selection handle ?-selection selection? ?-type type? ?-format format? window command
Creates a handler for selection requests, such that command will be executed whenever selection is owned by window and someone attempts to retrieve it in the form given by type (e.g. type is specified in the selection get command). Selection defaults to PRIMARY, type defaults to STRING, and format defaults to STRING. If command is an empty string then any existing handler for window, type, and selection is removed.
When selection is requested, window is the selection owner, and type is the requested type, command will be executed as a Tcl command with two additional numbers appended to it (with space separators). The two additional numbers are offset and maxChars: offset specifies a starting character position in the selection and maxChars gives the maximum number of characters to retrieve. The command should return a value consisting of at most maxChars of the selection, starting at position offset. For very large selections (larger than maxChars) the selection will be retrieved using several invocations of command with increasing offset values. If command returns a string whose length is less than maxChars, the return value is assumed to include all of the remainder of the selection; if the length of command 's result is equal to maxChars then command will be invoked again, until it eventually returns a result shorter than maxChars. The value of maxChars will always be relatively large (thousands of characters).
If command returns an error then the selection retrieval is rejected just as if the selection didn't exist at all.
The format argument specifies the representation that should be used to transmit the selection to the requester (the second column of Table 2 of the ICCCM), and defaults to STRING. If format is STRING, the selection is transmitted as 8-bit ASCII characters (i.e. just in the form returned by command). If format is ATOM, then the return value from command is divided into fields separated by white space; each field is converted to its atom value, and the 32-bit atom value is transmitted instead of the atom name. For any other format, the return value from command is divided into fields separated by white space and each field is converted to a 32-bit integer; an array of integers is transmitted to the selection requester.
The format argument is needed only for compatibility with selection requesters that don't use Tk. If Tk is being used to retrieve the selection then the value is converted back to a string at the requesting end, so format is irrelevant.