2种方式 ,推荐使用第2种

1. FadeInImage.assetNetwork

new FadeInImage.assetNetwork(
              placeholder: 'images/logo.png',
              image: imageUrl,
              width: 120,
              fit: BoxFit.fitWidth,
            )

2.CachedNetworkImage

new CachedNetworkImage(
                          imageUrl:
                             url,
                          placeholder: (context, url) => new Image.asset(
                              "image/news_big_default.png",
                              width: MediaQuery.of(context).size.width - 30,
                              height: (MediaQuery.of(context).size.width - 30) *9 /16),
                          errorWidget: (context, url, error) =>
                              new Image.asset("image/news_big_default.png"),
                        )