MeanShareAndBalanceModel should be an interface

[pixelzoom]

For code review #41 ...

MeanShareAndBalanceModel looks like this:

export default class MeanShareAndBalanceModel {

  public constructor( providedOptions: MeanShareAndBalanceModelOptions ) {
    // Here for potential future use.
  }

  public syncData(): void {
    // See subclass for implementation
  }

  /**
   * Resets the model.
   */
  public reset(): void {
    // See subclass for implementation
  }

  /**
   * Steps the model.
   * @param dt - time step, in seconds
   */
  public step( dt: number ): void {
    // See subclass for implementation
  }
}

MeanShareAndBalanceModel is not meant to be instantiated directly. It provides no properties, functionality, or default behavior. And (based on what I see in IntroModel) you're expecting the subclasses to override all of those empty methods. So MeanShareAndBalanceModel should be an interface.

As an interace, it would look like this:

interface MeanShareAndBalanceModel {

  syncData(): void;

  /**
   * Resets the model.
   */
  reset(): void;

  /**
   * Steps the model.
   * @param dt - time step, in seconds
   */
  step( dt: number ): void;
}

export default MeanShareAndBalanceModel;

And usages would look like this:

export default class IntroModel implements MeanShareAndBalanceModel {

Interfaces are sometime named with an "I" prefix, like axon's IProperty. But PhET does not have a clear convention on this, so you decide whether it should be MeanShareAndBalanceModel or IMeanShareAndBalanceModel.

[pixelzoom]

By the way... If MeanShareAndBalanceModel had properties or provided default behavior, then I'd be recommending that you make it an abstract class. And while that's not the case now, it might become the case as you add screens to this sim.

[marlitas]

@samreid suggested marking MeanShareAndBalanceModel as an abstract class for now as future screens get developed. We do anticipate adding methods or properties so will make abstract for the moment.

[marlitas]

MeanShareAndBalanceModel is now an abstract class. Closing.