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;
 
// 使用CachedNetworkImageProvider作为图片提供者
class CachedNetworkImage extends StatelessWidget {
  final String imageUrl;
  final double width;
  final double height;
  final BoxFit fit;
 
  const CachedNetworkImage({
    Key key,
    @required this.imageUrl,
    this.width,
    this.height,
    this.fit,
  }) : super(key: key);
 
  @override
  Widget build(BuildContext context) {
    return Image(
      image: CachedNetworkImageProvider(imageUrl: imageUrl),
      width: width,
      height: height,
      fit: fit,
    );
  }
}
 
class CachedNetworkImageProvider extends ImageProvider<CachedNetworkImageProvider> {
  final String imageUrl;
  final http.Client httpClient;
 
  CachedNetworkImageProvider({
    @required this.imageUrl,
    this.httpClient = http.Client(),
  });
 
  @override
  Future<CachedNetworkImageProvider> obtainKey(ImageConfiguration configuration) {
    return SynchronousFuture<CachedNetworkImageProvider>(this);
  }
 
  @override
  ImageStreamCompleter load(CachedNetworkImageProvider key) {
    final PaintingBinding binding = PaintingBinding.instance;
    final ImageStreamCompleter completer = ImageStreamCompleter();
 
    // 内存缓存逻辑
    if (binding.imageCache.putIfAbsent(
        key, () => completer.loader, onError: binding.imageCache.onError) != null) {
      return completer;
    }
 
    // 文件缓存逻辑
    final File tempFile = File(key.imageUrl);
    if (tempFile.existsSync()) {
      completer.setImage(binding.instantiateImageCodec, tempFile.readAsBytesSync());
    } else {
      httpClient
          .get(Uri.parse(key.imageUrl))
          .then((http.Response response) => response.bodyBytes)
          .then((Uint8List bytes) {
        if (completer.isCompleted) {
          return;
        }
        // 将图片数据写入文件缓存
        tempFile.writeAsBytes(bytes).then((_) {
          binding.instantiateImageCodec(bytes).then((codec) {
            if (completer.isCompleted) {
              return;
            }
            completer.setImage(codec);
          }, onError: binding.imageC