无法拍打屏幕截图 [英] Unable to take screenshot in flutter

问题描述

package:flutter/src/rendering/proxy_box.dart':失败的断言:2813行pos 12:'！debugNeedsPaint':不正确.

package:flutter/src/rendering/proxy_box.dart': Failed assertion: line 2813 pos 12: '!debugNeedsPaint': is not true.

我正试图拍张屏幕截图，但出现异常了.我访问了许多链接，但没有任何帮助.

I am trying to take screenshot in flutter, but I am getting exception. I visited many links but nothing worked.

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);
        var pngBytes = byteData.buffer.asUint8List();
        var bs64 = base64Encode(pngBytes);
        print(pngBytes);
        print(bs64);
        setState(() {});
        return pngBytes;
    } catch (e) {
        print(e);
    }
}

推荐答案

您可以找到官方 toImage 示例

You can find official toImage example here. But it looks like it doesn't work without delay between button tap and toImage call.

官方存储库中存在一个问题: https://github.com/flutter/flutter/issues/22308

There is an issue in the official repository: https://github.com/flutter/flutter/issues/22308

发生这种情况的原因:您的点击会初始化按钮的动画，并且递归调用包括父母在内的 RenderObject.markNeedsPaint ，因此您应等待 debugNeedsPaint 为假.在这种情况下， toImage 函数只会引发断言错误:

The cause for this: your tap initializes animation for a button and RenderObject.markNeedsPaint gets called recursively including parents, so you should wait while debugNeedsPaint will be false again. toImage function just throws assertion error in this case:

  Future<ui.Image> toImage({ double pixelRatio = 1.0 }) {
    assert(!debugNeedsPaint);
    final OffsetLayer offsetLayer = layer;
    return offsetLayer.toImage(Offset.zero & size, pixelRatio: pixelRatio);
  }

https://github.com/flutter/flutter/blob/f0553ba58e6455aa63fafcdca16100b81ff5c3ce/packages/flutter/lib/src/rendering/proxy_box.dart#L2857

  bool get debugNeedsPaint {
    bool result;
    assert(() {
      result = _needsPaint;
      return true;
    }());
    return result;
  }

https://github.com/flutter/flutter/blob/0ca5e71f281cd549f1b5284e339523ad93544c60/packages/flutter/lib/src/rendering/object.dart#L2011

实际上 assert 函数仅用于开发，因此您可以看到生产中的错误不会有麻烦.但是我不知道您会遇到什么样的麻烦，也许没有.)

Actually assert function is used only in development, so as you can see you will not have the trouble with the error in production. But I don't know what kind of troubles you can get instead, probably no ).

下一个代码不是很好，但是可以工作:

The next code is not great but it works:

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

  Future<Uint8List> _capturePng() async {
    RenderRepaintBoundary boundary = globalKey.currentContext.findRenderObject();

    if (boundary.debugNeedsPaint) {
      print("Waiting for boundary to be painted.");
      await Future.delayed(const Duration(milliseconds: 20));
      return _capturePng();
    }

    var image = await boundary.toImage();
    var byteData = await image.toByteData(format: ImageByteFormat.png);
    return byteData.buffer.asUint8List();
  }

  void _printPngBytes() async {
    var pngBytes = await _capturePng();
    var bs64 = base64Encode(pngBytes);
    print(pngBytes);
    print(bs64);
  }

  @override
  Widget build(BuildContext context) {
    return RepaintBoundary(
      key: globalKey,
      child: Center(
        child: FlatButton(
          color: Color.fromARGB(255, 255, 255, 255),
          child: Text('Capture Png', textDirection: TextDirection.ltr),
          onPressed: _printPngBytes
        ),
      ),
    );
  }
}

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