当imageUrl在颤动中动态变化时，CachedNetworkImage不显示占位符

import 'package:cached_network_image/cached_network_image.dart';
import 'package:flutter/material.dart';

class MyWidget extends StatefulWidget {
  final String imageUrl;

  MyWidget({Key key, this.imageUrl}) : super(key: key);

  @override
  _MyWidgetState createState() => _MyWidgetState();
}

class _MyWidgetState extends State<MyWidget> {
  Key _cachedNetworkImageKey;

  @override
  void initState() {
    super.initState();
    _cachedNetworkImageKey = UniqueKey();
  }

  @override
  void didUpdateWidget(MyWidget oldWidget) {
    super.didUpdateWidget(oldWidget);
    if (widget.imageUrl != oldWidget.imageUrl) {
      setState(() {
        _cachedNetworkImageKey = UniqueKey();
      });
    }
  }

  @override
  Widget build(BuildContext context) {
    return CachedNetworkImage(
      key: _cachedNetworkImageKey,
      imageUrl: widget.imageUrl,
      placeholder: (context, url) => CircularProgressIndicator(),
      errorWidget: (context, url, error) => Icon(Icons.error),
    );
  }
}