componentDidMount() {
    this._interval = window.setInterval(this.onTick, 20);
  }
When the component is mounted into the document - this is similar to a
constructor, but invoked when the instance is actually mounted into the
document. Here, we'll just set up an animation loop that invokes our
method. Binding of `this.onTick` is not needed because all React methods
are automatically bound before being mounted.