code: 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;
      }
    }
docs: Event handlers / actions 
  Handle user keyboard interactions
  @param {KeyboardEvent} event keydown event