Design Document for KStars SkyComponents
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1. Overview:

We use the Composite/Component design pattern to store all elements
that must be drawn on the SkyMap.  In this design pattern, elements to
be drawn in the map are called "Components", and they are
hierarchically organized into groups called "Composites".  A Composite
can contain other Composites, as well as Components.  From an
organizational point of view, you can think of Composites and
Components as being analogous to "Directories" and "Files" in a
computer filesystem. 

The advantage of this design pattern is that it minimizes code
duplication and maximizes modularity.  There is a set of operations
which all Components must perform periodically (such as drawing on
screen, or updating position), but each Component may perform its task
slightly differently.  Thus, each Component can implement its own
version of the functions, and each Composite calls on its child
Components to execute the desired function.  For example, if we wanted
to draw all Components in the SkyMap, we would simly need to call
"skyComposite()->draw()".  SkyComposite is the "toplevel" Composite;
its "draw()" function simply calls draw() on each of its children.
Its child Components will draw themselves, while its child Composites
will call "draw()" for each of *their* children.  Note that we don't
need to know *any* implementation details; when we need to draw the
elements, we just tell the toplevel Composite to spread the word.


2. SkyComposite Hierarchy:

In the following chart, Composites are preceeded by a "+", while
Components are preceeded by a "-".  The indentation indicates where
the Component belongs in the hierarchy.  The "baseclass" of a Component
is indicated in square brackets (see below for more on the base classes)

+ SkyComposite
  - StarComponent			[ListComponent]
  - DeepSkyComponent			[ListComponent]
  - ConstellationNamesComponent		[ListComponent]
  + SolarSystemComposite
    - SolarSystemSingleComponent (Sun		[SingleComponent]
    - SolarSystemSingleComponent (Moon)		[SingleComponent]
    - SolarSystemSingleComponent (Mercury)	[SingleComponent]
    - SolarSystemSingleComponent (Venus)	[SingleComponent]
    - SolarSystemSingleComponent (Mars)		[SingleComponent]
    - SolarSystemSingleComponent (Jupiter)	[SingleComponent]
    - SolarSystemSingleComponent (Saturn)	[SingleComponent]
    - SolarSystemSingleComponent (Uranus)	[SingleComponent]
    - SolarSystemSingleComponent (Neptune)	[SingleComponent]
    - SolarSystemSingleComponent (Pluto)	[SingleComponent]
    - AsteroidsComponent		[SolarSystemListComponent]
    - CometsComponent			[SolarSystemListComponent]
    - JupiterMoonsComponent		[SkyComponent]
  - EquatorComponent			[PointListComponent]
  - EclipticComponent			[PointListComponent]
  - HorizonComponent			[PointListComponent]
  - ConstellationBoundaryComponent	[SkyComponent]
  + ConstellationLinesComposite
    - CLineComponent	(one per constellation)	[PointListComponent]
  + CoordinateGridComposite
    - CoordinateGridComponent			[PointListComponent]
  + MilkyWayComposite
    - MilkyWayComponent		[PointListComponent]
  + CustomCatalogComposite
    - CustomCatalogComponent (one per defined catalog) [ListComponent]


3. Class Inheritance

The base class in this directory is SkyComponent; all other classes
are derived from it (directly or indirectly).  Its most important
derivative is SkyComposite, the base classes for all Composites.

From SkyComponent, we derive three important subclasses:
SingleComponent, ListComponent, and PointListComponent.
SingleComponent represents a sky element consisting of a single
SkyObject, such as the Sun.  ListComponent represents a list of
SkyObjects, such as the Asteroids.  PointListComponent represents a
list of SkyPoints (not SkyObjects), such as the Ecliptic.
Almost all other Components are derived from one of 
these three classes (two Components are derived directly from 
SkyComponent).

The Solar System bodies require some special treatment.  For example,
only solar system bodies may have attached trails, and they move
across the sky, so their positions must be updated frequently.  To
handle these features, we derive SolarSystemComposite from
SkyComposite to be the container for all solar system Components.  In
addition, we derive SolarSystemSingleComponent from SingleComponent,
and SolarSystemListComponent from ListComponent.  These classes simply
add functions to handle Trails, and to update the positions of the
bodies.  Also, they contain KSPlanetBase objects, instead of
SkyObjects.

The Sun, Moon and Pluto each get a unique Component class, derived
from SolarSystemSingleComponent (this is needed because each of these
bodies uses a unique class derived from KSPlanetBase: KSSun, KSMoon,
and KSPluto).  The remaining major planets are each represented with a
PlanetComponent object, which is also derived from
SolarSystemSingleComponent.  Finally, we have AsteroidsComponent and
CometsComponent, each of which is derived from
SolarSystemListComponent.

Next, we have the Components derived from PointListComponent.  These
Components represent imaginary guide lines in the sky, such as
constellation boundaries or the coordinate grid.  They are listed
above, and most of them don't require further comment.  The
GuidesComposite contains two sub-Composites: CoordinateGridComposite,
and ConstellationLinesComposite.  Both of these contain a number of
PointListComponents: CoordinateGridComposite contains one
PointsListComponent for each circle in the grid, and
ConstellationLineComposite contains one PointsListComponent for each
constellation (representing the "stick figure" of the constellation).

Finally, we have the Components which don't inherit from either
SingleComponent, ListComponent, or PointListComponent:
JupiterMoonsComponent inherits from SkyComponent directly, because 
the 4 Jupiter moons have their own class (JMoons) not derived 
directly from a SkyPoint.


4. Functions

TBD.  Describe important functions, and in which classes they are
defined (i.e., update() is only defined in SkyComponent[pure virtual],
SkyComposite, SingleComponent, ListComponent, and PointListComponent).
