source: src/documentation/constructs/qt-gui.dox@ 687ef1

/*
 * Project: MoleCuilder
 * Description: creates and alters molecular systems
 * Copyright (C)  2010 University of Bonn. All rights reserved.
 * Please see the LICENSE file or "Copyright notice" in builder.cpp for details.
 */

/**
 * \file qt-gui.dox
 *
 * Created on: Jan 5, 2012
 *    Author: heber
 */

/**
 * \page qt-gui Qt GUI
 *
 * The Qt GUI is the most advanced interface and thus the most complex.
 *
 * In the following we want to explain some of the details that are involved.
 *
 * \section qt-gui-general General Concepts
 *
 * Let us first discuss about the general concepts.
 *
 * MoleCuilder is about atoms, bonds and the molecules made up by them. But
 * there is more: There are fragments, potentials, shapes, and so on.
 *
 * In the Qt GUI all of these are displayed in certain areas of the screen
 * and also in a certain manner:
 * -# the 3D view represents a three-dimensional representation of all atoms,
 *    and their bonds or possibly the molecules they form alone. Also the
 *    bounding box is shown and all selected shapes. Atoms or molecules can
 *    be selected by clicking. The view can be manipulated through rotation
 *    and translation.
 * -# an element list shows all available elements of the period table.
 * -# a molecule list shows all present molecules sorted by their formula.
 * -# a fragment list shows all fragments with their energies and contributions
 * -# a potential list shows all currently instantiated potentials and
 *    gives a 2D plot.
 * -# a shape list displays all currently available shapes, allows to select
 *    them and buttons allow to combine them via boolean operation.
 * -# an info box informs about the current atom/molecule the mouse pointer
 *    is hovering over.
 *
 * So, there are many objects that need to be filled with information and
 * they need to access the World and other singletons in order to obtain
 * this information.
 *
 * One major obstacle, or rather THE major obstacle, is that Qt is threaded,
 * i.e. the Actions are processed in one thread and the Gui does its event
 * processing in another one. Qt's Signal/Slot system is handled via this
 * event system, i.e. a signal launched by one thread may be handled by
 * the slot function in another thread. The Observer/Observable system
 * of the CodePatterns which we used internally/outside Qt's scope does
 * not do this.
 *
 * Also, signals may get delayed. This can happen either deliberately, e.g.
 * there is a QTimer that only updates an object in regular intervals, or
 * because of asynchronous threads. Elsewhen, the slot callback for a
 * certain signal is called directly. For all of these cases we have to
 * accommodate. This is especially problematic with the instantiation and
 * destruction of objects.
 *
 * A clarifying example: Imagine an atom is constructed, the AtomObserver
 * notifies about it, but the information is not processed immediately.
 * Shortly after, the atom is destroyed again before its representation is
 * instantiated in the GUI. Afterwards the GUI attempts to instantiate it
 * but can not longer access the atom for its position and element.
 *
 * The only possible way out is to duplicate information. This is the usual
 * way how to deal with environments with multiple threads. I.e. all the
 * information that the GUI representants of information inside the World
 * needs to be doubled such that when the original information is destroyed
 * the representant can still be accessed as long as needed.
 *
 * \subsection qt-gui-general-observedvalue Observed Value
 *
 * These representants are called \a ObservedValue in CodePatterns and they
 * are used everywhere in the Qt Gui.
 *
 * They contain an internal information, e.g. a boolean, a Vector or even
 * a complex structure such as a Tesselation. They require an updater
 * function to obtain the derived information from the original source. And
 * they signOn to the source in order to be notified either generally on
 * updates or for specific channels only.
 *
 * The ObservedValue will automatically and immediately update its internal
 * representation of the derived information by calling the updater function
 * as soon as it has been informed about the update. Hence, the internal
 * information is always up-to-date and lives beyond the scope of the
 * source of the information until its own destruction. As updates are
 * processed immediately, this pattern only makes sense for "small" pieces
 * of information, i.e. when the updater function is very light-weight and
 * does not do much in terms of using computing resources.
