如何修改NetworkImage的Headers?

Question

如果设置Headers只会添加新的UA,原来的UA('dart:io')不会被删除

FadeInImage(
          placeholder:  MemoryImage(kTransparentImage),
          image: NetworkImage(
            playlist.coverImgUrl,
            headers: {
              'User-Agent':
                  'Mozilla/5.0 (Linux; U; Android 8.1.0; zh-cn; BLA-AL00 Build/HUAWEIBLA-AL00) AppleWebKit/537.36 (KHTML, like Gecko) Version/4.0 Chrome/57.0.2987.132 MQQBrowser/8.9 Mobile Safari/537.36',
            },
          ),
          fadeInDuration: const Duration(milliseconds: 300),
          fit: BoxFit.cover,
        )

部分NetworkImage源码(headers只会被添加但原来的不会被删除):

Future<ui.Codec> _loadAsync(
    NetworkImage key,
    StreamController<ImageChunkEvent> chunkEvents, {
    required _SimpleDecoderCallback decode,
  }) async {
    try {
      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) {
        // The network may be only temporarily unavailable, or the file will be
        // added on the server later. Avoid having future calls to resolve
        // fail to check the network again.
        await response.drain<List<int>>(<int>[]);
        throw image_provider.NetworkImageLoadException(statusCode: response.statusCode, uri: resolved);
      }

Answer 1

不限于某种语言提供给库使用者进行配置的方法（这是我能想到的）：

1. 方法的参数中
2. 提供接口 interface 只要实现接口的中方法就可以覆盖默认的方法
3. 继承重写
4. 提供类似AOP的方法

既然想删除一些内容那就继承后实现自己业务，功能和NetWorkImage一样；也就是说里面内容是从flutter3.19.5/packages/flutter/lib/src/painting/_network_image_io.dart这里抄的

import 'dart:async';
import 'dart:io';
import 'dart:ui' as ui;

import 'package:flutter/foundation.dart';

typedef _SimpleDecoderCallback = Future<ui.Codec> Function(
    ui.ImmutableBuffer buffer);

class SteNetworkImage extends ImageProvider<NetworkImage>
    implements NetworkImage {
  /// Creates an object that fetches the image at the given URL.
  const SteNetworkImage(this.url, {this.scale = 1.0, this.headers});

  @override
  final String url;

  @override
  final double scale;

  @override
  final Map<String, String>? headers;

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

  @override
  ImageStreamCompleter loadBuffer(
      NetworkImage key, DecoderBufferCallback decode) {
    // Ownership of this controller is handed off to [_loadAsync]; it is that
    // method's responsibility to close the controller's stream when the image
    // has been loaded or an error is thrown.
    final StreamController<ImageChunkEvent> chunkEvents =
        StreamController<ImageChunkEvent>();

    return MultiFrameImageStreamCompleter(
      codec: _loadAsync(key as NetworkImage, chunkEvents, decode: decode),
      chunkEvents: chunkEvents.stream,
      scale: key.scale,
      debugLabel: key.url,
      informationCollector: () => <DiagnosticsNode>[
        DiagnosticsProperty<ImageProvider>('Image provider', this),
        DiagnosticsProperty<NetworkImage>('Image key', key),
      ],
    );
  }

  @override
  ImageStreamCompleter loadImage(
      NetworkImage key, ImageDecoderCallback decode) {
    // Ownership of this controller is handed off to [_loadAsync]; it is that
    // method's responsibility to close the controller's stream when the image
    // has been loaded or an error is thrown.
    final StreamController<ImageChunkEvent> chunkEvents =
        StreamController<ImageChunkEvent>();

    return MultiFrameImageStreamCompleter(
      codec: _loadAsync(key as NetworkImage, chunkEvents, decode: decode),
      chunkEvents: chunkEvents.stream,
      scale: key.scale,
      debugLabel: key.url,
      informationCollector: () => <DiagnosticsNode>[
        DiagnosticsProperty<ImageProvider>('Image provider', this),
        DiagnosticsProperty<NetworkImage>('Image key', key),
      ],
    );
  }

  // Do not access this field directly; use [_httpClient] instead.
  // We set `autoUncompress` to false to ensure that we can trust the value of
  // the `Content-Length` HTTP header. We automatically uncompress the content
  // in our call to [consolidateHttpClientResponseBytes].
  static final HttpClient _sharedHttpClient = HttpClient()
    ..autoUncompress = false;

  static HttpClient get _httpClient {
    HttpClient? client;
    assert(() {
      if (debugNetworkImageHttpClientProvider != null) {
        client = debugNetworkImageHttpClientProvider!();
      }
      return true;
    }());
    return client ?? _sharedHttpClient;
  }

  Future<ui.Codec> _loadAsync(
    NetworkImage key,
    StreamController<ImageChunkEvent> chunkEvents, {
    required _SimpleDecoderCallback decode,
  }) async {
    try {
      assert(key == this);

      final Uri resolved = Uri.base.resolve(key.url);

      final HttpClientRequest request = await _httpClient.getUrl(resolved);
       
      ///这里实现你的逻辑
      headers?.forEach((String name, String value) {
        request.headers.add(name, value);
      });
      final HttpClientResponse response = await request.close();
      if (response.statusCode != HttpStatus.ok) {
        // The network may be only temporarily unavailable, or the file will be
        // added on the server later. Avoid having future calls to resolve
        // fail to check the network again.
        await response.drain<List<int>>(<int>[]);
        throw NetworkImageLoadException(
            statusCode: response.statusCode, uri: resolved);
      }

      final Uint8List bytes = await consolidateHttpClientResponseBytes(
        response,
        onBytesReceived: (int cumulative, int? total) {
          chunkEvents.add(ImageChunkEvent(
            cumulativeBytesLoaded: cumulative,
            expectedTotalBytes: total,
          ));
        },
      );
      if (bytes.lengthInBytes == 0) {
        throw Exception('SteNetworkImage is an empty file: $resolved');
      }

      return decode(await ui.ImmutableBuffer.fromUint8List(bytes));
    } catch (e) {
      // Depending on where the exception was thrown, the image cache may not
      // have had a chance to track the key in the cache at all.
      // Schedule a microtask to give the cache a chance to add the key.
      scheduleMicrotask(() {
        PaintingBinding.instance.imageCache.evict(key);
      });
      rethrow;
    } finally {
      chunkEvents.close();
    }
  }

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

  @override
  int get hashCode => Object.hash(url, scale);

  @override
  String toString() =>
      '${objectRuntimeType(this, 'SteNetworkImage')}("$url", scale: ${scale.toStringAsFixed(1)})';
}