Fall back to 'setTimeout' when 'requestAnimationFrame' is not called

fixtures/schedule/index.html

@@ -23,7 +23,7 @@ <h2>Tests:</h2>
     <ol>
       <li>
         <button onClick="runTestOne()">Run Test 1</button>
-        <p>Calls the callback with the frame when not blocked:</p>
+        <p>Calls the callback within the frame when not blocked:</p>
         <div><b>Expected:</b></div>
         <div id="test-1-expected">
         </div>
@@ -79,6 +79,15 @@ <h2>Tests:</h2>
         <p><b>IMPORTANT:</b> Open the console when you run this! Inspect the logs there!</p>
         <button onClick="runTestSix()">Run Test 6</button>
       </li>
+      <li>
+        <p>Continues calling callbacks even when user switches away from this tab</p>
+        <button onClick="runTestSeven()">Run Test 7</button>
+        <div><b>Click the button above, observe the counter, then switch to
+            another tab and switch back:</b></div>
+        <div id="test-7">
+        </div>
+        <div> If the counter advanced while you were away from this tab, it's correct.</div>
+      </li>
     </ol>
     <script src="../../build/dist/react-scheduler.development.js"></script>
     <script src="https://unpkg.com/babel-standalone@6/babel.js"></script>
@@ -464,6 +473,22 @@ <h2>Tests:</h2>
     console.log('scheduled cbE');
   };
 }
+
+function runTestSeven() {
+  // Test 7
+  // Calls callbacks, continues calling them even when this tab is in the
+  // background
+  clearTestResult(7);
+  let counter = -1;
+  function incrementCounterAndScheduleNextCallback() {
+    const counterNode = document.getElementById('test-7');
+    counter++;
+    counterNode.innerHTML = counter;
+    waitForTimeToPass(100);
+    scheduleWork(incrementCounterAndScheduleNextCallback);
+  }
+  scheduleWork(incrementCounterAndScheduleNextCallback);
+}
     </script type="text/babel">
   </body>
 </html>

packages/react-scheduler/src/ReactScheduler.js

@@ -117,6 +117,7 @@ if (!canUseDOM) {
   };
 } else {
   const localRequestAnimationFrame = requestAnimationFrame;
+  const localCancelAnimationFrame = cancelAnimationFrame;
 
   let headOfPendingCallbacksLinkedList: CallbackConfigType | null = null;
   let tailOfPendingCallbacksLinkedList: CallbackConfigType | null = null;
@@ -128,6 +129,27 @@ if (!canUseDOM) {
   let isIdleScheduled = false;
   let isAnimationFrameScheduled = false;
 
+  // requestAnimationFrame does not run when the tab is in the background.
+  // if we're backgrounded we prefer for that work to happen so that the page
+  // continues	to load in the background.
+  // so we also schedule a 'setTimeout' as a fallback.
+  const animationFrameTimeout = 100;
+  let rafID;
+  let timeoutID;
+  const scheduleAnimationFrameWithFallbackSupport = function(callback) {
+    // schedule rAF and also a setTimeout
+    rafID = localRequestAnimationFrame(function(...args) {
+      // cancel the setTimeout
+      localClearTimeout(timeoutID);
+      callback(...args);
+    });
+    timeoutID = localSetTimeout(function() {
+      // cancel the requestAnimationFrame
+      localCancelAnimationFrame(rafID);
+      callback(now());
+    }, animationFrameTimeout);
+  };
+
   let frameDeadline = 0;
   // We start out assuming that we run at 30fps but then the heuristic tracking
   // will adjust this value to a faster fps if we get more frequent animation
@@ -266,7 +288,7 @@ if (!canUseDOM) {
       if (!isAnimationFrameScheduled) {
         // Schedule another animation callback so we retry later.
         isAnimationFrameScheduled = true;
-        localRequestAnimationFrame(animationTick);
+        scheduleAnimationFrameWithFallbackSupport(animationTick);
       }
     }
   };
@@ -347,7 +369,7 @@ if (!canUseDOM) {
       // might want to still have setTimeout trigger scheduleWork as a backup to ensure
       // that we keep performing work.
       isAnimationFrameScheduled = true;
-      localRequestAnimationFrame(animationTick);
+      scheduleAnimationFrameWithFallbackSupport(animationTick);
     }
     return scheduledCallbackConfig;
   };