网上可能有很多实现的插件，有些动不动就上千行代码， 其实很简单 只需要在源码的基础上加一个本地缓存就行，

毕竟源码是最可靠的 

https://github.com/dikeboy/flutter-cache-image-local

Flutter  自带的有2个 图片的Widget ,Image 和FadeInImage, 从字面上就能理解 FadeIn是多了个预加载的图，

FadeInImage 有两个 方法 ，memoryNetwork 和assertNetwork  预加载图来源于二进制 或者 assert，如果是动态生成的 就需要用memoryNetwork

网络图片是通过NetworkImage来实现加载， 

文件  assert  memory  networkImage 都是继承自Image_provider,

Image_provider本身已经带了一个内存缓存，大概就类似安卓的Lrucache

 ImageStream resolve(ImageConfiguration configuration) {
    assert(configuration != null);
    final ImageStream stream = ImageStream();
    T obtainedKey;
    obtainKey(configuration).then<void>((T key) {
      obtainedKey = key;
      stream.setCompleter(PaintingBinding.instance.imageCache.putIfAbsent(key, () => load(key)));

这里的imageCache就是Flutter 自带的缓存

我们要实现缓存到本地   肯定需要 在内存缓存之后    网络请求之前

先来看下NetworkImage的源码

class NetworkImage extends ImageProvider<NetworkImage> {
  /// Creates an object that fetches the image at the given URL.
  ///
  /// The arguments must not be null.
  const NetworkImage(this.url, { this.scale = 1.0 , this.headers })
      : assert(url != null),
        assert(scale != null);

  /// The URL from which the image will be fetched.
  final String url;

  /// The scale to place in the [ImageInfo] object of the image.
  final double scale;

  /// The HTTP headers that will be used with [HttpClient.get] to fetch image from network.
  final Map<String, String> headers;

  @override
  Future<NetworkImage> obtainKey(ImageConfiguration configuration) {
    return SynchronousFuture<NetworkImage>(this);
  }

  @override
  ImageStreamCompleter load(NetworkImage key) {
    return MultiFrameImageStreamCompleter(
      codec: _loadAsync(key), //这个就是主要的图片网络加载方法 ， 我们主要是需要修改这个方法
      scale: key.scale,
      informationCollector: (StringBuffer information) {
        information.writeln('Image provider: $this');
        information.write('Image key: $key');
      }
    );
  }

  static final HttpClient _httpClient = HttpClient();
  方法很简单，拿回数据 转换为二进制返回/
  Future<ui.Codec> _loadAsync(NetworkImage key) async {
    assert(key == this);

    final Uri resolved = Uri.base.resolve(key.url);
    final HttpClientRequest request = await _httpClient.getUrl(resolved);
    headers?.forEach((String name, String value) {
      request.headers.add(name, value);
    });
    final HttpClientResponse response = await request.close();
    if (response.statusCode != HttpStatus.ok)
      throw Exception('HTTP request failed, statusCode: ${response?.statusCode}, $resolved');

    final Uint8List bytes = await consolidateHttpClientResponseBytes(response);
    if (bytes.lengthInBytes == 0)
      throw Exception('NetworkImage is an empty file: $resolved');

    return await PaintingBinding.instance.instantiateImageCodec(bytes);
  }

  @override
  bool operator ==(dynamic other) {
    if (other.runtimeType != runtimeType)
      return false;
    final NetworkImage typedOther = other;
    return url == typedOther.url
        && scale == typedOther.scale;
  }

  @override
  int get hashCode => hashValues(url, scale);

  @override
  String toString() => '$runtimeType("$url", scale: $scale)';
}

这里需要引入 几个插件   其实这几个就算不修改这里 项目中肯定也是必须的

http: ^0.12.0   网络请求 这个毫无疑问
transparent_image: ^0.1.0  //这是一个预加载的透明图 ，如果需要用assert图或者是自定义可以不需要  这里为了方便
path_provider: ^0.4.1   //这是获取 安卓  IOS 文件缓存路径  
crypto: ^2.0.6   //这个主要是MD5  base64之类 转码   我们根据图片的md5(url)做为key来缓存

下面直接贴修改后的 NetworkImage

class MyNetworkImage extends ImageProvider<MyNetworkImage> {
  /// Creates an object that fetches the image at the given URL.
  ///
  /// The arguments must not be null.
  const MyNetworkImage(this.url, { this.scale = 1.0 , this.headers,this.sdCache })
      : assert(url != null),
        assert(scale != null);

  /// The URL from which the image will be fetched.
  final String url;

  final bool sdCache;  //加一个标志为 是否需要缓存到sd卡

  /// The scale to place in the [ImageInfo] object of the image.
  final double scale;

  /// The HTTP headers that will be used with [HttpClient.get] to fetch image from network.
  final Map<String, String> headers;

  @override
  Future<MyNetworkImage> obtainKey(ImageConfiguration configuration) {
    return SynchronousFuture<MyNetworkImage>(this);
  }

  @override
  ImageStreamCompleter load(MyNetworkImage key) {
    return MultiFrameImageStreamCompleter(
        codec: _loadAsync(key),
        scale: key.scale,
        informationCollector: (StringBuffer information) {
          information.writeln('Image provider: $this');
          information.write('Image key: $key');
        }
    );
  }


  Future<Codec> _loadAsync(MyNetworkImage key) async {
    assert(key == this);
  //本地已经缓存过就直接返回图片
    if(sdCache!=null){ final Uint8List bytes =await _getFromSdcard(key.url); if (bytes!=null&&bytes.lengthInBytes!=null&&bytes.lengthInBytes!= 0) { print("success"); return await PaintingBinding.instance.instantiateImageCodec(bytes); } }
    final Uri resolved = Uri.base.resolve(key.url);
    http.Response response  = await http.get(resolved);

    if (response.statusCode != HttpStatus.ok)
      throw Exception('HTTP request failed, statusCode: ${response?.statusCode}, $resolved');

    final Uint8List bytes = await response.bodyBytes;
//网络请求结束后缓存图片到本地
    if(sdCache!=null&&bytes.lengthInBytes != 0){ _saveToImage(bytes, key.url); } if (bytes.lengthInBytes == 0)
      throw Exception('MyNetworkImage is an empty file: $resolved');

    return await PaintingBinding.instance.instantiateImageCodec(bytes);
  }

  @override
  bool operator ==(dynamic other) {
    if (other.runtimeType != runtimeType)
      return false;
    final MyNetworkImage typedOther = other;
    return url == typedOther.url
        && scale == typedOther.scale;
  }

  @override
  int get hashCode => hashValues(url, scale);

  @override
  String toString() => '$runtimeType("$url", scale: $scale)';

图片路径MD5一下 缓存到本地
  void _saveToImage(Uint8List mUint8List,String name) async { name = md5.convert(convert.utf8.encode(name)).toString(); Directory dir = await getTemporaryDirectory(); String path = dir.path +"/"+name; var file = File(path); bool exist =  await file.exists(); print("path =${path}"); if(!exist) File(path).writeAsBytesSync(mUint8List); }
   _getFromSdcard(String name) async{ name = md5.convert(convert.utf8.encode(name)).toString(); Directory dir = await getTemporaryDirectory(); String path = dir.path +"/"+name; var file = File(path); bool exist =  await file.exists(); if(exist){ final Uint8List bytes = await file.readAsBytes(); return bytes; } return null; }
}

如果需要直接修改FadeInImage 或者Image 直接把代码烤到一个新的dart文件  把NetworkImage 全部改成MyNetworkImage就行了