When & why is it a good idea to call findRenderObject?

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MrLarryQ

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Mar 19, 2019, 6:30:39 PM3/19/19

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I'm looking at some code someone has written, and had a question about GlobalKeys and the findRenderObject method. Here's the class in question:

class QrCodeImage extends StatelessWidget {

  final String product;
  GlobalKey _repaintQrImage = GlobalKey();

  QrCodeImage({Key key, this.product}) : super(key: key);

  Future<File> _captureByteData() async {
    RenderRepaintBoundary boundary = _repaintQrImage.currentContext.findRenderObject();
    ui.Image image = await boundary.toImage(
      pixelRatio: 3.0,
    );
    var byteData = await image.toByteData(
      format: ui.ImageByteFormat.png,
    );
    File imageFile = File(await FireStorage.getFilePath());
    return await FireStorage.writeQrImage(imageFile, byteData);
  }

  @override
  Widget build(BuildContext context) {
    if(product != null)
      return Expanded(
        child: Padding(
          padding: const EdgeInsets.only(
            top: 60,
          ),
          child: RepaintBoundary(
            key: _repaintQrImage,
            child: GestureDetector(
              onTap: () {
                _captureByteData().then((imageFile) {
                  FirebaseQrDetector(imageFile).detectQrCode();
                });
              },
              child: QrImage(
                backgroundColor: Colors.white,
                padding: EdgeInsets.all(40.0),
                data: product,
                size: 300,
              ),
            ),
          ),
        ),
      );
    else
      return Container();
  }
}

Basically what's happening is we capture an image from the screen and then manipulate it. That part isn't germane to my question however; I'm more interested in the use of GlobalKey() and the findRenderObject() method.

I've read up about global keys and my limited understanding is they give you a 'unique value' and so a quick way to identify a widget. However I'm not entirely sure why we're using a global key in this instance since a key is supposedly passed into the class's constructor?

That's one point, probably a minor one that I'm not quite getting. The larger one is why we create a RepaintBoundary widget and what the point of findRenderObject is. The author of the class, when explaining it in a lecture, vaguely referred to 'performance benefits' when he mentioned the code in question but never elaborated. I was hoping someone could provide a better explanation of what's going on and when I would want to do this in my own code.

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