一种方法是使用变量isBannerLoaded沿着BannerAdListener。

import 'dart:developer';

import 'package:flutter/material.dart';
import 'package:google_mobile_ads/google_mobile_ads.dart';
import 'package:provider/provider.dart';

class AdBanner extends StatefulWidget {
  final String adUnitId;
  final AdSize size;
  const AdBanner({this.adUnitId, this.size});

  @override
  State<AdBanner> createState() => _AdBannerState();

  static AdSize getBannerSize() {
    final width =
        MediaQueryData.fromWindow(WidgetsBinding.instance.window).size.width;
    final height =
        MediaQueryData.fromWindow(WidgetsBinding.instance.window).size.height;
    if ((width >= 460 && width <= 500) || (height >= 800 && height <= 900)) {
      return AdSize(width: 336, height: 280);
    } else if ((width >= 360 && width <= 375) ||
        (height >= 600 && height <= 700)) {
      return AdSize(width: 320, height: 100);
    } else if ((width >= 375 && width <= 414) ||
        (height >= 700 && height <= 800)) {
      return AdSize(width: 300, height: 250);
    } else if ((width >= 300 && width <= 360) ||
        (height >= 400 && height <= 600)) {
      return AdSize(width: 320, height: 50);
    } else {
      return AdSize(width: 320, height: 50);
    }
  }
}

class _AdBannerState extends State<AdBanner> {
  bool isBannerLoaded = false;
  BannerAd myBanner;
  @override
  Widget build(BuildContext context) {
    print('AdBanner build : ' + widget.adUnitId);
   
    final BannerAd bannerAd = myBanner;
    if (isBannerLoaded && bannerAd != null) {
      return Container(
          height: bannerAd.size.height.toDouble(),
          width: bannerAd.size.width.toDouble(),
          child: AdWidget(ad: bannerAd));
    } else {
      return Container(
      );
    }

   
  }

  @override
  void didChangeDependencies() {
    super.didChangeDependencies();
    myBanner = BannerAd(
      adUnitId: widget.adUnitId ?? '/234721286/NY_KB_Home_Micrec_23',
      size: widget.size ?? AdSize.banner,
      request: AdRequest(),
      listener: BannerAdListener(
        onAdLoaded: (Ad ad) {
          print('AdBanner onAdLoaded : ' + ad.adUnitId);
          setState(() {
            isBannerLoaded = true;
          });
        },
        // Called when an ad request failed.
        onAdFailedToLoad: (Ad ad, LoadAdError error) {
          print('inside failed');
          // Dispose the ad here to free resources.
          ad.dispose();
          print('failed to load: $error');
        },
        // Called when an ad opens an overlay that covers the screen.
        onAdOpened: (Ad ad) => print('Ad opened.'),
        // Called when an ad removes an overlay that covers the screen.
        onAdClosed: (Ad ad) => print('Ad closed.'),
        // Called when an impression occurs on the ad.
        onAdImpression: (Ad ad) => print('Ad impression.'),
      ),
    )..load();
  }

  @override
  void dispose() {
    super.dispose();
    myBanner.dispose();
  }
}

考虑到google_mobile_ads Flutter包确实使用了Google移动的Ads SDK for Android，这些优化应该适用。
使用这个有什么不好的地方吗？如果它只是使加载更好而没有任何潜在的“副作用”，为什么默认情况下不启用？
与described here一样，激活这些优化意味着需要延长处理时间的初始化和广告加载任务将被卸载到后台线程。
This thread没有提到任何缺点，参考这个"Get started" guide。
这意味着它不是默认的，因为这些优化仍处于Beta阶段。
昨天发布的new testing features for ad inspector可以帮助您验证这些优化效果。