Bangle.js: associates a point on the screen with the current device tilt

Bangle_TiltWatcher.js

//------------------------------------------------------------------------------
//--                               TiltWatcher                                --
//------------------------------------------------------------------------------

(function () {
  let _Sensitivity = 1;                // factor to amplify or damp acceleration

  const TakerList = [];

/**** setWatchFor ****/

  function setWatchFor (Taker) {
    if ((Taker == null) || (typeof Taker !== 'object')) {
      throw new TypeError('"Taker" should be an object');
    }

    let TakerIndex = TakerList.indexOf(Taker);
    if (TakerIndex < 0) {
      TakerList.push(Taker);
      if (TakerList.length === 1) {
        Bangle.on('accel',handleTilt);
      }
    }

    return this;
  }

/**** clearWatchFor ****/

  function clearWatchFor (Taker) {
    if ((Taker == null) || (typeof Taker !== 'object')) {
      throw new TypeError('"Taker" should be an object');
    }

    let TakerIndex = TakerList.indexOf(Taker);
    if (TakerIndex >= 0) {
      TakerList.splice(TakerIndex,1)
      if (TakerList.length === 0) {
        Bangle.removeListener('accel',handleTilt);
      }
    }

    return this;
  }
  
/**** clearWatches ****/

  function clearWatches () {
    if (TakerList.length > 0) {
      TakerList.splice(0,TakerList.length);
      Bangle.removeListener('accel',handleTilt);
    }
    return this;
  }

/**** informTakers ****/

  function informTakers (x,y) {
    for (let i = 0, l = TakerList.length; i < l; i++) {
      let Method = TakerList[i]['onTilt'];
      if (typeof Method === 'function') {
        try {
          Method.call(TakerList[i],x,y);
        } catch (Signal) { /* nop */ }
      }
    }
  }

/**** TiltWatcher object ****/

  TiltWatcher = {
    get Sensitivity () { return _Sensitivity },
    set Sensitivity (Value) {
      switch (true) {
        case (Value == null):
          _Sensitivity = 1;
          break;
        case (typeof Value === 'number') || (Value instanceof Number):
          _Sensitivity = (Value <= 0 ? 1 : Value);
          break;
        default:
          throw new TypeError('number expected');
      }
    },

    setWatchFor:setWatchFor, clearWatchFor:clearWatchFor,
    clearWatches:clearWatches
  };
  
/**** let TiltWatcher actually watch the device's tilt ****/

  function handleTilt (ValueSet) {
    let Width  = g.getWidth();
    let Height = g.getHeight();

    let x = E.clip(Math.round((1 + _Sensitivity*ValueSet.x)*Width/2),  0,Width);
    let y = E.clip(Math.round((1 + _Sensitivity*ValueSet.y)*Height/2), 0,Height);

    informTakers(x,y);
  }
})();