main.dart

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

import 'package:http/http.dart' as http;

import 'package:flutter/material.dart';
import 'package:flutter/rendering.dart';

void main() => runApp(MyApp());

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      home: MyHomePage(),
    );
  }
}

class MyHomePage extends StatefulWidget {
  @override
  _MyHomePageState createState() => _MyHomePageState();
}

class _MyHomePageState extends State<MyHomePage> with SingleTickerProviderStateMixin {
  final double width = 400;
  final double height = 400;
  final bool dropShadow = true;
  final bool useSnapshot = true; // CHANGE ME AND COLD RESTART TO SEE THE DIFFERENCE
  final String photo = 'https://github.com/flutter/website/blob/master/src/_includes/code/layout/lakes/images/lake.jpg?raw=true';

  GlobalKey _snapshotWidgetKey = new GlobalKey();
  Uint8List _snapshotByteData;
  Uint8List _photoBytes;

  bool _gpuImageCached = false;

  Animation<double> animation;
  AnimationController controller;

  void _fetchImage() async {
    http.Response response = await http.get(photo);
    setState((){
      _photoBytes = response.bodyBytes;
    });
  }

  void _trySnapshot() async {
    do {
      await Future.delayed(Duration(milliseconds: 20));
      if (_photoBytes == null || _snapshotWidgetKey.currentContext == null) {
        continue;
      }
      await Future.delayed(Duration(milliseconds: 200)); // wait for image to be shown
      RenderRepaintBoundary boundary = _snapshotWidgetKey.currentContext.findRenderObject();
      if (boundary == null) {
        continue;
      }
      ui.Image image = await boundary.toImage();
      _snapshotByteData = (await image.toByteData(format: ui.ImageByteFormat.png)).buffer.asUint8List();
    } while (_snapshotByteData == null);

    await Future.delayed(Duration(seconds: 1));
    _gpuImageCached = true;
  }

  Widget createSnapshotWidget() {
    if (useSnapshot && _snapshotByteData != null && _gpuImageCached) {
      return Image.memory(_snapshotByteData);
    }

    Widget cacheStack = Stack(
      children: <Widget>[
        _snapshotByteData == null ? Container() : Image.memory(_snapshotByteData),
        Container(color: Colors.white),
        _photoBytes == null ? Container() : Image.memory(_photoBytes),
      ],
    );

    return RepaintBoundary(
      key: _snapshotWidgetKey,
      child: Container(
        padding: EdgeInsets.all(10), // padding for box shadow
        child: Container(
          width: width,
          height: height,
          decoration: BoxDecoration(
            boxShadow: dropShadow
                ? [
              BoxShadow(
                  color: Colors.black.withOpacity(0.4),
                  blurRadius: 5.0,
                  offset: Offset(0.0, 1.0))
            ]
                : [],
            borderRadius: BorderRadius.all(Radius.circular(10.0)),
            color: Colors.white,
          ),
          child: ClipRRect(
            borderRadius: BorderRadius.circular(10.0),
            child: cacheStack,
          ),
        ),
      )
    );
  }

  @override
  void initState() {
    super.initState();
    _fetchImage();
    if (useSnapshot) {
      _trySnapshot();
    }
    controller = AnimationController(vsync: this);
    animation = Tween(begin: 0.0, end: 360.0).animate(controller)
      ..addListener((){
        setState((){});
      });
    controller.repeat(period: Duration(seconds: 2));
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text('Rotation Demo'),
      ),
      body: Center(
        child: MemberCard(
          snapshotWidget: createSnapshotWidget(),
          rotation: animation.value,
        ),
      ),
    );
  }
}

class MemberCard extends StatelessWidget {
  final double rotation;
  final Offset offset;
  final double scale;
  final Widget snapshotWidget;

  MemberCard({
    @required this.snapshotWidget,
    this.rotation = 0.0,
    this.offset = const Offset(0.0, 0.0),
    this.scale = 1.0,
  });

  @override
  Widget build(BuildContext context) {
    double angle = rotation * (3.1415926 / 180.0);
    var matrix = Matrix4.translationValues(offset.dx, offset.dy, 0.0);
    if (scale != 1.0) {
      matrix..scale(scale);
    }
    if (rotation != 0.0) {
      matrix.rotateZ(angle);
    }

    return Transform(
      alignment: Alignment.center,
      transform: matrix,
      child: Stack(
        children: <Widget>[
          snapshotWidget
        ],
      ),
    );
  }
}