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Tk_CreateItemType(3)	     Tk Library Procedures	  Tk_CreateItemType(3)

______________________________________________________________________________

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.
_________________________________________________________________

INTRODUCTION
       Tk_CreateItemType is invoked to	define	a  new	kind  of  canvas  item
       described  by the typePtr argument.  An item type corresponds to a par‐
       ticular 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  rectan‐
       gle  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.

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 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
       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 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.

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 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  fol‐
       lowing 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  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 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 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 argc,
		char **argv,
		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.  argc and argv con‐
       tain 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 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 argc,
		char **argv);
       The arguments interp, canvas, and itemPtr all have the  standard	 mean‐
       ings,  and  argc and argv describe the coordinate arguments.  For exam‐
       ple, 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 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 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
       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_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 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(
		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
       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 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; 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 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	 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 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->selec‐
       tionProc may be specified as NULL for non-textual item types.  The pro‐
       cedure 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
       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,
		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  follow‐
       ing 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(3)
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