This command provides an interface to the X selection mechanism and implements the full selection functionality described in the X Inter-Client Communication Conventions Manual (ICCCM).
The widget object used to invoke the methods below determines which display is used to access the selection. In order to avoid conflicts with selection methods of widget classes (e.g. Text) this set of methods uses the prefix Selection. The following methods are currently supported:
Type specifies the form in which the selection is to be returned (the desired ``target'' for conversion, in ICCCM terminology), and should be an atom name such as STRING or FILE_NAME; see the Inter-Client Communication Conventions Manual for complete details. Type defaults to STRING. The selection owner may choose to return the selection in any of several different representation formats, such as STRING, ATOM, INTEGER, etc. (this format is different than the selection type; see the ICCCM for all the confusing details).
If format is not STRING then things get messy, the following description is from the Tcl/Tk man page as yet incompetely translated for the perl version - it is misleading at best.
If the selection is returned in a non-string format, such as INTEGER or ATOM, the SelectionGet converts it to a list of perl values: atoms are converted to their textual names, and anything else is converted integers.
A goal of the perl port is to provide better handling of different formats than Tcl/Tk does, which should be possible given perl's wider range of ``types''. Although some thought went into this in very early days of perl/Tk what exactly happens is still "not quite right" and subject to change.
When selection is requested, $widget is the selection owner, and type is the requested type, callback will be executed with two additional arguments. The two additional arguments are offset and maxBytes: offset specifies a starting character position in the selection and maxBytes gives the maximum number of bytes to retrieve. The command should return a value consisting of at most maxBytes of the selection, starting at position offset. For very large selections (larger than maxBytes) the selection will be retrieved using several invocations of callback with increasing offset values. If callback returns a string whose length is less than maxBytes, the return value is assumed to include all of the remainder of the selection; if the length of callback's result is equal to maxBytes then callback will be invoked again, until it eventually returns a result shorter than maxBytes. The value of maxBytes will always be relatively large (thousands of bytes).
If callback returns an error (e.g. via die) 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 not STRING then things get messy, the following description is from the Tcl/Tk man page as yet untranslated for the perl version - it is misleading at best.
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 many selection requesters, except Tcl/Tk. If Tcl/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.
A goal of the perl port is to provide better handling of different formats than Tcl/Tk does, which should be possible given perl's wider range of ``types''. Although some thought went into this in very early days of perl/Tk what exactly happens is still "not quite right" and subject to change.