如何正方形裁剪Flutter相机预览

Question

我正试图在我的应用程序中拍摄方形图片。我正在使用camera包，我正在尝试显示CameraPreview小部件的中心正方形裁剪版本。

我的目标是显示预览的中心正方形（全宽），从顶部和底部裁剪出均匀的数量。

我努力让这个工作，所以我创建了一个使用固定图像的最小例子。 （为我在椅子上的沉闷画面道歉）：

import 'package:flutter/material.dart';

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

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Example',
      theme: ThemeData(),
      home: Scaffold(
        body: Example(),
      ),
    );
  }
}

class Example extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return Column(
      children: <Widget>[
        CroppedCameraPreview(),

        // Something to occupy the rest of the space
        Expanded(
          child: Container(),
        )
      ],
    );
  }
}

class CroppedCameraPreview extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    // We will pretend this is a camera preview (to make demo easier)
    var cameraImage = Image.network("https://i.imgur.com/gZfg4jm.jpg");
    var aspectRatio = 1280 / 720;

    return Container(
      width: MediaQuery.of(context).size.width,
      height: MediaQuery.of(context).size.width,
      child: ClipRect(
        child: new OverflowBox(
          alignment: Alignment.center,
          child: FittedBox(
            fit: BoxFit.fitWidth,
            child: cameraImage,
          ),
        ),
      ),
    );
  }
}

这很好 - 我得到一个全屏宽度的图像，中心裁剪并推到我的应用程序的顶部。

但是，如果我将此代码放入我现有的应用程序并用cameraImage替换CameraPreview，我会遇到很多布局错误：

flutter: ══╡ EXCEPTION CAUGHT BY RENDERING LIBRARY ╞═════════════════════════════════════════════════════════
flutter: The following assertion was thrown during performResize():
flutter: TextureBox object was given an infinite size during layout.
flutter: This probably means that it is a render object that tries to be as big as possible, but it was put
flutter: inside another render object that allows its children to pick their own size.
flutter: The nearest ancestor providing an unbounded width constraint is:
flutter:   RenderFittedBox#0bd54 NEEDS-LAYOUT NEEDS-PAINT
flutter:   creator: FittedBox ← OverflowBox ← ClipRect ← ConstrainedBox ← Container ← Stack ← ConstrainedBox
flutter:   ← Container ← CameraWidget ← Column ← CameraPage ← MediaQuery ← ⋯
flutter:   parentData: offset=Offset(0.0, 0.0) (can use size)
flutter:   constraints: BoxConstraints(w=320.0, h=320.0)
flutter:   size: MISSING
flutter:   fit: fitWidth
flutter:   alignment: center
flutter:   textDirection: ltr
flutter: The nearest ancestor providing an unbounded height constraint is:
flutter:   RenderFittedBox#0bd54 NEEDS-LAYOUT NEEDS-PAINT
flutter:   creator: FittedBox ← OverflowBox ← ClipRect ← ConstrainedBox ← Container ← Stack ← ConstrainedBox
flutter:   ← Container ← CameraWidget ← Column ← CameraPage ← MediaQuery ← ⋯
flutter:   parentData: offset=Offset(0.0, 0.0) (can use size)
flutter:   constraints: BoxConstraints(w=320.0, h=320.0)
flutter:   size: MISSING
flutter:   fit: fitWidth
flutter:   alignment: center
flutter:   textDirection: ltr
flutter: The constraints that applied to the TextureBox were:
flutter:   BoxConstraints(unconstrained)
flutter: The exact size it was given was:
flutter:   Size(Infinity, Infinity)
flutter: See https://flutter.io/layout/ for more information.

任何人都可以建议为什么我的预览错误以及如何避免它们？

Answer 1

我通过将它包装在CameraPreview中来为我的Container实例提供特定大小来解决这个问题：

  var size = MediaQuery.of(context).size.width;

  // ...

  Container(
    width: size,
    height: size,
    child: ClipRect(
      child: OverflowBox(
        alignment: Alignment.center,
        child: FittedBox(
          fit: BoxFit.fitWidth,
          child: Container(
            width: size,
            height:
                size / widget.cameraController.value.aspectRatio,
            child: camera, // this is my CameraPreview
          ),
        ),
      ),
    ),
  );

为了回应Luke的评论，我然后使用这个代码来裁剪生成的图像。 （因为即使预览是正方形，捕获的图像仍然是标准比率）。

  Future<String> _resizePhoto(String filePath) async {
      ImageProperties properties =
          await FlutterNativeImage.getImageProperties(filePath);

      int width = properties.width;
      var offset = (properties.height - properties.width) / 2;

      File croppedFile = await FlutterNativeImage.cropImage(
          filePath, 0, offset.round(), width, width);

      return croppedFile.path;
  }

这使用https://github.com/btastic/flutter_native_image。我使用这段代码已经有一段时间了 - 认为它目前只适用于肖像图像，但应该可以轻松扩展以处理景观。