how to assign future<> to widget in flutter? - dart

Question

Suppose I have a SingleChildScrollView, its content is read from a file:
singleChildScrollView(
padding: EdgeInsets.all(8.0),
child: nw Text(
getTextFromFile(), //<---read from file
style: TextStyle(
fontWeight: FontWeight.bold,
fontSize: 19.0,
),
));
Future<String> getFileData(String path) async {
return await rootBundle.loadString(path);
}
Future<String> getTextFromFile() async {
return getFileData("test.txt");
}
I got the following error:
The argument type 'Future<String>' can't be assigned to the parameter
type 'String'.
How to solve the issue?

Answer 1

Using a FutureBuilder should solve your problem. I modified you code so you can see how to use it. initialData is not required.
#override
Widget build(BuildContext context) {
return new FutureBuilder(
future: getTextFromFile(),
initialData: "Loading text..",
builder: (BuildContext context, AsyncSnapshot<String> text) {
return new SingleChildScrollView(
padding: new EdgeInsets.all(8.0),
child: new Text(
text.data,
style: new TextStyle(
fontWeight: FontWeight.bold,
fontSize: 19.0,
),
));
});
}
Future<String> getFileData(String path) async {
return await new Future(() => "test text");
}
Future<String> getTextFromFile() async {
return getFileData("test.txt");
}
}

Answer 2

StatefulWidget can be used for this purpose.
Declare a member variable String _textFromFile = ""; in your State class and update its value on future resolve by using setState() method.
I called your getTextFromFile() method from the constructor, but you may call it from anywhere.
Running code:
import 'package:flutter/material.dart';
import 'dart:async';
class StatefullWidgetDemo extends StatefulWidget {
#override
_StatefulWidgetDemoState createState() {
return new _StatefulWidgetDemoState();
}
}
class _StatefulWidgetDemoState extends State<StatefullWidgetDemo> {
String _textFromFile = "";
_StatefulWidgetDemoState() {
getTextFromFile().then((val) => setState(() {
_textFromFile = val;
}));
}
#override
Widget build(BuildContext context) {
return new Scaffold(
appBar: new AppBar(
title: new Text('Stateful Demo'),
),
body: new SingleChildScrollView(
padding: new EdgeInsets.all(8.0),
child: new Text(
_textFromFile,
style: new TextStyle(
fontWeight: FontWeight.bold,
fontSize: 19.0,
),
),
),
);
}
Future<String> getFileData(String path) async {
return "your data from file";
}
Future<String> getTextFromFile() async {
return await getFileData("test.txt");
}
}

Answer 3

Simple answer here=>
The class which calls the function:
#override
Widget build(BuildContext context) {
return new Scaffold(
child: FutureBuilder(
future: function(),
builder: (BuildContext context, AsyncSnapshot<String> text) {
return new Text(text.data);
});
)}
And the function:
Future<String> function() async {
return 'abc';
}

Answer 4

Here's a similar question:
flutter / dart error: The argument type 'Future<File>' can't be assigned to the parameter type 'File'
The solution proposed there is quite elegant and works properly. Where the IDE says it's expecting Type and not Future<Type>, put await in front of that argument

Answer 5

Another solution to get data on initialization would be to call getTextFromFile() in initState(), set state with new string and use that string in your widget:
String fileData = '';
Future<String> getFileData(String path) async {
return await rootBundle.loadString(path);
}
void getTextFromFile() async {
try {
String data = await getFileData("test.txt");
setState(() {
fileData = data;
});
} catch (ex) {
print(ex);
}
}
#override
void initState() {
super.initState();
getTextFromFile();
}
new singleChildScrollView(
padding: new EdgeInsets.all(8.0),
child: new Text(
fileData,
style: new TextStyle(
fontWeight: FontWeight.bold,
fontSize: 19.0,
),
));

Answer 6

I would like to add one more answer, as I tested other answers I got an error.
Moving the async part (plus setState) from the constructor to initState() solved that for me.
Enjoy
class TestAsyncInStateful extends StatefulWidget {
const TestAsyncInStateful({super.key});
#override
State<TestAsyncInStateful> createState() => _TestAsyncInStatefulState();
}
class _TestAsyncInStatefulState extends State<TestAsyncInStateful> {
#override
void initState() {
super.initState();
getTextFromServer().then(
(value) => setState(() {
textFromServer = value;
}),
);
}
String? textFromServer;
#override
Widget build(BuildContext context) {
return textFromServer == null? const SizedBox() :Text(textFromServer!);
}
}