Tk_AllocCursorFromObj C API

Maintain database of cursors

Also documents Tk_GetCursor, Tk_GetCursorFromObj, Tk_GetCursorFromData, Tk_NameOfCursor, Tk_FreeCursorFromObj, Tk_FreeCursor

literal type exactly as shown argument replace with your value ~~?optional?~~ may be omitted

Synopsis#

#include <tk.h>

Tk_Cursor Tk_AllocCursorFromObj(interp, tkwin, objPtr)#

Tk_Cursor Tk_GetCursor(interp, tkwin, name)#

Tk_Cursor Tk_GetCursorFromObj(tkwin, objPtr)#

Tk_Cursor Tk_GetCursorFromData(interp, tkwin, source, mask, width, height, xHot, yHot, fg, bg)#

const char * Tk_NameOfCursor(display, cursor)#

Tk_FreeCursorFromObj(tkwin, objPtr)#

Tk_FreeCursor(display, cursor)#

Arguments#

Tcl_Interp *interp (in): Interpreter to use for error reporting.
Tk_Window tkwin (in): Token for window in which the cursor will be used.
Tcl_Obj *objPtr (in/out): Description of cursor; see below for possible values. Internal rep will be modified to cache pointer to corresponding Tk_Cursor.
const char *name (in): Same as objPtr except description of cursor is passed as a string and resulting Tk_Cursor is not cached.
const char *source (in): Data for cursor cursor, in standard cursor format.
const char *mask (in): Data for mask cursor, in standard cursor format.
int width (in): Width of source and mask.
int height (in): Height of source and mask.
int xHot (in): X-location of cursor hot-spot.
int yHot (in): Y-location of cursor hot-spot.
const char *fg (in): Textual description of foreground color for cursor.
const char *bg (in): Textual description of background color for cursor.
Display *display (in): Display for which cursor was allocated.
Tk_Cursor cursor (in): Opaque Tk identifier for cursor. If passed to Tk_FreeCursor, must have been returned by some previous call to Tk_GetCursor or Tk_GetCursorFromData.

Description#

These procedures manage a collection of cursors being used by an application. The procedures allow cursors to be re-used efficiently, thereby avoiding server overhead, and also allow cursors to be named with character strings.

Tk_AllocCursorFromObj takes as argument an object describing a cursor, and returns an opaque Tk identifier for a cursor corresponding to the description. It re-uses an existing cursor if possible and creates a new one otherwise. Tk_AllocCursorFromObj caches information about the return value in objPtr, which speeds up future calls to procedures such as Tk_AllocCursorFromObj and Tk_GetCursorFromObj. If an error occurs in creating the cursor, such as when objPtr refers to a non-existent file, then None is returned and an error message will be stored in interp's result if interp is not NULL. ObjPtr must contain a standard Tcl list with one of the following forms:

name [fgColor [bgColor]]

Name is the name of a cursor in the standard X cursor cursor, i.e., any of the names defined in cursorcursor.h, without the XC_. Some example values are X_cursor, hand2, or left_ptr. Appendix B of “The X Window System” by Scheifler & Gettys has illustrations showing what each of these cursors looks like. If fgColor and bgColor are both specified, they give the foreground and background colors to use for the cursor (any of the forms acceptable to Tk_GetColor may be used). If only fgColor is specified, then there will be no background color: the background will be transparent. If no colors are specified, then the cursor will use black for its foreground color and white for its background color.

The Macintosh version of Tk supports all of the X cursors and will also accept any of the standard Mac cursors including ibeam, crosshair, watch, plus, and arrow. In addition, Tk will load Macintosh cursor resources of the types crsr (color) and CURS (black and white) by the name of the resource. The application and all its open dynamic library's resource files will be searched for the named cursor. If there are conflicts color cursors will always be loaded in preference to black and white cursors.

@sourceName maskName fgColor bgColor

In this form, sourceName and maskName are the names of files describing cursors for the cursor's source bits and mask. Each file must be in standard X11 cursor format. FgColor and bgColor indicate the colors to use for the cursor, in any of the forms acceptable to Tk_GetColor. This form of the command will not work on Macintosh or Windows computers.

