Flutter网络图片本地缓存的实现

一、问题：

Flutter原有的图片缓存机制，是通过PaintingBinding.instance!.imageCache来管理缓存的，这个缓存缓存到的是内存中，每次重新打开APP或者缓存被清理都会再次进行网络请求，大图片加载慢不友好，且增加服务器负担。

二、思路：

1、查看FadeInImage.assetNetwork、Image.network等几个网络请求的命名构造方法，初始化了ImageProvider。

  FadeInImage.assetNetwork({
    Key key,
    @required String placeholder,
    this.placeholderErrorBuilder,
    @required String image,
    this.imageErrorBuilder,
    AssetBundle bundle,
    double placeholderScale,
    double imageScale = 1.0,
    this.excludeFromSemantics = false,
    this.imageSemanticLabel,
    this.fadeOutDuration = const Duration(milliseconds: 300),
    this.fadeOutCurve = Curves.easeOut,
    this.fadeInDuration = const Duration(milliseconds: 700),
    this.fadeInCurve = Curves.easeIn,
    this.width,
    this.height,
    this.fit,
    this.alignment = Alignment.center,
    this.repeat = ImageRepeat.noRepeat,
    this.matchTextDirection = false,
    int placeholderCacheWidth,
    int placeholderCacheHeight,
    int imageCacheWidth,
    int imageCacheHeight,
  }) : assert(placeholder != null),
       assert(image != null),
       placeholder = placeholderScale != null
         ? ResizeImage.resizeIfNeeded(placeholderCacheWidth, placeholderCacheHeight, ExactAssetImage(placeholder, bundle: bundle, scale: placeholderScale))
         : ResizeImage.resizeIfNeeded(placeholderCacheWidth, placeholderCacheHeight, AssetImage(placeholder, bundle: bundle)),
       assert(imageScale != null),
       assert(fadeOutDuration != null),
       assert(fadeOutCurve != null),
       assert(fadeInDuration != null),
       assert(fadeInCurve != null),
       assert(alignment != null),
       assert(repeat != null),
       assert(matchTextDirection != null),
       image = ResizeImage.resizeIfNeeded(imageCacheWidth, imageCacheHeight, NetworkImage(image, scale: imageScale)),
       super(key: key);

  Image.network(
    String src, {
    Key key,
    double scale = 1.0,
    this.frameBuilder,
    this.loadingBuilder,
    this.errorBuilder,
    this.semanticLabel,
    this.excludeFromSemantics = false,
    this.width,
    this.height,
    this.color,
    this.colorBlendMode,
    this.fit,
    this.alignment = Alignment.center,
    this.repeat = ImageRepeat.noRepeat,
    this.centerSlice,
    this.matchTextDirection = false,
    this.gaplessPlayback = false,
    this.filterQuality = FilterQuality.low,
    this.isAntiAlias = false,
    Map headers,
    int cacheWidth,
    int cacheHeight,
  }) : image = ResizeImage.resizeIfNeeded(cacheWidth, cacheHeight, NetworkImage(src, scale: scale, headers: headers)),
       assert(alignment != null),
       assert(repeat != null),
       assert(matchTextDirection != null),
       assert(cacheWidth == null || cacheWidth > 0),
       assert(cacheHeight == null || cacheHeight > 0),
       assert(isAntiAlias != null),
       super(key: key);

其中：
image = ResizeImage.resizeIfNeeded(cacheWidth, cacheHeight, NetworkImage(src, scale: scale, headers: headers)),，使用ImageProvider类型的NetworkImage创建了ImageProvider类型的ResizeImage。

而NetworkImage是一个继承ImageProvider的抽象类。

abstract class NetworkImage extends ImageProvider {
  /// Creates an object that fetches the image at the given URL.
  ///
  /// The arguments [url] and [scale] must not be null.
  const factory NetworkImage(String url, { double scale, Map? headers }) = network_image.NetworkImage;

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

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

  /// The HTTP headers that will be used with [HttpClient.get] to fetch image from network.
  ///
  /// When running flutter on the web, headers are not used.
  Map? get headers;

  @override
  ImageStreamCompleter load(NetworkImage key, DecoderCallback decode);
}

其中工厂方法给了一个值，const factory NetworkImage(String url, { double scale, Map? headers }) = network_image.NetworkImage;

进入network_image.NetworkImage，到了_network_image_io.dart文件。

// Copyright 2014 The Flutter Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.


