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.\" ========================================================================
.\"
.IX Title "ERROR 1"
.TH ERROR 1 "2000-12-30" "perl v5.8.0" "User Contributed Perl Documentation"
.SH "NAME"
Tk::Error \- Method invoked to process background errors
.SH "SYNOPSIS"
.IX Header "SYNOPSIS"
Customization:
.PP
.Vb 1
\&    require Tk::ErrorDialog;
.Ve
.PP
or
.PP
.Vb 4
\&    sub Tk::Error
\&    {
\&      my ($widget,$error,@locations) = @_;
\&      ...
.Ve
.PP
.Vb 1
\&    }
.Ve
.SH "DESCRIPTION"
.IX Header "DESCRIPTION"
The \fBTk::Error\fR 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 \fBTk::Error\fR
method (e.g. as a perl sub) if they wish to handle background
errors in some other manner.
.PP
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 callback specified with a
bind or a 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.
.PP
When Tk detects a background error, it saves information about the
error and invokes the \fBTk::Error\fR method later when Tk is idle.
.PP
\&\fBTk::Error\fR is invoked by perl/Tk as if by the perl code:
.PP
\&\ \fI$mainwindow\fR\->\fBTk::Error\fR(\fI\*(L"error message\*(R"\fR, \fIlocation ...\fR);
.PP
\&\fI$mainwindow\fR is the \fBMainWindow\fR associated with widget which
detected the error, \fI\*(L"error message\*(R"\fR is a string describing the error
that has been detected, \fIlocation\fR is a list of one or more \*(L"locations\*(R"
which describe the call sequence at the point the error was detected.
.PP
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.
.PP
Tk will ignore any result returned by the \fBTk::Error\fR method.
If another error occurs within the \fBTk::Error\fR method
(for example if it calls \fBdie\fR) then Tk reports this error
itself by writing a message to stderr (this is to avoid infinite loops
due to any bugs in \fBTk::Error\fR).
.PP
If several background errors accumulate before \fBTk::Error\fR
is invoked to process them, \fBTk::Error\fR will be invoked once
for each error, in the order they occurred.
However, if \fBTk::Error\fR calls \fBTk\->break\fR, then
any remaining errors are skipped without calling \fBTk::Error\fR.
.PP
The \fBTk\fR module includes a default \fBTk::Error\fR subroutine
that simply reports the error on stderr.
.PP
An alternate definition is provided via :
.PP
\&\ \f(CW\*(C`require Tk::ErrorDialog;\*(C'\fR
.PP
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.
.SH "BUGS"
.IX Header "BUGS"
If \fBafter\fR or \fBfileevent\fR are not invoked as methods of a widget
then perl/Tk is unable to provide a \fI$mainwindow\fR argument.
To support such code from earlier versions of perl/Tk
perl/Tk therefore calls \fBTk::Error\fR with string 'Tk' instead:
\&\fBTk\->Tk::Error\e(...\e)\fR.
In this case the \fBTk::Error\fR in \fBTk::ErrorDialog\fR and similar
implementations cannot \*(L"popup\*(R" a window as they don't know which display
to use.  A mechanism to supply \fIthe\fR \fBMainWindow\fR in applications
which only have one (a very common case) should be provided.
.SH "SEE ALSO"
.IX Header "SEE ALSO"
Tk::bind
Tk::after
Tk::fileevent
.SH "KEYWORDS"
.IX Header "KEYWORDS"
background error, reporting