feat(ngMock/$httpBackend): flush requests in any order

Previously, requests were flushed in the order in which they were made.
With this change, it is possible to flush requests in any order. This is useful for simulating more
realistic scenarios, where parallel requests may be completed in any order.

Partially addresses #13717.

Closes #14967

Author: dizel3d

src/ngMock/angular-mocks.js

@@ -1789,24 +1789,30 @@ function createHttpBackendMock($rootScope, $timeout, $delegate, $browser) {
    * @ngdoc method
    * @name $httpBackend#flush
    * @description
-   * Flushes all pending requests using the trained responses.
-   *
-   * @param {number=} count Number of responses to flush (in the order they arrived). If undefined,
-   *   all pending requests will be flushed. If there are no pending requests when the flush method
-   *   is called an exception is thrown (as this typically a sign of programming error).
+   * Flushes pending requests in the order they arrived beginning at specified request using the trained responses.
+   * If there are no pending requests to flush when the method is called
+   * an exception is thrown (as this typically a sign of programming error).
+   *
+   * @param {number=} count Number of responses to flush. If undefined,
+   *   all pending requests from `skip` will be flushed.
+   * @param {number=} [skip=0] Number of pending requests to skip before flushing.
+   *   So it specifies the first request to flush.
    */
-  $httpBackend.flush = function(count, digest) {
+  $httpBackend.flush = function(count, skip, digest) {
     if (digest !== false) $rootScope.$digest();
-    if (!responses.length) throw new Error('No pending request to flush !');
+
+    skip = skip || 0;
+    if (skip >= responses.length) throw new Error('No pending request to flush !');
 
     if (angular.isDefined(count) && count !== null) {
       while (count--) {
-        if (!responses.length) throw new Error('No more pending request to flush !');
-        responses.shift()();
+        var part = responses.splice(skip, 1);
+        if (!part.length) throw new Error('No more pending request to flush !');
+        part[0]();
       }
     } else {
-      while (responses.length) {
-        responses.shift()();
+      while (responses.length > skip) {
+        responses.splice(skip, 1)[0]();
       }
     }
     $httpBackend.verifyNoOutstandingExpectation(digest);

test/ng/httpSpec.js

@@ -2212,7 +2212,7 @@ describe('$http with $applyAsync', function() {
     // Ensure requests are sent
     $rootScope.$digest();
 
-    $httpBackend.flush(null, false);
+    $httpBackend.flush(null, null, false);
     expect($rootScope.$applyAsync).toHaveBeenCalledOnce();
     expect(handler).not.toHaveBeenCalled();
 
@@ -2230,7 +2230,7 @@ describe('$http with $applyAsync', function() {
     // Ensure requests are sent
     $rootScope.$digest();
 
-    $httpBackend.flush(null, false);
+    $httpBackend.flush(null, null, false);
     expect(log).toEqual([]);
 
     $browser.defer.flush();
@@ -2255,7 +2255,7 @@ describe('$http with $applyAsync', function() {
     expect(log).toEqual(['response 1', 'response 2']);
 
     // Finally, third response is received, and a second coalesced $apply is started
-    $httpBackend.flush(null, false);
+    $httpBackend.flush(null, null, false);
     $browser.defer.flush();
     expect(log).toEqual(['response 1', 'response 2', 'response 3']);
   });

test/ngMock/angular-mocksSpec.js

@@ -1532,6 +1532,36 @@ describe('ngMock', function() {
       });
 
 
+      it('should flush given number of pending requests beginning at specified request', function() {
+        var dontCallMe = jasmine.createSpy('dontCallMe');
+
+        hb.when('GET').respond(200, '');
+        hb('GET', '/some', null, dontCallMe);
+        hb('GET', '/some', null, callback);
+        hb('GET', '/some', null, callback);
+        hb('GET', '/some', null, dontCallMe);
+
+        hb.flush(2, 1);
+        expect(dontCallMe).not.toHaveBeenCalled();
+        expect(callback).toHaveBeenCalledTimes(2);
+      });
+
+
+      it('should flush all pending requests beginning at specified request', function() {
+        var dontCallMe = jasmine.createSpy('dontCallMe');
+
+        hb.when('GET').respond(200, '');
+        hb('GET', '/some', null, dontCallMe);
+        hb('GET', '/some', null, dontCallMe);
+        hb('GET', '/some', null, callback);
+        hb('GET', '/some', null, callback);
+
+        hb.flush(null, 2);
+        expect(dontCallMe).not.toHaveBeenCalled();
+        expect(callback).toHaveBeenCalledTimes(2);
+      });
+
+
       it('should throw exception when flushing more requests than pending', function() {
         hb.when('GET').respond(200, '');
         hb('GET', '/url', null, callback);
@@ -1546,8 +1576,9 @@ describe('ngMock', function() {
 
         hb.when('GET').respond(200, '');
         hb('GET', '/some', null, callback);
-        hb.flush();
+        expect(function() {hb.flush(null, 1);}).toThrowError('No pending request to flush !');
 
+        hb.flush();
         expect(function() {hb.flush();}).toThrowError('No pending request to flush !');
       });