Flutter get screenshot from canvas layer

example.dart

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

import 'package:flutter/material.dart';
import 'package:gallery_saver/gallery_saver.dart';
import 'package:path/path.dart';
import 'package:path_provider/path_provider.dart';

import 'screenshot.dart';

void main() => runApp(const App());

class App extends StatelessWidget {
  const App({super.key});

  Future<void> saveScreenshot(ByteData data) async {
    File? file;
    try {
      final dir = await getTemporaryDirectory();
      file = File(join(dir.path, 'screenshot_${DateTime.now().millisecondsSinceEpoch.toRadixString(36)}.png'));
      await file.writeAsBytes(data.buffer.asUint8List(data.offsetInBytes, data.lengthInBytes));
      await GallerySaver.saveImage(file.path);
    } finally {
      await file?.delete();
    }
  }

  @override
  Widget build(BuildContext context) => ScreenshotScope(
        child: MaterialApp(
          title: 'Material App',
          home: Scaffold(
            appBar: AppBar(
              title: const Text('App Bar'),
            ),
            body: SafeArea(
              child: Center(
                child: SingleChildScrollView(
                  child: ScreenshotScope(
                    child: Builder(
                      builder: (context) => ElevatedButton(
                        onPressed: () => ScreenshotScope.takeScreenshot(context).then<void>(saveScreenshot),
                        child: Text('Hello World\n' * 100),
                      ),
                    ),
                  ),
                ),
              ),
            ),
          ),
        ),
      );
}

screenshot_scope.dart

/// How to take a screenshot of widgets
import 'dart:typed_data' as td;
import 'dart:ui' as ui;

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

class ScreenshotScope extends StatelessWidget {
  const ScreenshotScope({required this.child, super.key});

  final Widget child;

  static Future<td.ByteData> takeScreenshot(
    BuildContext context, {
    double pixelRatio = 1.0,
  }) async {
    RenderRepaintBoundary? boundary;
    context.visitAncestorElements((element) {
      if (element.widget is ScreenshotScope) return false;
      final renderObject = element.renderObject;
      if (renderObject is RenderRepaintBoundary) boundary = renderObject;
      return true;
    });
    if (boundary == null) throw UnsupportedError('No ScreenshotScope found');
    final image = await boundary!.toImage(pixelRatio: pixelRatio);
    final bytes = await image.toByteData(format: ui.ImageByteFormat.png);
    if (bytes is! td.ByteData) throw UnsupportedError('Error converting image to bytes');
    return bytes;
  }

  @override
  Widget build(BuildContext context) => RepaintBoundary(child: child);
}

Просто обернуть виджет в ScreenshotScope.

Получение скриншота из контекста:

final screenshot = await ScreenshotScope.takeScreenshot(context);

Ну и сохранить на диск png, например:

final dir = await getTemporaryDirectory();
final file = File(join(dir.path, 'screenshot_${DateTime.now().toIso8601String()}.png'));
await file.writeAsBytes(
  screenshot.buffer.asUint8List(screenshot.offsetInBytes, screenshot.lengthInBytes),
);