 *
 * Note that there is another concept that is opposite to the observed value,
 * namely the Cacheable. This pattern will update itself only when requested,
 * referred to as "lazy evaluation". Hence, this pattern is used for "large"
 * pieces of information that require more computing resources within the
 * updater. Also, the Cacheable's information can only be obtained as long
 * as the source of information is still alive.
 *
 * Both concepts can be used in threaded environments as mutexed are used to
 * protect read and write accesses.
 *
 * \subsection qt-gui-general-signalslot Observer/Observable and Signal/Slot
 *
 * In the following we refer to Observer/Observable as "O/O" and to Signal/Slot
 * as "S/S".
 *
 * One thing we need to do is to translate between update() or
 * recieveNotification() calls from an Observable and subsequent signal/slot
 * calls. The general idea is to use these ObservedValues as translation
 * points for small pieces of information and Cacheables for larger pieces.
 *
 * However, we need more of these translation points:
 * -# GLWorldView checks for
 *   -# World's MoleculeInserted
 *   -# World's SelectionChanged
 *   -# WorldTime's TimeChanged
 *   -# each molecule's AtomInserted and AtomRemoved
 *   -# AtomObservable's AtomChanged.
 *   -# ShapeRegistry's ShapedAdded, ShapeRemoved, and SelectionChanged
 * -# GLMoleculeObject_molecule checks for
 *   -# molecule's AtomInserted, AtomRemoved, AtomMoved, IndexChanged
 *   -# World's SelectionChanged
 *
 * \section qt-gui-qt3d Qt3D and the way to get atoms and bonds displayed
 *
 * By far the most difficult component of the Qt GUI is the 3D view. So,
 * let us explain it in detail.
 *
 * The general widget making up the view is called \a GLWorldView. It contains
 * the GLWorldScene (i.e. all atoms, bonds, molecules, and shapes). Also
 * the "dreibein" and the domain. It processes key presses and mouse events
 * to manipulate the view. And it also serves as the translator O/O to S/S
 * system.
 *
 * The GLWorldScene contains the actual nodes of the molecular system, i.e.
 * the atoms, bonds, molecules, and shapes. All of these are derived from
 * GLMoleculeObject and have their parent to the instance of the GLWorldScene
 * which goes through its list of children and to call draw() on them.
 *
 * The bottom-most structure is GLMoleculeObject_atom displaying a sphere
 * of an element-specific color at the atom's position. The atom relies
 * on its representants to be contain all required information but it
 * is also signOn() to the atom itself whose O/O are translated to S/S
 * for processing whenever desired.
 *
 * Next comes the GLMoleculeObject_bond which displays a cylinder between
 * two atoms. Actual, a true bond consists of two of these objects. If the
 * bond is between heterogeneous atoms each half will be displayed in the
 * color of the closer atom. These bond objects are not associated with
 * the atoms directly as the are linked to two atoms at the same time. They
 * rely on ObservedValues for position and element of either atom and for
 * the degree of the bond itself.
 *
 * Parallel to these are GLMoleculeObject_shape which display the surface
 * of a selected shape. A shape in general does not change after instantation,
 * hence the shape lives with the information it gets on instantiation till
 * it dies.
 *
 * Finally, the GLMoleculeObject_molecule owns both atoms and bonds. This
 * allows for switching the view between the classical ball-and-stick model
 * and the tesselated surface of the molecule. The latter uses a lot less
 * triangles and thus is faster. Also, it is especially suited for large
 * molecules. The molecule also needs ObservedValues for its bounding box
 * (used to show when it's selected), the index, the selection status,
 * and the list of atom ids. As Cacheable we use the tesselation structure.
 *
 * \section qt-gui-cases Sample cases
 *
 * Let us discuss some cases and how the different instances interact.
 *
 * \section qt-gui-cases-start Start
 *
 * When molecuilder is started, several singletons such as the World and
 * others are instantiated. No atoms are yet present, no bonds, no molecules.
 * Hence, nothing to display yet.
