MassIO looks overinstrumented and probably has unused parts

[samreid]

From #123

MassIO is currently:

  public static readonly MassIO = new IOType<Mass, MassIOStateObject>( 'MassIO', {
    valueType: Mass,
    documentation: 'Represents a mass that interacts in the scene, and can potentially float or displace fluid.',
    stateSchema: {
      matrix: Matrix3.Matrix3IO,
      stepMatrix: Matrix3.Matrix3IO,
      originalMatrix: Matrix3.Matrix3IO,
      canMove: BooleanIO,
      tag: MassTag.MassTagIO,
      massShape: EnumerationIO( MassShape ),

      // engine.bodyToStateObject
      position: Vector2.Vector2IO,
      velocity: Vector2.Vector2IO,
      force: Vector2.Vector2IO
    },
    toStateObject( mass: Mass ): MassIOStateObject {
      return combineOptions<MassIOStateObject>( {
        matrix: Matrix3.toStateObject( mass.matrix ),
        stepMatrix: Matrix3.toStateObject( mass.stepMatrix ),
        originalMatrix: Matrix3.toStateObject( mass.originalMatrix ),
        canMove: mass.canMove,
        tag: MassTag.MassTagIO.toStateObject( mass.tag ),
        massShape: EnumerationIO( MassShape ).toStateObject( mass.massShape )
      }, mass.engine.bodyToStateObject( mass.body ) );
    },
    applyState( mass: Mass, obj: MassIOStateObject ) {
      mass.matrix.set( Matrix3.fromStateObject( obj.matrix ) );
      mass.stepMatrix.set( Matrix3.fromStateObject( obj.stepMatrix ) );
      mass.originalMatrix.set( Matrix3.fromStateObject( obj.originalMatrix ) );
      mass.canMove = obj.canMove;
      MassTag.MassTagIO.applyState( mass.tag, obj.tag );
      mass.engine.bodyApplyState( mass.body, obj );
      mass.transformedEmitter.emit();
    },
    stateObjectToCreateElementArguments: ( stateObject: MassIOStateObject ) => [ EnumerationIO( MassShape ).fromStateObject( stateObject.massShape ) ]
  } );

It has 4-5 usages through the project.

• I suspect stateObjectToCreateElementArguments is unused, since this is not part of a dynamic element pattern.
• Many attributes like canMove: BooleanIO, and the tag are immutable and therefore not important to capture as part of the state unless for descriptive purposes.
• It probably just needs a sparse toStateObject and applyState, right?
• Can it be deleted? Or is this how we capture position state for the masses?
• If we just need 2-3 elements from it, should they be converted to Property?

[samreid]

Removed canMove/massShape and tag, closing.

[phet-dev]

Reopening because there is a TODO marked for this issue.

[samreid]

Fixed, closing.