Error(3)

NAME
Tk::Error - Method invoked to process background errors

SYNOPSIS
Customization:

    require Tk::ErrorDialog;

or

    sub Tk::Error
    {
      my ($widget,$error,@locations) = @_;
      ...

    }

DESCRIPTION
The 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 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 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.

When Tk detects a background error, it saves information about the error and invokes the Tk::Error method later when Tk is idle.

Tk::Error is invoked by perl/Tk as if by the perl code:

$mainwindow->Tk::Error("error message", location ...);

$mainwindow is the MainWindow associated with widget which detected the error, "error message" is a string describing the error that has been detected, 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 Tk::Error method. If another error occurs within the Tk::Error method (for example if it calls die) then Tk reports this error itself by writing a message to stderr (this is to avoid infinite loops due to any bugs in Tk::Error).

If several background errors accumulate before Tk::Error is invoked to process them, Tk::Error will be invoked once for each error, in the order they occurred. However, if Tk::Error calls Tk->break, then any remaining errors are skipped without calling Tk::Error.

The Tk module includes a default Tk::Error subroutine that simply reports the error on stderr.

Tk::ErrorDialog
An alternate definition is provided via:

"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 bgerror, with a twist: since there is only one 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 -cleanupcode and -appendtraceback ErrorDialog options you must call the constructor and create it manually.

The ErrorDialog object essentially consists of two subwidgets: a Dialog widget to display the background error and a Text widget for the traceback information. If required, you can invoke various widget methods to customize these subwidgets - their advertised names are described below.

$mw->ErrorDialog(-cleanupcode => code, -appendtraceback => bool);

$mw is a window reference.

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 Tk::Error was invoked with.

bool is a boolean indicating whether or not to append successive tracebacks (default is 1, do append).

  Advertised ErrorDialog widgets
error_dialog is the Dialog widget reference.

text is the Text widget reference containing the traceback information.

BUGS
If after or fileevent are not invoked as methods of a widget then perl/Tk is unable to provide a $mainwindow argument. To support such code from earlier versions of perl/Tk perl/Tk therefore calls Tk::Error with string 'Tk' instead: Tk->Tk::Error\(...\). In this case the Tk::Error in Tk::ErrorDialog and similar implementations cannot "popup" a window as they don't know which display to use. A mechanism to supply the MainWindow in applications which only have one (a very common case) should be provided.

SEE ALSO
Tk::bind Tk::after Tk::fileevent

KEYWORDS
background error, reporting