flutter中可以通過RepaintBoundary widget中的toImage方法將頁面中的widget轉為圖片儲存。

如何使用？

首先要在全域性定義一個global key，分配給RepaintBoundary。然後將要轉化為圖片的widget用RepaintBoundary包裹。

關鍵程式碼：

RenderRepaintBoundary boundary = _globalKey.currentContext.findRenderObject(); // 獲取頁面渲染物件

獲取到頁面渲染物件之後，就可以通過toImage方法將物件轉化為raw image data,然後通過下面三步可以將位元組資料轉化為base64字元：

ui.Image image = await boundary.toImage(pixelRatio: 3.0);

ByteData byteData = await image.toByteData(format: ui.ImageByteFormat.png);
Uint8List pngBytes = byteData.buffer.asUint8List();
String bs64 = base64Encode(pngBytes);

完整可執行程式碼：

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

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

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

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return new MaterialApp(
      title: 'Flutter Demo',
      theme: 
 
new ThemeData(
        primarySwatch: Colors.blue,
      ),
      home: new MyHomePage(),
    );
  }
}

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

class _MyHomePageState extends State<MyHomePage> {
  GlobalKey _globalKey = new GlobalKey();

  Future<Uint8List> _capturePng() async {
    try {
      print('inside');
      RenderRepaintBoundary boundary =
      _globalKey.currentContext.findRenderObject();
      ui.Image image = await boundary.toImage(pixelRatio: 3.0);
      ByteData byteData =
      await image.toByteData(format: ui.ImageByteFormat.png);
      Uint8List pngBytes = byteData.buffer.asUint8List();
      String bs64 = base64Encode(pngBytes);
      print(pngBytes);
      print(bs64);
      setState(() {});
      return pngBytes;
    } catch (e) {
      print(e);
    }
  }

  @override
  Widget build(BuildContext context) {
    return RepaintBoundary(
      key: _globalKey,
      child: new Scaffold(
        appBar: new AppBar(
          title: new Text('Widget To Image demo'),
        ),
        body: new Center(
          child: new Column(
            mainAxisAlignment: MainAxisAlignment.center,
            children: <Widget>[
              new Text(
                '點選下面按鈕將widget轉為圖片',
              ),
              new RaisedButton(
                child: Text('點我'),
                onPressed: _capturePng,
              ),
            ],
          ),
        ),
      ),
    );
  }
}