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

import 'package:flutter/foundation.dart';

import 'binding.dart';
import 'debug.dart';
import 'image_provider.dart' as image_provider;
import 'image_stream.dart';

/// The dart:io implementation of [image_provider.NetworkImage].
@immutable
class NetworkImage extends image_provider.ImageProvider implements image_provider.NetworkImage {
  /// Creates an object that fetches the image at the given URL.
  ///
  /// The arguments [url] and [scale] must not be null.
  const NetworkImage(this.url, { this.scale = 1.0, this.headers })
    : assert(url != null),
      assert(scale != null);

  @override
  final String url;

  @override
  final double scale;

  @override
  final Map? headers;

  @override
  Future obtainKey(image_provider.ImageConfiguration configuration) {
    return SynchronousFuture(this);
  }

  @override
  ImageStreamCompleter load(image_provider.NetworkImage key, image_provider.DecoderCallback 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 chunkEvents = StreamController();

    return MultiFrameImageStreamCompleter(
      codec: _loadAsync(key as NetworkImage, chunkEvents, decode),
      chunkEvents: chunkEvents.stream,
      scale: key.scale,
      debugLabel: key.url,
      informationCollector: () {
        return [
          DiagnosticsProperty('Image provider', this),
          DiagnosticsProperty('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 = _sharedHttpClient;
    assert(() {
      if (debugNetworkImageHttpClientProvider != null)
        client = debugNetworkImageHttpClientProvider!();
      return true;
    }());
    return client;
  }

  Future _loadAsync(
    NetworkImage key,
    StreamController chunkEvents,
    image_provider.DecoderCallback 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.
        throw image_provider.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('NetworkImage is an empty file: $resolved');

      return decode(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 => ui.hashValues(url, scale);

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

对其中的_loadAsync方法进行修改，实现图片的本地存储和获取，即可。

三、实现

1、新建一个文件my_local_cache_network_image.dart，将_network_image_io.dart内容复制过来，进行修改。
2、全部文件内容如下（非空安全版本）：

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

import 'package:crypto/crypto.dart';
import 'package:flutter/foundation.dart';
import 'package:flutter/material.dart';
import 'package:path_provider/path_provider.dart';

/// The dart:io implementation of [image_provider.NetworkImage].
@immutable
class MyLocalCacheNetworkImage extends ImageProvider implements NetworkImage {
  /// Creates an object that fetches the image at the given URL.
  ///
  /// The arguments [url] and [scale] must not be null.
  const MyLocalCacheNetworkImage(
    this.url, {
    this.scale = 1.0,
    this.headers,
    this.isLocalCache = false,
  })  : assert(url != null),
        assert(scale != null);

  @override
  final String url;

  @override
  final double scale;

  @override
  final Map headers;

  final bool isLocalCache;

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

  @override
  ImageStreamCompleter load(NetworkImage key, DecoderCallback 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 chunkEvents = StreamController();

    return MultiFrameImageStreamCompleter(
      codec: _loadAsync(key, chunkEvents, decode),
      chunkEvents: chunkEvents.stream,
      scale: key.scale,
      debugLabel: key.url,
      informationCollector: () {
        return [
          DiagnosticsProperty('Image provider', this),
          DiagnosticsProperty('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 = _sharedHttpClient;
    assert(() {
      if (debugNetworkImageHttpClientProvider != null) client = debugNetworkImageHttpClientProvider();
      return true;
    }());
    return client;
  }

  Future _loadAsync(
    NetworkImage key,
    StreamController chunkEvents,
    DecoderCallback decode,
  ) async {
    try {
      assert(key == this);

      /// 如果本地缓存过图片，直接返回图片
      if (isLocalCache != null && isLocalCache == true) {
        final Uint8List bytes = await _getImageFromLocal(key.url);
        if (bytes != null && bytes.lengthInBytes != null && bytes.lengthInBytes != 0) {
          return await PaintingBinding.instance.instantiateImageCodec(bytes);
        }
      }

      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.
        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,
          ));
        },
      );

      /// 网络请求结束后，将图片缓存到本地
      if (isLocalCache != null && isLocalCache == true && bytes.lengthInBytes != 0) {
        _saveImageToLocal(bytes, key.url);
      }

