====================================================================
      Remarks for MuseScore Version 3
      design goals/changes
====================================================================

- Optimize horizontal layout by allowing overlapping segments.
      - Compute an outline (Shape) for every Staff/Segment.
      - Use the outline to compute the minimum distance between Segments.
            - this also avoids collisions between Lyrics and ChordNames

- Automatically increase vertical space between staves to avoid collisions.
      - Use the segment shapes to compute the minimum distance between staves.
        Use the minimum distance if its bigger than the style distance;

- Implement a way to (re-)layout only the modified part of the score.
  A reorganization of layout is needed for this. Layout should work in one pass in
  an incremental way.
  The pattern to trigger incremental layout is:
            score->startCmd();
            ...
            score->setLayout(tick1);      // tick position of score change
            ...
            score->setLayout(tickn);      // another change at a different position
            ...
            score->endCmd();
   setLayout(int tick) records the min+max value of all calls and feeds it
   to score->doLayoutRange(int minTick, int maxTick);


- Do not allow more than one System on a line. In 2.x a horizontal box splits
  a line into two systems. In 3.x a horizontal box is handled as a special measure.
  This simplifies page layout a lot.

- System bar lines are moved into measures in a special segment type "BeginBarLine".

- Do not undo/redo add/removal of "created" elements.
      - It speeds up doLayout() and consumes less memory.

      In 2.x all Segments changes must be on the undo/redo stack to keep the history
      consistent, This was necessary as Segments were referring to
      previous/next segments to find the proper insertion point.

      In 3.x  Undo knows were to (re-)insert Segments by only checking type and tick
      position.

- evaluate the possibility of implementing "Continuous View" as a real "view" change
  without changing the score (no undo/redo change)

- move compatibility code into separate modules.


====================================================================
      New Features
====================================================================

Timewise insert/delete notes/rests
-----------------------------------

      - Ctrl+Shift+[cdefg]
        Insert note with current duration.
        This increases the duration of the actual measure independent of the
        current time signature making it an irregular measure.

      - Ctrl + left mouse button in note entry mode
        Inserts note/rest with current duration

      - Ctrl+Del
        Removes the selected Chord/Rest decreasing the duration of the measure.
        Extend this to selection ranges.

      - drop barline
        This inserts a barline into the current measure or changes the current barline.

      - ctrl + drop barline
        Splits the current measure at the new barline.

      - ctrl + delete barline
        This joins the current measure with the next measure.

      - show irregular measures
        This displays a "+" or "-" in the upper right corner of a measure if the measure
        length is different from the current time signature.

      Functions which may be removed:
            - split measure: replaced by dropping a barline
            - join measure:  replaced by deleting a barline
            - adjusting the actual measure length in measure properties; this
              can be achieved by inserting/deleting notes/rests in the measure.
            - delete measure timewise: replaced by Ctrl+Del of selection ranges


Palette handling
-------------------------------
      - shift+drag allows to move a palette element in the palette
      - ctrl+drag is drag operation with special semantic

Lyrics
-------------------------------
      - lyrics can be placed above a staff by changing the Placement property to "Above".
        Verse numbers are counted from top to bottom. If you change for example the placement
        of a lyric in verse 3 to Above then all lyrics in verse 1-3 are placed above, verse 4+
        will stay below.
      - 'x' (flip) changes the placement of a lyric.
      - up/down moves a lyric to the previous/next verse. If there is already a lyric, the text
        is swapped.
      - the style values "Position above" and "Position below" determine the default lyric position.
      - lyrics are move up or down when a collision with other score elements are detected. The style
        value "Autoplace vertical align range" tells mscore to align other lyrics measure or system wide.



====================================================================
      Programming Style Changes in MuseScore 3
====================================================================

      * Instead of
            if (e->type() == Element::Type::Chord)
                  ...
        write
            if (e->isChord())
                  ...

