在屏幕外构建小部件

Question

我需要构建一个小部件来获取它的位图。我不在乎小部件出现在屏幕上。

所以我的问题是：我可以在不使用屏幕视图层次结构的情况下以某种方式“在侧面”构建小部件吗？

我没有找到办法。因此，如果这不可能，我可以在屏幕上构建小部件，但实际上不显示它。 我试过Visibility 但这会使RenderObject 为空。使用 Offstage 在断言上调用 toImage() 时会失败：Failed assertion: line 2752 pos 12: ‘!debugNeedsPaint’: is not true.

Answer 1

编辑：看起来这在最新版本的 Flutter 中出现了问题。不知道为什么，但我猜当 Flutter 确定覆盖层根本不可见时，它现在会避免绘制。这个方法还是可以用的，但是需要结合 translate 才能移出屏幕：https://gist.github.com/itsJoKr/ce5ec57bd6dedf74d1737c1f39481913

有人推荐使用OverlayEntry，它看起来是最好的解决方案。

您可以将OverlayEntry 放在当前屏幕下方，使其不可见，而maintainState: true 将被构建。 一个很大的优势是它更容易实现，因为它不会与当前的小部件树混合。

OverlayState overlayState = Overlay.of(context);
OverlayEntry entry = OverlayEntry(builder: (context) {
  return RepaintBoundary(key: key, child: yourWidget,); // Using RepaintBoundary to get RenderObject and convert to image
}, maintainState: true);
overlayState.insert(entry);
// doesn't work anymore
// overlayState.rearrange([entry], above: entry); // Didn't find how to insert it at the bottom of current overlays, so this should rearrange it so that our entry is at the bottom

Answer 2

这应该可以完成工作。我们创建了一个屏幕大小的区域。但是（在屏幕外），它仍然可以作为树的一部分被捕获。

也可以在这里找到：https://gist.github.com/slightfoot/8eeadd8028c373df87f3a47bd4a35e36

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

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

void main() {
  runApp(
    MaterialApp(
      theme: ThemeData(
        primarySwatch: Colors.indigo,
        accentColor: Colors.pinkAccent,
      ),
      home: ExampleScreen(),
    ),
  );
}

class ExampleScreen extends StatefulWidget {
  @override
  _ExampleScreenState createState() => new _ExampleScreenState();
}

class _ExampleScreenState extends State<ExampleScreen> {
  final _captureKey = GlobalKey<CaptureWidgetState>();
  Future<CaptureResult> _image;

  void _onCapturePressed() {
    setState(() {
      _image = _captureKey.currentState.captureImage();
    });
  }

  @override
  Widget build(BuildContext context) {
    return CaptureWidget(
      key: _captureKey,
      capture: Material(
        child: Padding(
          padding: const EdgeInsets.all(8.0),
          child: Column(
            mainAxisSize: MainAxisSize.min,
            children: <Widget>[
              Text(
                'These widgets are not visible on the screen yet can still be captured by a RepaintBoundary.',
              ),
              SizedBox(height: 12.0),
              Container(
                width: 25.0,
                height: 25.0,
                color: Colors.red,
              ),
            ],
          ),
        ),
      ),
      child: Scaffold(
        appBar: AppBar(
          title: Text('Widget To Image Demo'),
        ),
        body: FutureBuilder<CaptureResult>(
          future: _image,
          builder: (BuildContext context, AsyncSnapshot<CaptureResult> snapshot) {
            return SingleChildScrollView(
              child: Column(
                mainAxisAlignment: MainAxisAlignment.center,
                children: <Widget>[
                  Center(
                    child: RaisedButton(
                      child: Text('Capture Image'),
                      onPressed: _onCapturePressed,
                    ),
                  ),
                  if (snapshot.connectionState == ConnectionState.waiting)
                    Center(
                      child: CircularProgressIndicator(),
                    )
                  else if (snapshot.hasData) ...[
                    Text(
                      '${snapshot.data.width} x ${snapshot.data.height}',
                      textAlign: TextAlign.center,
                    ),
                    Container(
                      margin: const EdgeInsets.all(12.0),
                      decoration: BoxDecoration(
                        border: Border.all(color: Colors.grey.shade300, width: 2.0),
                      ),
                      child: Image.memory(
                        snapshot.data.data,
                        scale: MediaQuery.of(context).devicePixelRatio,
                      ),
                    ),
                  ],
                ],
              ),
            );
          },
        ),
      ),
    );
  }
}

class CaptureWidget extends StatefulWidget {
  final Widget child;
  final Widget capture;

  const CaptureWidget({
    Key key,
    this.capture,
    this.child,
  }) : super(key: key);

  @override
  CaptureWidgetState createState() => CaptureWidgetState();
}

class CaptureWidgetState extends State<CaptureWidget> {
  final _boundaryKey = GlobalKey();

  Future<CaptureResult> captureImage() async {
    final pixelRatio = MediaQuery.of(context).devicePixelRatio;
    final boundary = _boundaryKey.currentContext.findRenderObject() as RenderRepaintBoundary;
    final image = await boundary.toImage(pixelRatio: pixelRatio);
    final data = await image.toByteData(format: ui.ImageByteFormat.png);
    return CaptureResult(data.buffer.asUint8List(), image.width, image.height);
  }

  @override
  Widget build(BuildContext context) {
    return LayoutBuilder(
      builder: (BuildContext context, BoxConstraints constraints) {
        final height = constraints.maxHeight * 2;
        return OverflowBox(
          alignment: Alignment.topLeft,
          minHeight: height,
          maxHeight: height,
          child: Column(
            children: <Widget>[
              Expanded(
                child: widget.child,
              ),
              Expanded(
                child: Center(
                  child: RepaintBoundary(
                    key: _boundaryKey,
                    child: widget.capture,
                  ),
                ),
              ),
            ],
          ),
        );
      },
    );
  }
}

class CaptureResult {
  final Uint8List data;
  final int width;
  final int height;

  const CaptureResult(this.data, this.width, this.height);
}

Answer 3

在任何地方调用captureImage() 方法之前，请执行此操作

        if (boundary.debugNeedsPaint) {
            print("Waiting for boundary to be painted.");
            await Future.delayed(const Duration(milliseconds: 100));
            return captureImage();
          }