1.ui.Image转ImageProvider

最近用photo_view做图片预览时，有个需求是加一张canvas生成的海报图片，因为CustomPainter返回的时候可以toImage，于是就有了这些东西。

import 'package:flutter/cupertino.dart';
import 'package:flutter/foundation.dart';
import 'dart:ui' as ui;

/**
 * 学知乎大佬思路，
 * 模仿NetworkImage制作一个自定义ImageProvider
 */
class LoadImage extends ImageProvider<LoadImage> {

  const LoadImage(this.image, { this.scale = 1.0 })
      : assert(image != null),
        assert(scale != null);

  final ui.Image image;
  final double scale;

  @override
  ImageStreamCompleter load(LoadImage key, decode) {
    return MultiFrameImageStreamCompleter(
      codec: _loadAsync(key),
      scale: key.scale,
    );
  }

  Future<ui.Codec> _loadAsync(LoadImage key) async {
    assert(key == this);
    //image转ByteData
    final a = await image.toByteData(format: ui.ImageByteFormat.png);
    var codec = await PaintingBinding.instance
        .instantiateImageCodec(a.buffer.asUint8List());
    return codec;
  }

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

  @override
  bool operator ==(dynamic other) {
    if (other.runtimeType != runtimeType) return false;
    final LoadImage typedOther = other;
    return image == typedOther.image && scale == typedOther.scale;
  }

  @override
  int get hashCode => hashValues(image.hashCode, scale);

  @override
  String toString() =>
      '$runtimeType(${describeIdentity(image)}, scale: $scale)';
}

实际使用
LoadImage loadImage = LoadImage(image);

ImageProvider转ui.Image

 static Future<ui.Image> loadImageByProvider(
      ImageProvider provider, {
        ImageConfiguration config = ImageConfiguration.empty,
      }) async {
    Completer<ui.Image> completer = Completer<ui.Image>(); //完成的回调
    ImageStreamListener listener;
    ImageStream stream = provider.resolve(config); //获取图片流
    listener = ImageStreamListener((ImageInfo frame, bool sync) {
//监听
      final ui.Image image = frame.image;
      completer.complete(image); //完成
      stream.removeListener(listener); //移除监听
    });
    stream.addListener(listener); //添加监听
    return completer.future; //返回
  }

//实际使用 ImageProvider转ui.Image
 ui.Image image = await ImageUtils.loadImageByProvider(
        CachedNetworkImageProvider(imgUrl));

ui.Image转asUint8List

// 转asUint8List
var a = await image.toByteData(format: ui.ImageByteFormat.png);
  print(a.buffer.asUint8List());

ByteData转Widget

Widget image = Image.memory(a.buffer.asUint8List())

2.ByteData、ui.Image互转

//ui.Image转ByteData
final a = await image.toByteData(format: ImageByteFormat.png);
var codec = await ui.instantiateImageCodec(a.buffer.asUint8List());
//ByteData转ui.Image
FrameInfo fi = await codec.getNextFrame();
ui.Image image = fi.image;