Commits on Oct 20, 2020

1. fix(core): markDirty() should only mark flags when really scheduling … …

   …tick.
   
   Close angular#39296
   
   Fix an issue that `markDirty()` will not trigger change detection.
   
   The case is for example we have the following component.
   
   ```
   export class AppComponent implements OnInit {
     constructor(private router: Router) {}
   
     ngOnInit() {
       this.router.events
         .pipe(filter((e) => e instanceof NavigationEnd))
         .subscribe(() => ɵmarkDirty(this));
     }
   }
   
   export class CounterComponent implements OnInit, OnDestroy {
     ngOnInit() {
       this.countSubject.pipe(takeUntil(this.destroy)).subscribe((count) => {
         this.count = count;
         ɵmarkDirty(this);
       });
     }
   ```
   
   Then the app navigate from `AppComponent` to `CounterComponent`,
   so there are 2 `markDirty()` call at in a row.
   
   The `1st` call is from `AppComponent` when router changed, the
   `2nd` call is from `CounterComponent.ngOnInit()`.
   
   And the `markDirty()->scheduleTick()` code look like this
   
   ```
   function scheduleTick(rootContext, flags) {
       const nothingScheduled = rootContext.flags === 0 /* Empty */;
       rootContext.flags |= flags;
       if (nothingScheduled && rootContext.clean == _CLEAN_PROMISE) {
         rootContext.schedule(() => {
   	...
           if (rootContext.flags & RootContextFlags.DetectChanges)
             rootContext.flags &= ~RootContextFlags.DetectChanges;
             tickContext();
   
           rootContext.clean = _CLEAN_PROMISE;
           ...
         });
   ```
   
   So in this case, the `1st` markDirty() will
   1. set rootContext.flags = 1
   2. before `tickContext()`, reset rootContext.flags = 0
   3. inside `tickContext()`, it will call `CounterComponent.ngOnint()`,
      so the `2nd` markDirty() is called.
   4. and the `2nd` scheduleTick is called, `nothingScheduled` is true,
      but rootContext.clean is not `_CLEAN_PROMISE` yet, since the `1st` markDirty tick
      is still running.
   5. So nowhere will reset the `rootContext.flags`.
   6. then in the future, any other `markDirty()` call will not trigger the tick, since
      `nothingScheduled` is always false.
   
   So `nothingScheduled` means no tick is scheduled, `rootContext.clean === _CLEAN_PROMISE`
   means no tick is running.
   So we should set the flags to `rootContext` only when `no tick is scheudled or running`.

   

   JiaLiPassion committed Oct 20, 2020
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