docs: function(event) {
      var touches = event.touches[0];
  
      // Reset the states.
      this.touchEnd(event);
  
      // Determine which key to simulate.
      if (touches.pageY < window.innerHeight * 0.3) {
        this.key(true, {keyCode: 38});
      } else if (touches.pageY > window.innerHeight * 0.7) {
        this.key(true, {keyCode: 40});
      } else if (touches.pageX < window.innerWidth * 0.5) {
        this.key(true, {keyCode: 37});
      } else if (touches.pageX > window.innerWidth * 0.5) {
        this.key(true, {keyCode: 39});
      }
    }