in the components of Angular dart, in hook such as, ngAfterViewInit ngAfterContentInit when onInit, the result of querySelector is that null,ngAfterContentChecked will be called three times, and the correct result can only be obtained the third time. It seems that the old version of AngularDart has a ShadowRoot callback that can accurately know the dom ready. This method can no longer be found in the new version of Angular. Find a similar method.