        This is shorter and easier to read. Similar
            if ((s.segmentType() == Segment::Type::ChordRest))
                  ...
        can be written as
            if (s.isChordRestType())
                  ...

      * Use safer type conversion: instead of
            Chord* chord = static_cast<Chord*>(e);

        write
            Chord* chord = toChord(e);

        This adds an Q_ASSERT to make sure e is really a Chord().

      * Prefer vector container over list:
            - Instead of QList use QVector or even better std::vector if possible.

      * Prefer stl style over Qt style when using container:
            - use list.push_back(xx) instead of list.append(xx) or
            - use list.empty() instead of list.isEmpty()
            - etc.

        (see https://marcmutz.wordpress.com/effective-qt/containers/)

        Caution when replacing Qt container with stl container as the semantic
        may be different. Especially in a "for (xxx : yyy)" loop the Qt container
        is copied (copy on write) and the stl container not. That means that you can modify a
        Qt container (inserting/deleting elements) in this for loop. This will usually not
        work for a stl container.


      * In iterating a SegmentList instead of
            for (Segment* s = segments->first(); s; s = s->next())
                  ...

        you can write
            for (Segment& s : segments)
                  ...

      * enums
            To export enums for scripting we collect them in types.h and use the new
            qt feature Q_NAMESPACE.

      * debug messages
            - use qWarning(), qCritical() and qFatal()
            - if debug messages are used, it should be possible to switch them off and they
              should be removed in release mode
            - don't use qDebug(), instead use a log category and call qCDebug(logCategory, ...)
              (see undo.h undo.cpp for example usage)
              TODO: check if they can be removed in system mode

      * path separator
            Don't use QDir::separator(), but "/", see https://doc.qt.io/qt-5/qdir.html#separator
            and http://agateau.com/2015/qdir-separator-considered-harmful/


====================================================================
      some rules
====================================================================

Element type system
      Every class derived from ScoreElement which implements
            virtual ElementType type() const = 0;
      should have set the class attribute "final" to make sure no other class will be derived
      from this one. This avoids ambiguities in the internal type system.

Layout

      - After loading a score, the score data can only be modified in a "undo" function during editing.
        The score is "dirty" if there is an undo stack entry.

      - Flags/data which describe what kind of relayout/update the score needs are collected in
        Score->CmdState. They can only be set in an "undo" function.

Score File

      - "created" elements are not written out into the score file. For example bar lines are usually created
        by layout. They are marked as created and therefore do not appear in the score file.

      - Element properties are only written if they differ from their default value which usually is the
        style value if they are styled.

paint() method in Element
      paint() should never paint something or not conditionally. If you want something not to be
      painted, then set the bounding box of the element to zero in layout() and paint will not
      be called. This works better with smart layout and collision detection.

====================================================================
      Implementation Details
====================================================================

Measure layout
      A measure is laid out in four different contexts:
            - first measure in a system
                  - the measure is prepended by a system header

            - last measure in a system
                  - the measure has a system trailer

            - in the middle of a system

            - is the only measure in a system
                  - the measure has a system header and a system trailer

BarLine handling

      A measure can have four kind of barlines, contained in a special typed Segment:

            - BeginBarLine
                  this barline is also called the "systemic barline"
                  - automatically created for every System
                  - a BarLine can be dropped on a horizontal frame which adds
                    a BeginBarLine to the next measure
                  - overrides the EndBarLine of a previous measure in the system

            - StartRepeatBarLine
                  - after a horizontal frame it overrides a BeginBarLine
                  - is prepended by a system header if in the first system measure
                  - sets the _repeatStart measure flag

            - BarLine
                  - is in the middle of a measure and has no semantic (repeat etc.)
                    (tick > start tick of measure and tick < end tick of measure)

            - EndBarLine
                  - is created automatically
                  - maybe not the last Segment in a Measure (cautionary elements can follow)


Ottava
      Ottavas have no placement style. Placement depends on the ottava subtype and is hardcoded.

