画像ロード状態監視可能な CachedNetworkImageProvider

better_cached_network_image_provider.dart

import 'dart:async' show Future;
import 'dart:io' show File;
import 'dart:typed_data';
import 'dart:ui' as ui show instantiateImageCodec, Codec;

import 'package:common/util/util.dart';
import 'package:flutter/foundation.dart';
import 'package:flutter/material.dart';
import 'package:flutter/scheduler.dart';
import 'package:flutter_cache_manager/flutter_cache_manager.dart';

typedef void Function() StatusListener(
    BetterCachedNetworkImageProviderStatus status);

enum BetterCachedNetworkImageProviderStatus { none, fetching, error, fetched }

/// Original source: https://github.com/renefloor/flutter_cached_network_image/blob/9579401d835061be8b53bb7e78a5bcf667ddde68/lib/src/cached_network_image_provider.dart
class BetterCachedNetworkImageProvider
    extends ImageProvider<BetterCachedNetworkImageProvider> {
  const BetterCachedNetworkImageProvider(
    this.url, {
    this.scale = 1.0,
    this.statusNotifier,
    this.headers,
    this.cacheManager,
  })  : assert(url != null),
        assert(scale != null);

  final BaseCacheManager cacheManager;
  final String url;
  final double scale;
  final ValueNotifier<BetterCachedNetworkImageProviderStatus> statusNotifier;
  final Map<String, String> headers;

  @override
  Future<BetterCachedNetworkImageProvider> obtainKey(
          ImageConfiguration configuration) =>
      SynchronousFuture<BetterCachedNetworkImageProvider>(this);

  @override
  ImageStreamCompleter load(BetterCachedNetworkImageProvider key) {
    return MultiFrameImageStreamCompleter(
      codec: _loadAsync(key),
      scale: key.scale,
// TODO enable information collector on next stable release of flutter
//      informationCollector: () sync* {
//        yield DiagnosticsProperty<ImageProvider>(
//          'Image provider: $this \n Image key: $key',
//          this,
//          style: DiagnosticsTreeStyle.errorProperty,
//        );
//      },
    );
  }

  Future<ui.Codec> _loadAsync(BetterCachedNetworkImageProvider key) async {
    final manager = cacheManager ?? CacheManager();
    // build内で使っている場合に即座に呼ばれて、この変更起点にsetState
    // されるような処理を書いているとエラーが起こるのでその対処
    SchedulerBinding.instance.addPostFrameCallback((_) {
      statusNotifier?.value = BetterCachedNetworkImageProviderStatus.fetching;
    });
    final file = await manager.getSingleFile(url, headers: headers);
    if (file == null) {
      statusNotifier?.value = BetterCachedNetworkImageProviderStatus.error;
      return Future<ui.Codec>.error('Couldn\'t download or retrieve file.');
    }
    return await _loadAsyncFromFile(key, file);
  }

  Future<ui.Codec> _loadAsyncFromFile(
      BetterCachedNetworkImageProvider key, File file) async {
    assert(key == this);

    final bytes = Uint8List.fromList(await file.readAsBytes());

    if (bytes.lengthInBytes == 0) {
      statusNotifier?.value = BetterCachedNetworkImageProviderStatus.error;
      throw Exception('File was empty');
    }

    final result = await ui.instantiateImageCodec(bytes);
    statusNotifier?.value = BetterCachedNetworkImageProviderStatus.fetched;
    return result;
  }

  @override
  bool operator ==(Object other) =>
      identical(this, other) ||
      other is BetterCachedNetworkImageProvider &&
          runtimeType == other.runtimeType &&
          url == other.url &&
          scale == other.scale;

  @override
  int get hashCode => url.hashCode ^ scale.hashCode;

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