Architectural Overview of Konsole for KDE 4.0

Authors:        Robert Knight <robertknight@gmail.com>
Last Updated:   14-02-2007
Summary:        This document describes the basic architecture of Konsole for KDE 4.0

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Contents:
    1.  Introduction
    2.  Back-end
    3.  Front-end
    3.1 Terminal display
    3.2 User interface


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1.  Introduction

Konsole consists of two main components, the back-end, which performs the terminal
emulation, and the front-end, which renders the terminal display, handles user input
and provides the user interface.

2.  Back-end

The back-end provides the interface for the terminal emulation and an implementation of 
a DEC VT102 terminal.

The task of the back-end is to take a serial stream of characters arriving from the 
program currently running in the terminal, decode and interpret them to produce
an image of characters which can be rendered by the front-end.  The incoming
stream of characters can be encoded in a variety of formats, and may include
'escape-sequences'.  These are special sequences of characters which can:
    - Turn various formatting options on or off ( text color, bold, underline etc. )
    - Move the cursor around ( new characters are inserted at the current cursor position )
    - Insert, delete or replace lines on screen
    - Change various aspects of the terminal state 
    - ... and other tasks

When the back-end receives input from the program running in the terminal, it first decodes
it into the standard Unicode format used internally.  This is done in a generic fashion which
is independant of the kind of terminal actually being emulated.  Once the characters have
been decoded into a format that Konsole can understand, the interpretation of the characters is
performed by an implementation of a specific type of terminal.  The terminal emulation has
access to one or more screens, which act as a canvas which the emulation can draw characters
on, with various associated formatting information.  Each screen may have a history scroll
associated with it.  When new lines are added to the screen, older lines will be transferred
to the history scroll.  Different history scroll implementations are available, and the number
of lines they can store and where they store those lines varies between them.  The history scroll
provides the facility which allows the user to scroll the display up and down to look at previous
output.

The terminal emulation is aware of the displays which are connected to it, and when it 
receives input, it schedules an update of the attached displays.  If several batches
of input are received in quick succession, only one update will be performed.  The update
itself consists of the emulation getting the final character image from the current screen
and sending it to the terminal display, which can then render the updates.

The back-end is also responsible for encoding key-presses and other input from the user
and sending it as a serial character stream to the standard input of the program
currently running in the terminal.  The translation of key-presses into character
sequences which are sent to the terminal is customiseable using so-called 'keytrans' files,
a number of which are provided with Konsole.

Key Classes:    TEmulation  - The generic terminal emulation implementation
                TEmuVt102   - The DEC VT102 + xterm extensions implementation
                TEScreen    - The screen which acts as a canvas for the emulation to draw on
                TEHistory   - Interface for history scroll classes

3.  Front-end

The front-end consists primarily of the user interface and the terminal displays.

3.1  Terminal display

The terminal display is a widget which renders an image of characters on screen or to another device
such as a printer.  Each back-end may have one or more terminal displays connected to it.

3.2  User interface