\section{Relating frames}			\label{sec:application}

Applications may consist of multiple \classs{frame}, either permanent,
or temporary such as for opening a `settings' window. This section
discusses how frame references can be found, as well as how frames
can force the user to deal with this frame first, in favour of all
the other frames of the application: \idx{modal} frames.

\subsection{Class application}

The class \class{application} is the key to this section. An
\class{application} is a subclass of \class{visual}, and optionally
located between \class{display} and \class{frame} in the visual
consists-of hierarchy. It defines the `application <-member' method to
located named frames, and thus supports locating frames similar to other
graphicals as described in \secref{whoiswho}.

A \class{frame} is made part of an \class{application} using the method
`frame ->application'.  The application to which a frame is related may
be changed.  Frames are not required to be part of an application.

\begin{description}
    \sendmethod{frame}{application}{application*}
Changes the application object the receiver belongs too. Combining
multiple frame objects in an application allows for finding frames as
well as defining modal relations between frames. See `frame->modal'.

The application to which a frame belongs may be changed at any time.
Using the @nil argument, to frame is detached from any application.

This method invokes `application ->delete' to the application currently
holding the frame (if any) and `application ->append' to the application
receiving the frame.  These methods may be redefined if an application
wants to keep track of its associated frames.
    \getmethod{application}{member}{name}{frame}
Return the frame for which `frame <-name' returns the argument name. See
also `device <-member' and \secref{whoiswho}.
\end{description}


\subsection{Transient frames}

\index{transient,frame}%
The term \jargon{transient} window is taken from X11. A transient window
(frame) is a frame that supports another frame.  Transient windows are
normally used for prompting.  The related method is:

\begin{description}
    \sendmethod{frame}{transient_for}{frame}
Make the receiver a transient window for the argument.  This notion is
handed to the X11 window manager, but the support varies.  Therefore
\product{} ensures that:
    \begin{itemlist}
	\item [The transient window stays on top]
	Whenever the main window is raised, the transient window is
	raised too, ensuring the transient window does not get hidden
	behind the main window.
	\item [Synchronise status change and destruction]
	Status change (see `frame ->status: \{unmapped,hidden,iconic,open\}')
	of the main window are forwarded to the transient windows. If
	the main window is destroyed, so is the transient window.
    \end{itemlist}
\end{description}


\subsection{Modal operation}

\index{modal}%
The method `frame ->modal' works in combination with class
\class{application} and transient frames to define what frames are
temporary insensitive to events, forcing the user to operate on the
modal frame first.

\begin{description}
    \sendmethod{frame}{modal}{\{application,transient\}*}
Operate as a modal frame for all frames in the <-application, the frame
I am <-transient_for, or none. A common sequence to display a modal
dialog window centered on a frame is below. Note that, instead of `frame
->open_centered', one could also have used `frame <-confirm_centered'.

\begin{code}
settings(Frame) :->
	"Open settings dialog"::		      
	new(D, dialog(settings)),
	send(D, transient_for, Frame),
	send(D, modal, transient),
	...,
	<fill the dialog>,
	...,
	send(D, open_centered, Frame?area?center).
\end{code}

Instead of using the center of a frame, the method can also use the
location of @event to position itself.  The code fragment for that is:

\begin{code}
	...,
	(   send(@event, instance_of, event)
	->  get(@event, position, @display, EventPos)
	;   EventPos = @default)
	),
	send(D, open_centered, EventPos).
\end{code}
\end{description}