Update of /project/mcclim/cvsroot/mcclim/Doc In directory clnet:/tmp/cvs-serv27169

Modified Files: manual.tex Log Message: Section on how CLIM is different from traditional GUI toolkits.

--- /project/mcclim/cvsroot/mcclim/Doc/manual.tex 2006/10/30 06:26:53 1.32 +++ /project/mcclim/cvsroot/mcclim/Doc/manual.tex 2006/10/30 08:21:01 1.33 @@ -95,6 +95,78 @@ and not well described. It will be noted here when they are partially implemented in McCLIM or not implemented at all.

+\section{How CLIM is different} + +Many new users of CLIM have a hard time trying to understand how it +works and how to use it. A large part of the problem is that many +such users are used to more traditional GUI toolkits, and they try to +fit CLIM into their mental model of how GUI toolkits should work. + +But CLIM is much more than just a GUI toolkit, as suggested by its +name, it is an \emph{interface manager}, i.e. it is a complete +mediator between application ``business logic'' and the way the user +interacts with objects of the application. In fact, CLIM doesn't have +to be used with graphics output at all, as it contains a large +collection of functionality to manage text. + +Traditional GUI toolkits have an \emph{event loop}. Events are +delivered to GUI elements called \emph{gadgets} (or \emph{widgets}), +and the programmer attaches \emph{event handlers} to those gadgets in +order to invoke the functionality of the application logic. While +this way of structuring code is sometimes presented as a virtue +(``Event-driven programming''), it has an unfortunate side effect, +namely that event handlers are executed in a null context, so that it +becomes hard to even remember two consecutive events. The effect of +event-driven programming is that applications written that way have +very rudimentary interaction policies. + +At the lowest level, CLIM also has an event loop, but most application +programmers never have any reason to program at that level with CLIM. +Instead, CLIM has a \emph{command loop} at a much higher level than +the event loop. At each iteration of the command loop: + +\begin{itemize} +\item A command is acquired. You might satisfy this demand by + clicking on a menu item, by typing the name of a command, by hitting + some kind of keystroke, by pressing a button, or by pressing some + visible object with a command associated with it; +\item Arguments that are required by the command are acquired. Each + argument is often associated with a \emph{presentation type}, and + visible objects of the right presentation type can be clicked on + to satisfy this demand. You can also type a textual + representation of the argument, using completion, or you can use a + context menu; +\item The command is called on the arguments, usually resulting in + some significant modification of the data structure representing + your application logic; +\item A \emph{display routine} is called to update the views of the + application logic. The display routine may use features such as + incremental redisplay. +\end{itemize} + +Instead of attaching event handlers to gadgets, writing a CLIM +application therefore consists of: + +\begin{itemize} +\item writing CLIM commands that modify the application data + structures independently of how those commands are invoked, and + which may take application objects as arguments; +\item writing display routines that turn the application data + structures (and possibly some "view" object) into a collection of + visible representations (having presentation types) of application + objects; +\item writing completion routines that allow you to type in + application objects (of a certain presentation type) using + completions; +\item independently deciding how commands are to be invoked (menus, + buttons, presentations, textual commands, etc). +\end{itemize} + +By using CLIM as a mediator of command invocation and argument +acquisition, you can obtain some very modular code. Application logic +is completely separate from interaction policies, and the two can +evolve separately and independently. + \part{Getting Started}

\chapter{CLIM Demos and Applications}