are Mass initialDisplacementProperty and initialVelocityProperty necessary?

[pixelzoom]

Related to #2 (code review).

In Mass.js:

      // @public {Property.<Vector2>} mass current displacement (from the equilibrium position)
      this.displacementProperty = new Vector2Property( new Vector2( 0, 0 ), {
        tandem: tandem.createTandem( 'displacementProperty' )
      } );

      // @public {Property.<Vector2>} mass initial displacement
      this.initialDisplacementProperty = new Vector2Property( new Vector2( 0, 0 ), {
        tandem: tandem.createTandem( 'initialDisplacementProperty' )
      } );

      // @public {Property.<Vector2>} mass current velocity
      this.velocityProperty = new Vector2Property( new Vector2( 0, 0 ), {
        tandem: tandem.createTandem( 'velocityProperty' )
      } );

      // @public {Property.<Vector2>} mass initial velocity
      this.initialVelocityProperty = new Vector2Property( new Vector2( 0, 0 ), {
        tandem: tandem.createTandem( 'initialVelocityProperty' )
      } );

Every Property stores it's initial value, and the initial value can be restored by calling reset. So I wonder if this.initialDisplacementProperty and this.initialVelocityProperty are really necessary.

On the other hand, I see initialDisplacementProperty.set and this.initialVelocityProperty called, but don't understand why:

// OneDimensionModel.js
this.masses[ i ].initialDisplacementProperty.set( this.masses[ i ].displacementProperty.get() );
this.masses[ i ].initialVelocityProperty.set( this.masses[ i ].velocityProperty.get() );

// TwoDimensionsModel.js
this.masses[ i ][ j ].initialDisplacementProperty.set( this.masses[ i ][ j ].displacementProperty.get() );
this.masses[ i ][ j ].initialVelocityProperty.set( this.masses[ i ][ j ].velocityProperty.get() );

Recommended to:

• Verify that initialDisplacementProperty and initialVelocityProperty are really needed.
• If they are needed, document why. If not, remove them.

[tmildemberger]

They were indeed not needed. Thanks for pointing it out. Closing.