handler(ev) {
    let eventType = MOUSE_INPUT_MAP[ev.type];

    // on start we want to have the left mouse button down
    if (eventType & INPUT_START && ev.button === 0) {
      this.pressed = true;
    }

    if (eventType & INPUT_MOVE && ev.which !== 1) {
      eventType = INPUT_END;
    }

    // mouse must be down
    if (!this.pressed) {
      return;
    }

    if (eventType & INPUT_END) {
      this.pressed = false;
    }

    this.callback(this.manager, eventType, {
      pointers: [ev],
      changedPointers: [ev],
      pointerType: INPUT_TYPE_MOUSE,
      srcEvent: ev
    });
  }
@private
handle mouse events
@param {Object} ev