function keydown (event) {
    switch (event.keyCode) {
      case $mdConstant.KEY_CODE.LEFT_ARROW:
        event.preventDefault();
        incrementIndex(-1, true);
        break;
      case $mdConstant.KEY_CODE.RIGHT_ARROW:
        event.preventDefault();
        incrementIndex(1, true);
        break;
      case $mdConstant.KEY_CODE.SPACE:
      case $mdConstant.KEY_CODE.ENTER:
        event.preventDefault();
        if (!locked) select(ctrl.focusIndex);
        break;
      case $mdConstant.KEY_CODE.TAB:
        // On tabbing out of the tablist, reset hasFocus to reset ng-focused and
        // its md-focused class if the focused tab is not the active tab.
        if (ctrl.focusIndex !== ctrl.selectedIndex) {
          ctrl.focusIndex = ctrl.selectedIndex;
        }
        break;
    }
  }
Handle user keyboard interactions
@param {KeyboardEvent} event keydown event