Remove wasteful checks from shouldYield

`shouldYield` will currently return `true` if there's a higher priority task in the Scheduler queue. Since we yield every 5ms anyway, this doesn't really have any practical benefit. On the contrary, the extra checks on every `shouldYield` call are wasteful.

Author: acdlite

packages/react/src/__tests__/ReactProfiler-test.internal.js

@@ -2481,15 +2481,11 @@ describe('Profiler', () => {
 // Errors that happen inside of a subscriber should throw,
 throwInOnWorkStarted=true;
 expect(Scheduler).toFlushAndThrow('Expected error onWorkStarted');
-// Rendering was interrupted by the error that was thrown
-expect(Scheduler).toHaveYielded([]);
-// Rendering continues in the next task
-expect(Scheduler).toFlushAndYield(['Component:text']);
 throwInOnWorkStarted=false;
+// Rendering was interrupted by the error that was thrown, then
+// continued and finished in the next task.
+expect(Scheduler).toHaveYielded(['Component:text']);
 expect(onWorkStarted).toHaveBeenCalled();
-
-// But the React work should have still been processed.
-expect(Scheduler).toFlushAndYield([]);
 consttree=renderer.toTree();
 expect(tree.type).toBe(Component);
 expect(tree.props.children).toBe('text');

packages/scheduler/src/Scheduler.js

@@ -393,21 +393,6 @@ function unstable_getCurrentPriorityLevel() {
 returncurrentPriorityLevel;
 }
 
-functionunstable_shouldYield(){
-constcurrentTime=getCurrentTime();
-advanceTimers(currentTime);
-constfirstTask=peek(taskQueue);
-return(
-(firstTask!==currentTask&&
-currentTask!==null&&
-firstTask!==null&&
-firstTask.callback!==null&&
-firstTask.startTime<=currentTime&&
-firstTask.expirationTime<currentTask.expirationTime)||
-shouldYieldToHost()
-);
-}
-
 constunstable_requestPaint=requestPaint;
 
 export{
@@ -422,7 +407,7 @@ export {
 unstable_cancelCallback,
 unstable_wrapCallback,
 unstable_getCurrentPriorityLevel,
-unstable_shouldYield,
+shouldYieldToHostasunstable_shouldYield,
 unstable_requestPaint,
 unstable_continueExecution,
 unstable_pauseExecution,

packages/scheduler/src/__tests__/Scheduler-test.js

@@ -249,7 +249,7 @@ describe('Scheduler', () => {
 });
 
 it(
-'continuations are interrupted by higher priority work scheduled '+
+'continuations do not block higher priority work scheduled '+
 'inside an executing callback',
 ()=>{
 consttasks=[
@@ -272,8 +272,8 @@ describe('Scheduler', () => {
 Scheduler.unstable_yieldValue('High pri');
 });
 }
-if(tasks.length>0&&shouldYield()){
-Scheduler.unstable_yieldValue('Yield!');
+if(tasks.length>0){
+// Return a continuation
 returnwork;
 }
 }
@@ -283,9 +283,8 @@ describe('Scheduler', () => {
 'A',
 'B',
 'Schedule high pri',
-// Even though there's time left in the frame, the low pri callback
-// should yield to the high pri callback
-'Yield!',
+// The high pri callback should fire before the continuation of the
+// lower pri work
 'High pri',
 // Continue low pri work
 'C',
@@ -662,7 +661,7 @@ describe('Scheduler', () => {
 const[label,ms]=task;
 Scheduler.unstable_advanceTime(ms);
 Scheduler.unstable_yieldValue(label);
-if(tasks.length>0&&shouldYield()){
+if(tasks.length>0){
 returnwork;
 }
 }