Dynamics
      Default vertical reference position for Placement "below" (dynamicPosBelow = 0.0) is the last
      staff line + text line height.


==============================================================
      Styles
==============================================================

A style is part of a score. Its a list of style values.
Style values are identified by the enumeration Sid.
A style is initialized with a set of build in hard coded values. This build in style can be
modified by a style set in preferences (if set).
The style associated with a score can be modified by the user in the UI.
If the user resets a style value in the UI, it is set to the hardcoded build in value;
Only style values which differ from the build in values are saved in a score.

==============================================================
      Properties
==============================================================

Properties are identified by the enumeration Pid.

Some properties are associated with a Style. This styled properties have
a state (enumeration PropertyFlags):

      NOSTYLE:
            The property has no associated style.

      STYLED
            The property has the value of the associated Style. If the style changes, the value
            of all styled properties also change.
            If the user changes a styled property in the inspector it is set to UNSTYLED. That means
            that it does not follow any style changes anymore but stays on the user modified value.
            A reset of the property in the inspector resets it to its styled value and also sets
            its state from UNSTYLED to STYLED.

      UNSTYLED
            The property has an associated style but does not follow any style changes.
            Only the local value of the property is valid.

Every element has a list of styled properties. Usually this list points to an element type specific
static list. Some element types can change their style and therefore have a local copy of the static list.


Element Interface

Setter/Getter

      virtual QVariant getProperty(P_ID) const;
      virtual bool setProperty(P_ID, const QVariant&);

Default

      A property can have a default value.

      virtual QVariant propertyDefault(P_ID) const;
      virtual void resetProperty(P_ID id);
            Reset property to default value. Also if the property has an associated style value,
            sets the property state to STYLED.

Undo/Redo:

      virtual void undoChangeProperty(P_ID id, const QVariant&, PropertyFlags ps);
      void undoChangeProperty(P_ID id, const QVariant&);
      void undoResetProperty(P_ID id);

      void undoPushProperty(P_ID);

Serialization:

      void ScoreElement::writeProperty(XmlWriter& xml, P_ID id) const;

      bool Element::readProperty(const QStringRef&, XmlReader&, P_ID);
            This sets the property state to UNSTYLED if it is a styled property.

Human-readable representation:

      virtual QString propertyUserValue(Pid) const;
            Returns the value of the property in a human-readable representation.

Styled properties:

      virtual StyleIdx getPropertyStyle(P_ID) const;
      virtual void styleChanged();

      Helper functions:
      virtual PropertyFlags& propertyFlags(P_ID);
      virtual void setPropertyFlags(P_ID, PropertyFlags);

Helper functions:

      void Element::initSubStyle(SubStyle st)
      bool Element::custom(P_ID id) const
            return true if property is != default value

      virtual const StyledProperty* ScoreElement::styledProperties() const;
      virtual PropertyFlags* ScoreElement::propertyFlagsList();


Macros:

    M_PROPERTY (type, getter_name, setter_name)
       helper macro to define a styled ScoreElement property

    usage example:
    class Text : public Element {
          M_PROPERTY(bool, bold, setBold)
          ...
          };
    this defines:
          bool _bold;
          const bool& bold() const { return _bold; }
          void setBold(const a& val) { _bold = val; }


Functions which must be implemented for any new ScoreElement:

      virtual QVariant getProperty(P_ID propertyId) const override;
      virtual bool setProperty(P_ID propertyId, const QVariant&) override;
      virtual QVariant propertyDefault(P_ID) const override;


==============================================================
      Element position
==============================================================

The position of an element is notated relative to its parent element so it has
a local coordinate system starting with zero. The draw position of an element is the
sum of Element->_pos + Element->_offset.

For some element types _offset is styled. The style value gives the default position of an
element. Each element can separately moved by the user by changing the _offset value.

If an element is "autoplaced" then Element->_pos is changed to move the element to a position without
colliding with other element.

