2. Preliminaries: Key concepts and relations

The purpose of this chapter to explain some of key concepts and relations you need to understand before reading the following chapters. These include modules explained in section 2.1 and the Notion class and object hierarchies, section 2.2.

2.1 Modules

Notion has been designed so that the 'notion' executable only implements some basic services on top of which very different kinds of window managers could be build by loading the appropriate 'modules'. On modern system these modules are simply dynamically loaded .so libraries. On more primitive systems, or if you want to squeeze total size of the executable and libraries, the modules can optionally be statically linked to the main binary, but must nevertheless be loaded with the dopath function. Modules may also include Lua code.

Currently Ion provides the following modules:

Tilings for workspaces of the original tiled Notion kind.
Queries (for starting programs and so on) and message boxes.
Support for menus, both pull-down and keyboard-operated in-frame menus.
Module that implements a statusbar that can be adaptively embedded in each workspace's layout.
Module for docking Window Maker dock-apps. The dock can both float and be embedded as the statusbar.
This module implements a scratchpad frame that can be toggled on/off everywhere. Think of the 'console' in some first-person shooters.
Session management support module. Loaded automatically when needed!

So-called drawing engines are also implemented as a modules, but they are not discussed here; see chapter 4.

The stock configuration for the notion executable loads all of the modules mentioned above except mod_dock. The stock configuration for the pwm3 executable (which differs from the notion executable in a few configuration details) loads another set of modules.

2.2 Class and object hierarchies

While Notion does not have a truly object-oriented design2.1, things that appear on the computer screen are, however, quite naturally expressed as such ``objects''. Therefore Notion implements a rather primitive OO system for these screen objects and some other things.

It is essential for the module writer to learn this object system, but also people who write their own binding configuration files necessarily come into contact with the class and object hierarchies - you need to know which binding setup routines apply where, and what functions can be used as handlers in which bindings. It is the purpose of this section to attempt to explain these hierarchies. If you do not wish the read the full section, at least read the summary at the end of it, so that you understand the very basic relations.

For simplicity we consider only the essential-for-basic-configuration Notioncore, mod_tiling and mod_query classes. See Appendix B for the full class hierarchy visible to Lua side.

2.2.1 Class hierarchy

One of the most important principles of object-oriented design methodology is inheritance; roughly how classes (objects are instances of classes) extend on others' features. Inheritance gives rise to class hierarchy. In the case of single-inheritance this hierarchy can be expressed as a tree where the class at the root is inherited by all others below it and so on. Figure 2.1 lists out the Notion class hierarchy and below we explain what features of Notion the classes implement.

Figure 2.1: Partial Notioncore, mod_tiling and mod_query class hierarchy.
     |    |-->WClientWin
     |    |-->WWindow
     |    |    |-->WRootWin
     |    |    |-->WMPlex
     |    |    |    |-->WFrame
     |    |    |    `-->WScreen
     |    |    `-->WInput (mod_query)
     |    |         |-->WEdln (mod_query)
     |    |         `-->WMessage (mod_query)
     |    |-->WGroup
     |    |    |-->WGroupWS
     |    |    `-->WGroupCW
     |    `-->WTiling (mod_tiling)
     `-->WSplit (mod_tiling)

The core classes:

Is the base of Notion's object system.

is the base class for everything corresponding to something on the screen. Each object of type WRegion has a size and position relative to the parent WRegion. While a big part of Notion operates on these instead of more specialised classes, WRegion is a ``virtual'' base class in that there are no objects of ``pure'' type WRegion; all concrete regions are objects of some class that inherits WRegion.

is a class for client window objects, the objects that window managers are supposed to manage.

is the base class for all internal objects having an X window associated to them (WClientWins also have X windows associated to them).

is a base class for all regions that ``multiplex'' other regions. This means that of the regions managed by the multiplexer, only one can be displayed at a time.

is an instance of WMPlex for screens.

is the class for root windows of X screens. Note that an ``X screen'' or root window is not necessarily a single physical screen as a root window may be split over multiple screens when techniques such as Xinerama are used. (Actually there can be only one root window when Xinerama is used.)

is the class for frames. While most Notion's objects have no graphical presentation, frames basically add to WMPlexes the decorations around client windows (borders, tabs).

is the base class for groups. Particular types of groups are workspaces (WGroupWS) and groups of client windows (WGroupCW).

