Add experimental safariYieldStrategy: "time" config option

This greatly improves speed, at least in jasmine-core's own tests.

spec/core/ConfigurationSpec.js

@@ -15,7 +15,8 @@ describe('Configuration', function() {
     'seed',
     'specFilter',
     'extraItStackFrames',
-    'extraDescribeStackFrames'
+    'extraDescribeStackFrames',
+    'safariYieldStrategy'
   ];
   Object.freeze(standardBooleanKeys);
   Object.freeze(allKeys);
@@ -36,6 +37,7 @@ describe('Configuration', function() {
     expect(subject.detectLateRejectionHandling).toEqual(false);
     expect(subject.extraItStackFrames).toEqual(0);
     expect(subject.extraDescribeStackFrames).toEqual(0);
+    expect(subject.safariYieldStrategy).toEqual('count');
   });
 
   describe('copy()', function() {
@@ -137,5 +139,28 @@ describe('Configuration', function() {
       subject.update({ extraDescribeStackFrames: 100000 });
       expect(subject.extraDescribeStackFrames).toEqual(100000);
     });
+
+    it('sets safariYieldStrategy when valid', function() {
+      const subject = new privateUnderTest.Configuration();
+
+      subject.update({ safariYieldStrategy: undefined });
+      expect(subject.safariYieldStrategy).toEqual('count');
+
+      subject.update({ safariYieldStrategy: 'time' });
+      expect(subject.safariYieldStrategy).toEqual('time');
+
+      subject.update({ safariYieldStrategy: 'count' });
+      expect(subject.safariYieldStrategy).toEqual('count');
+    });
+
+    it('rejcts invalid safariYieldStrategy values', function() {
+      const subject = new privateUnderTest.Configuration();
+
+      expect(function() {
+        subject.update({ safariYieldStrategy: 'thyme' });
+      }).toThrowError(
+        "Invalid safariYieldStrategy value. Valid values are 'count' and 'time'."
+      );
+    });
   });
 });

spec/core/StackClearerSpec.js

@@ -180,30 +180,82 @@ describe('StackClearer', function() {
       expect(called).toBe(true);
     });
 
-    it('uses setTimeout instead of queueMicrotask every 10 calls to make sure we release the CPU', function() {
-      const queueMicrotask = jasmine.createSpy('queueMicrotask');
-      const setTimeout = jasmine.createSpy('setTimeout');
-      const global = {
-        ...makeGlobal(),
-        queueMicrotask,
-        setTimeout
-      };
-      const { clearStack } = privateUnderTest.getStackClearer(global);
+    function hasSetTimeoutBehavior(configure) {
+      it('uses setTimeout instead of queueMicrotask every 10 calls', function() {
+        const queueMicrotask = jasmine.createSpy('queueMicrotask');
+        const setTimeout = jasmine.createSpy('setTimeout');
+        const global = {
+          ...makeGlobal(),
+          queueMicrotask,
+          setTimeout
+        };
+        const stackClearer = privateUnderTest.getStackClearer(global);
 
-      for (let i = 0; i < 9; i++) {
-        clearStack(function() {});
-      }
+        if (configure) {
+          configure(stackClearer);
+        }
 
-      expect(queueMicrotask).toHaveBeenCalled();
-      expect(setTimeout).not.toHaveBeenCalled();
+        for (let i = 0; i < 9; i++) {
+          stackClearer.clearStack(function() {});
+        }
 
-      clearStack(function() {});
-      expect(queueMicrotask).toHaveBeenCalledTimes(9);
-      expect(setTimeout).toHaveBeenCalledTimes(1);
+        expect(queueMicrotask).toHaveBeenCalled();
+        expect(setTimeout).not.toHaveBeenCalled();
 
-      clearStack(function() {});
-      expect(queueMicrotask).toHaveBeenCalledTimes(10);
-      expect(setTimeout).toHaveBeenCalledTimes(1);
+        stackClearer.clearStack(function() {});
+        expect(queueMicrotask).toHaveBeenCalledTimes(9);
+        expect(setTimeout).toHaveBeenCalledTimes(1);
+
+        stackClearer.clearStack(function() {});
+        expect(queueMicrotask).toHaveBeenCalledTimes(10);
+        expect(setTimeout).toHaveBeenCalledTimes(1);
+      });
+    }
+
+    hasSetTimeoutBehavior();
+
+    describe('With yield strategy explicitly set to count', function() {
+      hasSetTimeoutBehavior(function(stackClearer) {
+        stackClearer.setSafariYieldStrategy('count');
+      });
+    });
+
+    describe('With yield strategy set to time', function() {
+      beforeEach(function() {
+        jasmine.clock().install();
+        jasmine.clock().mockDate();
+      });
+
+      afterEach(function() {
+        jasmine.clock().uninstall();
+      });
+
+      it('uses setTimeout instead of queueMicrotask every 25 milliseconds', function() {
+        const queueMicrotask = jasmine.createSpy('queueMicrotask');
+        const setTimeout = jasmine.createSpy('setTimeout');
+        const global = {
+          ...makeGlobal(),
+          queueMicrotask,
+          setTimeout
+        };
+        const stackClearer = privateUnderTest.getStackClearer(global);
+        stackClearer.setSafariYieldStrategy('time');
+
+        // 10+ counts should not trigger a setTimeout if they happen fast enough
+        jasmine.clock().tick(24);
+        for (let i = 0; i < 11; i++) {
+          stackClearer.clearStack(function() {});
+        }
+
+        expect(queueMicrotask).toHaveBeenCalled();
+        expect(setTimeout).not.toHaveBeenCalled();
+
+        queueMicrotask.calls.reset();
+        jasmine.clock().tick(1);
+        stackClearer.clearStack(function() {});
+        expect(queueMicrotask).not.toHaveBeenCalled();
+        expect(setTimeout).toHaveBeenCalledTimes(1);
+      });
     });
   }
 

spec/support/jasmine-browser.js

@@ -28,7 +28,8 @@ module.exports = {
     'helpers/resetEnv.js'
   ],
   env: {
-    forbidDuplicateNames: true
+    forbidDuplicateNames: true,
+    safariYieldStrategy: 'time'
   },
   random: true,
   browser: {