 *
 * Before launching any Action the ActionQueue is forced to wait till the
 * GUI is finished instantiating. This is to ensure that GLWorldView and
 * others are in place to receive signals from the O/O system.
 *
 * When a molecule is loaded, the instantiation of a GLMoleculeObject_molecule
 * does not happen immediately. Hence, GLWorldView listens to the World's
 * MoleculeInserted. On receiving it, it also signOn()s to the molecule
 * to get its subjectKilled(). It translates then these and also all
 * AtomInserted and AtomRemoved to the S/S system as moleculeInserted,
 * moleculeRemoved and atomInserted/atomRemoved respectively, which are
 * processed by the GLWorldScene.
 *
 * The GLWorldScene records any atomInserted/atomRemoved until the molecule
 * has been instantiated. On instantiation all recorded events are played.
 * This is to ensure that there is no overlap in instantiation and signOn()
 * to the molecule. If we would simply get all atoms which are present
 * on processing the molecule's instantiation we might stumble over a signal
 * of a molecule of a just added atom. This occurs frequently as both
 * are very much correlated.
 *
 * GLWorldView keep track of all ObservedMolecules. And GLWorldScene keeps
 * track of all shapes and molecules in the scene. Each
 * GLMoleculeObject_molecule in turn keeps track of all atoms and bonds in
 * its part of the scene.
 *
 * \section QtElementList
 *
 * Lists for each element how often it occurs in the world. Selecting an entry
 * calls SelectionAtomByElementAction to select all atoms of that particular
 * element.
 *
 * Initially, it fills itself by looking at all elements in the World's
 * periodentafel. It also listens to AtomObserver's ElementChanged to know
 * when to update a certain element in its list. By using an internal list
 * for each atom's element, it can update each element's occurrence.
 *
 * \section QtMoleculeList
 *
 * Lists all the molecules currently in the world grouped by their formula.
 * Selecting an entry calls the SelectionMoleculeByIdAction.
 *
 * The QtMoleculeList is also a rather complex beast. It is a tree of
 * rows and each row consists of a number of elements. There are two
 * levels, the group level where the common formula for all molecules
 * is given, and the molecule level where are molecules of this specific
 * formula are summarized.
 *
 * The group items are QStandardItems. Sadly, they are not derived from
 * QObject and hence do not use the S/S system. The group items are
 * directly controlled by the QtMoleculeList.
 *
 * However, the molecule items are different. They are derived from
 * QtMoleculeList and use an ObservedValue internally to contain an always
 * valid copy of the required information. They inform the QtMoleculeList on
 * updates via a callback (as QStandardItem, from which they are also derived,
 * does not use the S/S system). The callback takes care of then also updating
 * the group items and possibly moving the molecule items around, e.g. if
 * their formula has changed they suddenly belong to another group.
 *
 * All items are instantiated by the QtMoleculeItemFactory.
 *
 * QtMoleculeList uses an internal QTimer to only update itself at regular
 * intervals. Hence, updates are processed rather lazily. We keep lists
 * of changes, separated for group and molecule items. And these are processed
 * one after the other at the intervals dictated by the QTimer in
 * updateItemStates().
 *
 * \section QtShapeController
 *
 * This is the interface for the ShapeRegistry. It lists all the shapes in the
 * registry and lets the user select them. It also features buttons to call
 * actions creating and manipulating the selected shapes.
 *
 * As an Observer it handles the following messages from ShapeRegistry:
 *  - ShapeRegistry::ShapeInserted
 *  - ShapeRegistry::ShapeRemoved
 *  - ShapeRegistry::SelectionChanged
 *
 * \section QtInfoBox
 *
 * Shows information about the atom and molecule the cursor is currently hovering
 * over inside the GLWorldView.
 *
 * GLWorldView emits hoverChanged signals (via QT's signal slot mechanism) which
 * the QtInfoBox receives. QtInfoBox then creates its info pages for the atom
 * being transmitted as the signal's parameter.
 *
 * The info pages are Observers for the atom/molecule. When recieving subjectKilled
 * they automatically clear the info box.
 *
 * \date 2015-07-15
 */