.
This table will normally be used by typePtr->createProc and type-
Ptr->configProc, but Tk also uses it directly to retrieve option infor-
mation in the itemcget and itemconfigure widget commands. type-
Ptr->configSpecs must point to the configuration table for this type.
Note: Tk provides a custom option type tk_CanvasTagsOption for imple-
menting the -tags option; see an existing type manager for an example
of how to use it in configSpecs.
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 objc,
Tcl_Obj *const objv[],
int flags);
The interp argument identifies the interpreter in which the widget com-
mand was invoked, canvas is a handle for the canvas widget, and itemPtr
is a pointer to the item being configured. objc and objv contain the
configuration options. Note that if TK_CONFIG_OBJS is not set in the
typePtr->alwaysRedraw field, the objv parameter will actually contain a
pointer to an array of constant strings. For example, if the following
command is invoked:
.c itemconfigure 2 -fill red -outline black
objc is 4 and objv contains the string objects -fill through black.
objc will always be an even value. The flags argument contains flags
to pass to Tk_ConfigureWidget; currently this value is always TK_CON-
FIG_ARGV_ONLY when Tk invokes typePtr->configProc, but the type man-
ager's createProc procedure will usually invoke configProc with differ-
ent flag values.
typePtr->configProc returns a standard Tcl completion code and leaves
an error message in the interpreter result if an error occurs. It must
update the item's bounding box to reflect the new configuration op-
tions.
COORDPROC
typePtr->coordProc is invoked by Tk to implement the coords widget com-
mand for an item. It must match the following prototype:
typedef int Tk_ItemCoordProc(
Tcl_Interp *interp,
Tk_Canvas canvas,
Tk_Item *itemPtr,
int objc,
Tcl_Obj *const objv[]);
The arguments interp, canvas, and itemPtr all have the standard mean-
ings, and objc and objv describe the coordinate arguments. Note that
if TK_CONFIG_OBJS is not set in the typePtr->alwaysRedraw field, the
objv parameter will actually contain a pointer to an array of constant
strings. For example, if the following widget command is invoked:
.c coords 2 30 90
objc will be 2 and objv will contain the integer objects 30 and 90.
The coordProc procedure should process the new coordinates, update the
item appropriately (e.g., it must reset the bounding box in the item's
header), and return a standard Tcl completion code. If an error oc-
curs, coordProc must leave an error message in the interpreter result.
DELETEPROC
typePtr->deleteProc is invoked by Tk to delete an item and free any re-
sources 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
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 iden-
tifies 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 in-
formation 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_Canvas-
DrawableCoords to transform coordinates from those of the canvas to
those of dst.
Normally an item's displayProc is only invoked if the item overlaps the
area being displayed. However, if bit zero of typePtr->alwaysRedraw is
1, (i.e. “typePtr->alwaysRedraw & 1 == 1”) then displayProc is invoked
during every redisplay operation, even if the item does not overlap the
area of redisplay; this is useful for cases such as window items, where
the subwindow needs to be unmapped when it is off the 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 lo-
cating 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 in-
side and partially outside the area, and 1 if it lies entirely inside
the area.
POSTSCRIPTPROC
typePtr->postscriptProc is invoked by Tk to generate Postscript 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(
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 success-
fully, it should append Postscript for the item to the information in
the interpreter result (e.g. by calling Tcl_AppendResult, not Tcl_Se-
tResult) 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 postscript-
Proc. 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 Post-
script, 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_CanvasPsFont); 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 proto-
type:
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 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 dur-
ing 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 speci-
fication into a numerical index, for example during the index widget
command. It is only relevant for item types that support indexable
text or coordinates; typePtr->indexProc may be specified as NULL for
non-textual item types if they do not support detailed coordinate ad-
dressing. The procedure must match the following prototype:
typedef int Tk_ItemIndexProc(
Tcl_Interp *interp,
Tk_Canvas canvas,
Tk_Item *itemPtr,
Tcl_Obj *indexObj,
int *indexPtr);
The interp, canvas, and itemPtr arguments all have the usual meaning.
indexObj contains a textual description of an index, and indexPtr
points to an integer value that should be filled in with a numerical
index. Note that if TK_CONFIG_OBJS is not set in the typePtr->al-
waysRedraw field, the indexObj parameter will actually contain a
pointer to a constant string. 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 the
interpreter 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 in a textual item. It is
only relevant for item types that support an insertion cursor; type-
Ptr->icursorProc may be specified as NULL for item types that do not
support an insertion cursor. The procedure must match the following
prototype:
typedef void Tk_ItemCursorProc(
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 dis-
play 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->selection-
Proc may be specified as NULL for non-textual item types. The proce-
dure 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 selec-
tion; 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 re-
quested 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 are not offset+maxBytes bytes in the selection.
INSERTPROC
typePtr->insertProc is invoked by Tk during the insert widget command
to insert new text or coordinates into a canvas item. It is only rele-
vant for item types that support the insert method; typePtr->insertProc
may be specified as NULL for other item types. The procedure must
match the following prototype:
typedef void Tk_ItemInsertProc(
Tk_Canvas canvas,
Tk_Item *itemPtr,
int index,
Tcl_Obj *obj);
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
obj contains new text to insert just before the character given by in-
dex. Note that if TK_CONFIG_OBJS is not set in the typePtr->alwaysRe-
draw field, the obj parameter will actually contain a pointer to a con-
stant string to be inserted. If the item supports modification of the
coordinates list by this
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 or a range of coordinates
from a pathed item. It is only relevant for item types that support
text; typePtr->dCharsProc may be specified as NULL for non-textual item
types that do not want to support coordinate deletion. 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 pre-
vious 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
Tk 4.0 Tk_CreateItemType(3tk)
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