In this tutorial, we’re gonna build a Flutter App that can read file and write data to file for later use. To do this, we need to combine path_provider plugin with dart:io library.
Contents
Flutter App Overview
Our Flutter App has a Text field, when we click on Write to File button, the String in Text field will be written to text.txt file (appending text) and display on Screen.
Everytime we launch the App, it read text.txt file and show contents inside.
We can also clear content of the file by Clear Contents button.
Read/Write Files
Find Local Path
We need a place to write data on disk and read it again when the app loads.
>> So we use path_provider plugin to access Documents directory (on iOS, this corresponds to NSDocumentDirectory, on Android, this is the AppData directory).
1 2 3 4 | Future<String> get _localPath async { final directory = await getApplicationDocumentsDirectory(); return directory.path; } |
Reference to File
To create a reference to the File full location (in our case, the text.txt file), we use File class from the dart:io library.
1 2 3 4 | Future<File> get _localFile async { final path = await _localPath; return File('$path/text.txt'); } |
Write data to File
We will write a string to a file using File writeAsString() method. It returns a Future<File> that completes with this File object once the entire operation has completed.
By default, writeAsString() creates the file and truncates the file if it already exists.
>> To append data to existing file, we pass FileMode.append mode as second parameter.
1 2 3 4 | Future<File> writeFile(String text) async { final file = await _localFile; return file.writeAsString('$text\r\n', mode: FileMode.append); } |
Read data from File
We use File readAsString() method to read the entire contents as a string. It returns a Future<String> that completes with the string once contents has been read.
1 2 3 4 5 6 7 8 9 10 | Future<String> readFile() async { try { final file = await _localFile; String content = await file.readAsString(); return content; } catch (e) { return ''; } } |
Complete Code
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 132 133 134 135 136 137 138 139 140 141 142 143 144 | import 'package:flutter/material.dart'; import 'package:path_provider/path_provider.dart'; import 'dart:async'; import 'dart:io'; void main() { runApp( MaterialApp( title: 'Read/Write Files', home: MyApp(storage: TextStorage()), ), ); } class TextStorage { Future<String> get _localPath async { final directory = await getApplicationDocumentsDirectory(); return directory.path; } Future<File> get _localFile async { final path = await _localPath; return File('$path/text.txt'); } Future<String> readFile() async { try { final file = await _localFile; String content = await file.readAsString(); return content; } catch (e) { return ''; } } Future<File> writeFile(String text) async { final file = await _localFile; return file.writeAsString('$text\r\n', mode: FileMode.append); } Future<File> cleanFile() async { final file = await _localFile; return file.writeAsString(''); } } class MyApp extends StatefulWidget { final TextStorage storage; MyApp({Key key, @required this.storage}) : super(key: key); @override _MyAppState createState() => _MyAppState(); } class _MyAppState extends State<MyApp> { TextEditingController _textField = new TextEditingController(); String _content = ''; @override void initState() { super.initState(); widget.storage.readFile().then((String text) { setState(() { _content = text; }); }); } Future<File> _writeStringToTextFile(String text) async { setState(() { _content += text + '\r\n'; }); return widget.storage.writeFile(text); } Future<File> _clearContentsInTextFile() async { setState(() { _content = ''; }); return widget.storage.cleanFile(); } @override Widget build(BuildContext context) { return Scaffold( appBar: AppBar( title: Text('Read/Write File Example'), backgroundColor: Colors.blue, ), body: Container( padding: EdgeInsets.all(20.0), child: Column( children: <Widget>[ TextField( controller: _textField, ), Padding( padding: EdgeInsets.all(20.0), child: RaisedButton( child: Text('Write to File'), onPressed: () { if (_textField.text.isNotEmpty) { _writeStringToTextFile(_textField.text); _textField.clear(); } }, ), ), Padding( padding: EdgeInsets.only(bottom: 20.0), child: RaisedButton( child: Text( 'Clear Contents', style: TextStyle(color: Colors.white), ), color: Colors.redAccent, onPressed: () { _clearContentsInTextFile(); }, ), ), Expanded( flex: 1, child: new SingleChildScrollView( child: Text( '$_content', style: TextStyle( color: Colors.blue, fontSize: 22.0, ), ), ), ), ], ), ), ); } } |