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

      return decode(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();
    }
  }

  /// 图片路径通过MD5处理，然后缓存到本地
  void _saveImageToLocal(Uint8List mUInt8List, String name) async {
    String path = await _getCachePathString(name);
    var file = File(path);
    bool exist = await file.exists();
    if (!exist) {
      File(path).writeAsBytesSync(mUInt8List);
    }
  }

  /// 从本地拿图片
  Future _getImageFromLocal(String name) async {
    String path = await _getCachePathString(name);
    var file = File(path);
    bool exist = await file.exists();
    if (exist) {
      final Uint8List bytes = await file.readAsBytes();
      return bytes;
    }
    return null;
  }

  /// 获取图片的缓存路径并创建
  Future _getCachePathString(String name) async {
    // 获取图片的名称
    String filePathFileName = md5.convert(convert.utf8.encode(name)).toString();
    String extensionName = name.split('/').last.split('.').last;

    // print('图片url:$name');
    // print('filePathFileName:$filePathFileName');
    // print('extensionName:$extensionName');

    // 生成、获取结果存储路径
    final tempDic = await getTemporaryDirectory();
    Directory directory = Directory(tempDic.path + '/CacheImage/');
    bool isFoldExist = await directory.exists();
    if (!isFoldExist) {
      await directory.create();
    }
    return directory.path + filePathFileName + '.$extensionName';
  }

  @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 => ui.hashValues(url, scale);

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

主要修改有：
1、从本地获取缓存并返回

      /// 如果本地缓存过图片，直接返回图片
      if (isLocalCache != null && isLocalCache == true) {
        final Uint8List bytes = await _getImageFromLocal(key.url);
        if (bytes != null && bytes.lengthInBytes != null && bytes.lengthInBytes != 0) {
          return await PaintingBinding.instance.instantiateImageCodec(bytes);
        }
      }

2、图片网络情请求完之后，存储到本地

      /// 网络请求结束后，将图片缓存到本地
      if (isLocalCache != null && isLocalCache == true && bytes.lengthInBytes != 0) {
        _saveImageToLocal(bytes, key.url);
      }

3、保存到本地、从本地获取图片、获取并创建本地缓存路径的具体实现，主要是最其中图片网络请求获取到的bytes和图片的url进行存储等操作。

  /// 图片路径通过MD5处理，然后缓存到本地
  void _saveImageToLocal(Uint8List mUInt8List, String name) async {
    String path = await _getCachePathString(name);
    var file = File(path);
    bool exist = await file.exists();
    if (!exist) {
      File(path).writeAsBytesSync(mUInt8List);
    }
  }

  /// 从本地拿图片
  Future _getImageFromLocal(String name) async {
    String path = await _getCachePathString(name);
    var file = File(path);
    bool exist = await file.exists();
    if (exist) {
      final Uint8List bytes = await file.readAsBytes();
      return bytes;
    }
    return null;
  }

  /// 获取图片的缓存路径并创建
  Future _getCachePathString(String name) async {
    // 获取图片的名称
    String filePathFileName = md5.convert(convert.utf8.encode(name)).toString();
    String extensionName = name.split('/').last.split('.').last;

    // print('图片url:$name');
    // print('filePathFileName:$filePathFileName');
    // print('extensionName:$extensionName');

    // 生成、获取结果存储路径
    final tempDic = await getTemporaryDirectory();
    Directory directory = Directory(tempDic.path + '/CacheImage/');
    bool isFoldExist = await directory.exists();
    if (!isFoldExist) {
      await directory.create();
    }
    return directory.path + filePathFileName + '.$extensionName';
  }

四、使用

将上面的命名构造方法复制出来，创建一个自己的命名构造方法，比如（部分代码）：

class CustomFadeInImage extends StatelessWidget {
  CustomFadeInImage.assetNetwork({
    @required this.image,
    this.placeholder,
    this.width,
    this.height,
    this.fit,
    this.alignment = Alignment.center,
    this.imageScale = 1.0,
    this.imageCacheWidth,
    this.imageCacheHeight,
  }) : imageProvider = ResizeImage.resizeIfNeeded(
            imageCacheWidth, imageCacheHeight, MyLocalCacheNetworkImage(image, scale: imageScale, isLocalCache: true));

将ResizeImage.resizeIfNeeded中的NetworkImage替换为MyLocalCacheNetworkImage即可。

五、缓存清理

清空对应的缓存目录里的图片即可。