Tk_GetCursor, Tk_GetCursorFromData, Tk_NameOfCursor, Tk_FreeCursor - maintain database of cursors
Tk_GetCursor(interp, tkwin, nameId)
Tk_GetCursorFromData(interp, tkwin, source, mask, width, height, xHot, yHot, fg, bg)
These procedures manage a collection of cursors being used by an application. The procedures allow cursors to be reused efficiently, thereby avoiding server overhead, and also allow cursors to be named with character strings (actually Tk_Uids).
Tk_GetCursor takes as argument a Tk_Uid 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. NameId must be 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 font, i.e., any of the names defined in cursorfont.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 also supports all of the X cursors. Tk on the Mac 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 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 bitmaps for the cursor's source bits and mask. Each file must be in standard X11 or X10 bitmap 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.
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.
Tk_GetCursorFromData allows cursors to be created from inmemory descriptions of their source and mask bitmaps. Source points to standard bitmap data for the cursor's source bits, and mask points to standard bitmap 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:
cursor = Tk_GetCursorFromData(interp, tkwin, source_bits, mask_bits, source_width, source_height, source_x_hot, source_y_hot, Tk_GetUid("red"), Tk_GetUid("blue"));
Under normal conditions, Tk_GetCursor and Tk_GetCursorFromData will return an identifier for the requested cursor. If an error occurs in creating the cursor, such as when nameId refers to a non-existent file, then None is returned and an error message will be stored in interp->result.
Tk_GetCursor and Tk_GetCursorFromData maintain a database of all the cursors they have created. Whenever possible, a call to 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 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 nameId 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: 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_GetCursor or Tk_GetCursorFromData is no longer needed, Tk_FreeCursor should be called to release it. There should be exactly one call to Tk_FreeCursor for each call to Tk_GetCursor or Tk_GetCursorFromData. When a cursor is no longer in use anywhere (i.e. it has been freed as many times as it has been gotten) Tk_FreeCursor will release it to the X server and remove it from the database.
In determining whether an existing cursor can be used to satisfy a new request, 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.
Table of Contents