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On successful completion, the widget constructor functions return the
symbol passed in as the first argument. It will now have a functional
binding. It is an error to pass in a symbol which already corresponds
to a widget, without first calling the destroy
command. On failure,
an error is signalled.
The widget functions themselves, do not normally return any value. Indeed the lisp process does not wait for them to return, but merely dispatches the commands, such as to change the text in themselves. Sometimes however you either wish to wait, in order to synchronize, or you wish to see if your command fails or succeeds. You request values by passing the keyword :return and a value indicating the type.
(.hello :configure :text "Bye World" :return 'string) ==> "" ==> T
the empty string is returned as first value, and the second value
T
indicates that the new text value was successfully set. LISP
will not continue until the tkclsrv process indicates back that the
function call has succeeded. While waiting of course LISP will continue
to process other graphics events which arrive, since otherwise a
deadlock would arise: the user for instance might click on a mouse, just after
we had decided to wait for a return value from the .hello
function.
More generally a user program may be running in GCL and be interrupted
to receive and act on communications from the gcltksrv
process. If an error occurred then the second return value of the
lisp function will be NIL. In this case the first value, the string
is usually an informative message about the type of error.
A special variable tk::*break-on-errors*
which if not
nil
, requests that that LISP signal an error when a message
is received indicating a function failed. Whenever a command fails,
whether a return value was requested or not, gcltksrv returns a
message indicating failure. The default is to not go into the
debugger. When debugging your windows it may be convenient however to
set this variable to T
to track down incorrect messages.
The gcltksrv process always returns strings as values.
If :return
type is specified, then conversion to type
is accomplished by calling
(coerce-result return-string type)
Here type must be a symbol with a coercion-functions
property.
The builtin return types which may be requested are:
T
in which case the string passed back from the gcltksrv process, will be read by the lisp reader.
number
the string is converted to a number using the current *read-base*
list-strings
(coerce-result "a b {c d} e" 'list-strings) ==> ("a" "b" "c d" "e")
boolean
(coerce-result "1" ’boolean) ==> T (coerce-result "0" ’boolean) ==> NIL
The above symbols are in the TK
or LISP
package.
It would be possible to add new types just as the :return t
is done:
(setf (get 't 'coercion-functions) (cons #'(lambda (x) (our-read-from-string x 0)) #'(lambda (x) (format nil "~s" x))))
The coercion-functions
property of a symbol, is a cons whose
car
is the coercion form from a string to some possibly different
lisp object, and whose cdr
is a function which builds a string
to send to the graphics server. Often the two functions are inverse
functions one of the other up to equal.
The control funcions (see Control) do not return a value
or wait unless requested to do so, using the :return
keyword.
The types and method of specification are the same as for the
Widget Functions in the previous section.
(winfo :width '.hello :return 'number) ==> 120
indicates that the .hello
button is actually 120 pixels
wide.
Next: Argument Lists, Previous: Common Features of Widgets, Up: General [Contents]