Flutter中网络图片加载和缓存源码分析（内存缓存+文件缓存

import 'dart:io';
import 'package:flutter/foundation.dart';
import 'package:flutter/widgets.dart';
import 'package:http/http.dart' as http;
 
// 图片提供者，用于加载网络图片并缓存
class NetworkImageProvider extends ImageProvider<NetworkImageProvider> {
  final String url;
  final http.Client httpClient;
  final bool cacheMemory;
  final bool cacheFile;
 
  const NetworkImageProvider(this.url,
      {this.httpClient = const http.Client(), this.cacheMemory = true, this.cacheFile = true});
 
  @override
  Future<NetworkImageProvider> obtainKey(ImageConfiguration configuration) {
    return SynchronousFuture<NetworkImageProvider>(this);
  }
 
  @override
  ImageStreamCompleter load(NetworkImageProvider key) {
    return MultiFrameImageStreamCompleter(
      codec: _loadAsync(key),
      scale: key.scale,
      informationCollector: (information) {
        information.add(DiagnosticsProperty<ImageProvider>('Image provider', this));
        information.add(DiagnosticsProperty<NetworkImageProvider>('Image key', key));
      },
    );
  }
 
  // 异步加载图片
  Future<ui.Codec> _loadAsync(NetworkImageProvider key) async {
    final bytes = await _getBytes(key.url, key.httpClient);
    if (bytes.isEmpty) {
      throw Exception('无法加载图片');
    }
    return await ui.instantiateImageCodec(bytes);
  }
 
  // 获取图片字节
  Future<Uint8List> _getBytes(String url, http.Client client) async {
    final response = await client.get(Uri.parse(url));
    if (response.statusCode == 200) {
      return response.bodyBytes;
    } else {
      throw Exception('请求图片失败，状态码：${response.statusCode}');
    }
  }
 
  @override
  bool operator ==(Object other) {
    if (other.runtimeType != runtimeType) {
      return false;
    }
    return other is NetworkImageProvider && other.url == url;
  }
 
  @override
  int get hashCode => url.hashCode;
 
  @override
  String toString() => 'NetworkImageProvider("$url")';
}

这段代码实现了一个简单的图片提供者，用于加载网络上的图片并进行缓存。它使用http包来发送HTTP请求，并使用Flutter框架提供的ui库来解码图片。图片的缓存可以是内存缓存也可以是文件缓存，具体取决于cacheMemory和cacheFile标志。这个实现展示了如何创建一个自定义的ImageProvider，并且如何使用MultiFrameImageStreamCompleter来处理异步图片加载。