Handles changes to the searchText property.
@param searchText
@param previousSearchText
function handleSearchText (searchText, previousSearchText) {
    ctrl.index = getDefaultIndex();

    // do nothing on init
    if (searchText === previousSearchText) return;

    updateModelValidators();

    getDisplayValue($scope.selectedItem).then(function (val) {
      // clear selected item if search text no longer matches it
      if (searchText !== val) {
        $scope.selectedItem = null;


        // trigger change event if available
        if (searchText !== previousSearchText) announceTextChange();

        // cancel results if search text is not long enough
        if (!isMinLengthMet()) {
          ctrl.matches = [];

          setLoading(false);
          reportMessages(false, ReportType.Count);

        } else {
          handleQuery();
        }
      }
    });

  }