Angular 如何渲染ngFor

这是ngFor的源代码，这个方法是用Angular 差异标识的变化来调用的，它通知对象或数组中的变化.但如果你注意到代码，there is a 101 which run for each element and inside it we notice for new records, there is a if condition where we run 102, so each row is created one by one!

private _applyChanges(changes: IterableChanges<T>) {
    const viewContainer = this._viewContainer;
    changes.forEachOperation(
        (item: IterableChangeRecord<T>, adjustedPreviousIndex: number|null,
         currentIndex: number|null) => {
          if (item.previousIndex == null) {
            // NgForOf is never "null" or "undefined" here because the differ detected
            // that a new item needs to be inserted from the iterable. This implies that
            // there is an iterable value for "_ngForOf".
            viewContainer.createEmbeddedView(
                this._template, new NgForOfContext<T, U>(item.item, this._ngForOf!, -1, -1),
                currentIndex === null ? undefined : currentIndex);
          } else if (currentIndex == null) {
            viewContainer.remove(
                adjustedPreviousIndex === null ? undefined : adjustedPreviousIndex);
          } else if (adjustedPreviousIndex !== null) {
            const view = viewContainer.get(adjustedPreviousIndex)!;
            viewContainer.move(view, currentIndex);
            applyViewChange(view as EmbeddedViewRef<NgForOfContext<T, U>>, item);
          }
        });

    for (let i = 0, ilen = viewContainer.length; i < ilen; i++) {
      const viewRef = <EmbeddedViewRef<NgForOfContext<T, U>>>viewContainer.get(i);
      const context = viewRef.context;
      context.index = i;
      context.count = ilen;
      context.ngForOf = this._ngForOf!;
    }

    changes.forEachIdentityChange((record: any) => {
      const viewRef = <EmbeddedViewRef<NgForOfContext<T, U>>>viewContainer.get(record.currentIndex);
      applyViewChange(viewRef, record);
    });
  }