dependencies:
  ...
  image_picker:
  image_cropper

将UCropActivity添加到AndroidManifest.xml中

 <activity android:name="com.yalantis.ucrop.UCropActivity" android:screenOrientation="portrait" android:theme="@style/Theme.AppCompat.Light.NoActionBar"/>

添加AndroidX兼容

org.gradle.jvmargs=-Xmx1536M
android.enableJetifier=true
android.useAndroidX=true


import 'package:flutter/material.dart';
import 'dart:async';
import 'dart:io';

import 'package:image_cropper/image_cropper.dart';
import 'package:image_picker/image_picker.dart';

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

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'ImageCropper',
      home: MyHomePage(
        title: 'ImageCropper',
      ),
    );
  }
}

class MyHomePage extends StatefulWidget {
  final String title;

  MyHomePage({this.title});

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

enum AppState {
  free,
  picked,
  cropped,
}

class _MyHomePageState extends State<MyHomePage> {
  AppState state;
  File imageFile;

  @override
  void initState() {
    super.initState();
    state = AppState.free;
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text(widget.title),
      ),
      body: Center(
        child: imageFile != null ? Image.file(imageFile) : Container(),
      ),
      floatingActionButton: FloatingActionButton(
        onPressed: () {
          if (state == AppState.free)
            _pickImage();
          else if (state == AppState.picked)
            _cropImage();
          else if (state == AppState.cropped) _clearImage();
        },
        child: _buildButtonIcon(),
      ),
    );
  }

  Widget _buildButtonIcon() {
    if (state == AppState.free)
      return Icon(Icons.add);
    else if (state == AppState.picked)
      return Icon(Icons.crop);
    else if (state == AppState.cropped)
      return Icon(Icons.clear);
    else
      return Container();
  }

  Future<Null> _pickImage() async {
    imageFile = await ImagePicker.pickImage(source: ImageSource.gallery);
    if (imageFile != null) {
      setState(() {
        state = AppState.picked;
      });
    }
  }

  Future<Null> _cropImage() async {
    File croppedFile = await ImageCropper.cropImage(
      sourcePath: imageFile.path,
      toolbarTitle: 'Cropper',
      toolbarColor: Colors.blue,
      toolbarWidgetColor: Colors.white,
    );
    if (croppedFile != null) {
      imageFile = croppedFile;
      setState(() {
        state = AppState.cropped;
      });
    }
  }

  Future<Null> _saveImage() async {}

  void _clearImage() {
    imageFile = null;
    setState(() {
      state = AppState.free;
    });
  }
}

• Image Cropper
• image_picker