feat(typeahead): add min-length === 0 support

- Adds support for `typeahead-min-length` being equal to `0`

Closes #764
Closes #2324
Closes #4789

Author: chenyuzhcy

Diff for: src/typeahead/docs/readme.md

@@ -45,7 +45,7 @@ The typeahead directives provide several attributes:
 
 * `typeahead-min-length` <i class="glyphicon glyphicon-eye-open"></i>
    _(Defaults: 1)_ :
-   Minimal no of characters that needs to be entered before typeahead kicks-in. Must be greater than or equal to 1.
+   Minimal no of characters that needs to be entered before typeahead kicks-in. Must be greater than or equal to 0.
 
 * `typeahead-no-results` <i class="glyphicon glyphicon-eye-open"></i>
    _(Defaults: angular.noop)_ :

Diff for: src/typeahead/test/typeahead.spec.js

@@ -1204,6 +1204,14 @@ describe('typeahead tests', function() {
       changeInputValueTo(element, '');
       expect(element).toBeOpenWithActive(3, 0);
     });
+
+    it('should open typeahead when input is focused and value is empty if defined threshold is 0', function () {
+      var element = prepareInputEl('<div><input ng-model="result" uib-typeahead="item for item in source | filter:$viewValue" typeahead-min-length="0"></div>');
+      var inputEl = findInput(element);
+      inputEl.focus();
+      $scope.$digest();
+      expect(element).toBeOpenWithActive(3, 0);
+    });
   });
 
   describe('event listeners', function() {

Diff for: src/typeahead/typeahead.js

@@ -385,6 +385,13 @@ angular.module('ui.bootstrap.typeahead', ['ui.bootstrap.position'])
       }
     });
 
+    element.bind('focus', function () {
+      hasFocus = true;
+      if (minLength === 0 && !modelCtrl.$viewValue) {
+        getMatchesAsync(modelCtrl.$viewValue);
+      }
+    });
+
     element.bind('blur', function() {
       if (isSelectOnBlur && scope.matches.length && scope.activeIdx !== -1 && !selected) {
         selected = true;
@@ -393,7 +400,8 @@ angular.module('ui.bootstrap.typeahead', ['ui.bootstrap.position'])
         });
       }
       if (!isEditable && modelCtrl.$error.editable) {
-          element.val('');
+        modelCtrl.$viewValue = '';
+        element.val('');
       }
       hasFocus = false;
       selected = false;