@sourceName fgColor

This form is similar to the one above, except that the source is used as mask also. This means that the cursor's background is transparent. This form of the command will not work on Macintosh or Windows computers.

@sourceName

This form only works on Windows, and will load a Windows system cursor (.ani or .cur) from the file specified in sourceName.

Tk_GetCursor is identical to Tk_AllocCursorFromObj except that the description of the cursor is specified with a string instead of an object. This prevents Tk_GetCursor from caching the return value, so Tk_GetCursor is less efficient than Tk_AllocCursorFromObj.

Tk_GetCursorFromObj returns the token for an existing cursor, given the window and description used to create the cursor. Tk_GetCursorFromObj does not actually create the cursor; the cursor must already have been created with a previous call to Tk_AllocCursorFromObj or Tk_GetCursor. The return value is cached in objPtr, which speeds up future calls to Tk_GetCursorFromObj with the same objPtr and tkwin.

Tk_GetCursorFromData allows cursors to be created from in-memory descriptions of their source and mask cursors. Source points to standard cursor data for the cursor's source bits, and mask points to standard cursor data describing which pixels of source are to be drawn and which are to be considered transparent. Width and height give the dimensions of the cursor, xHot and yHot indicate the location of the cursor's hot-spot (the point that is reported when an event occurs), and fg and bg describe the cursor's foreground and background colors textually (any of the forms suitable for Tk_GetColor may be used). Typically, the arguments to Tk_GetCursorFromData are created by including a cursor file directly into the source code for a program, as in the following example:

Tk_Cursor cursor;
#include "source.cursor"
#include "mask.cursor"
cursor = Tk_GetCursorFromData(interp, tkwin, source_bits,
    mask_bits, source_width, source_height, source_x_hot,
    source_y_hot, "red", "blue");

Under normal conditions Tk_GetCursorFromData will return an identifier for the requested cursor. If an error occurs in creating the cursor then None is returned and an error message will be stored in interp's result.

Tk_AllocCursorFromObj, Tk_GetCursor, and Tk_GetCursorFromData maintain a database of all the cursors they have created. Whenever possible, a call to Tk_AllocCursorFromObj, Tk_GetCursor, or Tk_GetCursorFromData will return an existing cursor rather than creating a new one. This approach can substantially reduce server overhead, so the Tk procedures should generally be used in preference to Xlib procedures like XCreateFontCursor or XCreatePixmapCursor, which create a new cursor on each call. The Tk procedures are also more portable than the lower-level X procedures.

The procedure Tk_NameOfCursor is roughly the inverse of Tk_GetCursor. If its cursor argument was created by Tk_GetCursor, then the return value is the name argument that was passed to Tk_GetCursor to create the cursor. If cursor was created by a call to Tk_GetCursorFromData, or by any other mechanism, then the return value is a hexadecimal string giving the X identifier for the cursor. Note that the string returned by Tk_NameOfCursor is only guaranteed to persist until the next call to Tk_NameOfCursor. Also, this call is not portable except for cursors returned by Tk_GetCursor.

When a cursor returned by Tk_AllocCursorFromObj, Tk_GetCursor, or Tk_GetCursorFromData is no longer needed, Tk_FreeCursorFromObj or Tk_FreeCursor should be called to release it. For Tk_FreeCursorFromObj the cursor to release is specified with the same information used to create it; for Tk_FreeCursor the cursor to release is specified with its Tk_Cursor token. There should be exactly one call to Tk_FreeCursor for each call to Tk_AllocCursorFromObj, Tk_GetCursor, or Tk_GetCursorFromData.

Bugs#

In determining whether an existing cursor can be used to satisfy a new request, Tk_AllocCursorFromObj, Tk_GetCursor, and Tk_GetCursorFromData consider only the immediate values of their arguments. For example, when a file name is passed to Tk_GetCursor, Tk_GetCursor will assume it is safe to re-use an existing cursor created from the same file name: it will not check to see whether the file itself has changed, or whether the current directory has changed, thereby causing the name to refer to a different file. Similarly, Tk_GetCursorFromData assumes that if the same source pointer is used in two different calls, then the pointers refer to the same data; it does not check to see if the actual data values have changed.

Keywords#