View file File name : Error.pod Content :# Copyright (c) 1990-1994 The Regents of the University of California. # Copyright (c) 1994-1996 Sun Microsystems, Inc. # See the file "license.terms" for information on usage and redistribution # of this file, and for a DISCLAIMER OF ALL WARRANTIES. # # =head1 NAME Tk::Error - Method invoked to process background errors =for category Binding Events and Callbacks =head1 SYNOPSIS Customization: require Tk::ErrorDialog; or sub Tk::Error { my ($widget,$error,@locations) = @_; ... } =head1 DESCRIPTION The B<Tk::Error> method is invoked by perl/Tk when a background error occurs. Two possible implementations are provided in the distribution and individual applications or users can (re)define a B<Tk::Error> method (e.g. as a perl sub) if they wish to handle background errors in some other manner. A background error is one that occurs in a command that didn't originate with the application. For example, if an error occurs while executing a L<callback|Tk::callbacks> specified with a L<bind|Tk::bind> or a L<after|Tk::after> command, then it is a background error. For a non-background error, the error can simply be returned up through nested subroutines until it reaches the top-level code in the application; then the application can report the error in whatever way it wishes. When a background error occurs, the unwinding ends in the Tk library and there is no obvious way for Tk to report the error. When Tk detects a background error, it saves information about the error and invokes the B<Tk::Error> method later when Tk is idle. B<Tk::Error> is invoked by perl/Tk as if by the perl code: S< >I<$mainwindow>-E<gt>B<Tk::Error>(I<"error message">, I<location ...>); I<$mainwindow> is the B<MainWindow> associated with widget which detected the error, I<"error message"> is a string describing the error that has been detected, I<location> is a list of one or more "locations" which describe the call sequence at the point the error was detected. The locations are a typically a mixture of perl location reports giving script name and line number, and simple strings describing locations in core Tk or perl/Tk C code. Tk will ignore any result returned by the B<Tk::Error> method. If another error occurs within the B<Tk::Error> method (for example if it calls B<die>) then Tk reports this error itself by writing a message to stderr (this is to avoid infinite loops due to any bugs in B<Tk::Error>). If several background errors accumulate before B<Tk::Error> is invoked to process them, B<Tk::Error> will be invoked once for each error, in the order they occurred. However, if B<Tk::Error> calls B<Tk-E<gt>break>, then any remaining errors are skipped without calling B<Tk::Error>. The B<Tk> module includes a default B<Tk::Error> subroutine that simply reports the error on stderr. =head1 Tk::ErrorDialog An alternate definition is provided via: S< >C<require Tk::ErrorDialog;> that posts a dialog box containing the error message and offers the user a chance to see a stack trace showing where the error occurred. This is an OO implementation of the Tcl/Tk command B<bgerror>, with a twist: since there is only one B<ErrorDialog> widget, you aren't required to invoke the constructor to create it; it will be created automatically when the first background error occurs. However, in order to configure the I<-cleanupcode> and I<-appendtraceback> B<ErrorDialog> options you must call the constructor and create it manually. The B<ErrorDialog> object essentially consists of two subwidgets: a B<Dialog> widget to display the background error and a B<Text> widget for the traceback information. If required, you can invoke various widget methods to customize these subwidgets - their advertised names are described below. S< >I<$mw>-E<gt>B<ErrorDialog>(-cleanupcode => I<code>, -appendtraceback => I<bool>); $mw is a window reference. I<code> is a CODE reference if special post-background error processing is required (default is undefined). The callback subroutine is called with @_ having the same arguments that B<Tk::Error> was invoked with. I<bool> is a boolean indicating whether or not to append successive tracebacks (default is 1, do append). =head2 Advertised ErrorDialog widgets I<error_dialog> is the Dialog widget reference. I<text> is the Text widget reference containing the traceback information. =head1 BUGS If B<after> or B<fileevent> are not invoked as methods of a widget then perl/Tk is unable to provide a I<$mainwindow> argument. To support such code from earlier versions of perl/Tk perl/Tk therefore calls B<Tk::Error> with string 'Tk' instead: B<Tk-E<gt>Tk::Error\(...\)>. In this case the B<Tk::Error> in B<Tk::ErrorDialog> and similar implementations cannot "popup" a window as they don't know which display to use. A mechanism to supply I<the> B<MainWindow> in applications which only have one (a very common case) should be provided. =head1 SEE ALSO L<Tk::bind|Tk::bind> L<Tk::after|Tk::after> L<Tk::fileevent|Tk::fileevent> =head1 KEYWORDS background error, reporting =cut