Classes implemented by the mod_tiling module:

is the class for tilings of frames.
(or, more specifically, classes that inherit it) encode the WTiling tree structure.

Classes implemented by the mod_query module:

is a virtual base class for the two classes below.
is the class for the ``queries'', the text inputs that usually appear at bottoms of frames and sometimes screens. Queries are the functional equivalent of ``mini buffers'' in many text editors.
implements the boxes for warning and other messages that Notion may wish to display to the user. These also usually appear at bottoms of frames.

There are also some other ``proxy'' classes that do not refer to objects on the screen. The only important one of these for basic configuration is WMoveresMode that is used for binding callbacks in the move and resize mode. Run-time access to types

Even though it often indicates a design mistake, sometimes it can be useful to have run-time access to the types of objects.

For example, to check wether a given object is of type WMPlex, the following C code can be used:

    if(obj_is((Obj*)cwin, &CLASSDESCR(WMPlex)))

Its lua counterpart is:

    if(obj_is(cwin, "WMPlex"))

While there's also an 'obj_cast' method available, C structs can be freely cast to their 'superclass' using a regular C cast, for example:

bool input_fitrep(WInput *input, WWindow *par, const WFitParams *fp)
    if(par!=NULL && !region_same_rootwin((WRegion*)input, (WRegion*)par))
        return FALSE;

2.2.2 Object hierarchies: WRegion parents and managers Parent-child relations

Each object of type WRegion has a parent and possibly a manager associated to it. The parent for an object is always a WWindow and for WRegion with an X window (WClientWin, WWindow) the parent WWindow is given by the same relation of the X windows. For other WRegions the relation is not as clear. There is generally very few restrictions other than the above on the parent--child relation but the most common is as described in Figure 2.2.

Figure 2.2: Most common parent-child relations
     |-->WClientWins in full screen mode
          |-->WFrames for transients
          `-->a possible WEdln or WMessage Manager-managed relations

WRegions have very little control over their children as a parent. The manager WRegion has much more control over its managed WRegions. Managers, for example, handle resize requests, focusing and displaying of the managed regions. Indeed the manager--managed relationship gives a better picture of the logical ordering of objects on the screen. Again, there are generally few limits, but the most common hierarchy is given in Figure 2.3. Note that sometimes the parent and manager are the same object and not all regions may have a manager, but all non-screen regions have a parent--a screen if not anything else.

Figure 2.3: Most common manager-managed relations
     |-->WGroupCWs for full screen WClientWins
     |    |-->WClientWins
     |    `-->WFrames for transients (dialogs)
     |         `--> WClientWin
     |-->WGroupWSs for workspaces
     |    |-->WTiling
     |    |    |-->WFrames
     |    |    |    `-->WGroupCWs (with contents as above)
     |    |    `-->possibly a WStatusBar or WDock
     |    |-->WFrames for floating content
     |    |-->possibly a WEdln, WMessage or WMenu
     |    `-->possibly a WStatusBar or WDock (if no tiling)
     `-->WFrames for sticky stuff, such as the scratchpad

Note that a workspace can manage another workspace. This can be achieved with the WGroup.attach_new function, and allows you to nest workspaces as deep as you want.

In the presense of Xinerama there may be WScreen objects at the top of the manager-managed tree instead of WRootWin.

2.2.3 Summary

In the standard setup, keeping queries, messages and menus out of consideration:


... design2.1
the author doesn't like such artificial designs