feat(typeahead): add dynamic min length support

- Add support for dynamically changed min length

Closes #5363

src/typeahead/test/typeahead.spec.js

@@ -196,6 +196,30 @@ describe('typeahead tests', function() {
       expect(element).toBeClosed();
     });
 
+
+    it('should support changing min-length', function() {
+        $scope.typeAheadMinLength = 2;	
+        var element = prepareInputEl('<div><input ng-model="result" uib-typeahead="item for item in source | filter:$viewValue" typeahead-min-length="typeAheadMinLength"></div>');
+
+        changeInputValueTo(element, 'b');
+
+        expect(element).toBeClosed();
+
+        $scope.typeAheadMinLength = 0;
+        $scope.$digest();
+        changeInputValueTo(element, '');
+
+        expect(element).toBeOpenWithActive(3, 0);
+
+        $scope.typeAheadMinLength = 2;
+        $scope.$digest();
+        changeInputValueTo(element, 'b');
+
+        expect(element).toBeClosed();
+    });
+
+
+
     it('should support custom model selecting function', function() {
       $scope.updaterFn = function(selectedItem) {
         return 'prefix' + selectedItem;

src/typeahead/typeahead.js

@@ -39,6 +39,10 @@ angular.module('ui.bootstrap.typeahead', ['ui.bootstrap.debounce', 'ui.bootstrap
       minLength = 1;
     }
 
+    originalScope.$watch(attrs.typeaheadMinLength, function (newVal) {
+        minLength = !newVal && newVal !== 0 ? 1 : newVal;
+    });
+
     //minimal wait time after last character typed before typeahead kicks-in
     var waitTime = originalScope.$eval(attrs.typeaheadWaitMs) || 0;