getApplicationDocumentsDirectory().then((value) => print(value));

>>> '/var/mobile/Containers/Data/Application/EAE0BE0B-9AE2-4A42-9693-295BD398BA73/Documents'

getLibraryDirectory().then((value) => print(value));

getExternalStorageDirectory().then((value) => print(value));

// 获取外部存储目录列表
getExternalStorageDirectories().then((value) => print(value));

Future<Directory> _tempDirectory;
Future<Directory> _appSupportDirectory;
Future<Directory> _appLibraryDirectory;
Future<Directory> _appDocumentsDirectory;
Future<Directory> _externalStorageDirectory;
Future<List<Directory>> _externalStorageDirectories;
Future<List<Directory>> _externalCacheDirectories;
Future<Directory> _downloadDirectory;

_tempDirectory = getTemporaryDirectory();
_appSupportDirectory = getApplicationSupportDirectory();
_appLibraryDirectory = getLibraryDirectory();
_appDocumentsDirectory = getApplicationDocumentsDirectory();
_externalStorageDirectory = getExternalStorageDirectory();
_externalCacheDirectories = getExternalCacheDirectories();
_externalStorageDirectories = getExternalStorageDirectories();
_downloadDirectory = getDownloadsDirectory();

import 'dart:io';       // Directory所在的包
import 'dart:convert';  

/// 属性
// String path 获取此目录的路径
print(dir.path);

// Uri 返回表示目录位置的uri
print(dir.uri);

// 父目录的Directory对象
print(dir.parent);

// absolute 返回一个绝对路径的Directory对象
print(dir.absolute);

// 判断是否是绝对路径
print(dir.isAbsolute);

/// 创建实例
new File('path');

/// 方法
// 打印一下file实例对象
print(file);   // File: 'assets\files\index.txt'

// 返回绝对路径的File实例
print(file.absolute); // File: 'C:\Users\Administrator.PW6NUH21X31RM79\Desktop\Dart\assets\files\index.txt'

// 获取文件路径
print(file.path); // assets\files\index.txt

// 是否是绝对路径
print(file.isAbsolute);  // fasle

// 获取父目录的Directory对象
print(file.parent);  // Directory: 'assets\files'

// 获取uri
print(file.uri);   // assets/files/index.txt

// 根据路径字符创建目录对象
RaisedButton(
  onPressed: () async {
    final path = await getApplicationDocumentsDirectory();
    final directory = Directory(path.path+'/base/');
    print(path.path);
    directory.create(recursive: true);
  },
  child: Text('创建目录'),
),

directory.createSync();

final file = File("_path");
final String content = await file.readAsString(); // 

// final file = File('');
// await file.writeAsString(DateTime.now().toString());

RaisedButton(
  onPressed: () async{
    final path = await getApplicationDocumentsDirectory();
    final file = new File(path.path+'/hi.txt');
    print(file.path);
    file.writeAsString('Hello World');
  },
  child: Text('创建文件'),
),

<key>LSSupportsOpeningDocumentsInPlace</key>
<true/>
<key>UIFileSharingEnabled</key>
<true/>