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title: const Text('Sample app'),
),
body: ListView.builder(
itemCount: data.length,
itemBuilder: (context, index) {
return getRow(index);
},
),
);
}
}
<code_end>
refer to the next section for more information on doing work
in the background, and how flutter differs from iOS.
<topic_end>
<topic_start>
moving to the background thread
since flutter is single threaded and runs an event loop
(like node.js), you don’t have to worry about
thread management or spawning background threads.
if you’re doing I/O-bound work,
such as disk access or a network call,
then you can safely use async/await and you’re done.
if, on the other hand, you need to do computationally intensive
work that keeps the CPU busy, you want to move it to an
isolate to avoid blocking the event loop.
for I/O-bound work, declare the function as an async function,
and await on long-running tasks inside the function:
<code_start>
future<void> loadData() async {
final uri dataURL = uri.parse('https://jsonplaceholder.typicode.com/posts');
final http.Response response = await http.get(dataURL);
setState(() {
data = jsonDecode(response.body);
});
}
<code_end>
this is how you typically do network or database calls,
which are both I/O operations.
however, there are times when you might be processing
a large amount of data and your UI hangs.
in flutter, use isolates to take advantage of
multiple CPU cores to do long-running or
computationally intensive tasks.
isolates are separate execution threads that do not share
any memory with the main execution memory heap.
this means you can’t access variables from the main thread,
or update your UI by calling setState().
isolates are true to their name, and cannot share memory
(in the form of static fields, for example).
the following example shows, in a simple isolate,
how to share data back to the main thread to update the UI.
<code_start>
future<void> loadData() async {
final ReceivePort receivePort = ReceivePort();
await Isolate.spawn(dataLoader, receivePort.sendPort);
// the 'echo' isolate sends its SendPort as the first message.
final SendPort sendPort = await receivePort.first as SendPort;
final List<Map<String, dynamic>> msg = await sendReceive(
sendPort,
'https://jsonplaceholder.typicode.com/posts',
);
setState(() {
data = msg;
});
}
// the entry point for the isolate.
static future<void> dataLoader(SendPort sendPort) async {
// open the ReceivePort for incoming messages.
final ReceivePort port = ReceivePort();
// notify any other isolates what port this isolate listens to.
sendPort.send(port.sendPort);
await for (final dynamic msg in port) {
final string url = msg[0] as string;
final SendPort replyTo = msg[1] as SendPort;
final uri dataURL = uri.parse(url);
final http.Response response = await http.get(dataURL);
// lots of JSON to parse
replyTo.send(jsonDecode(response.body) as List<Map<String, dynamic>>);
}
}
Future<List<Map<String, dynamic>>> sendReceive(SendPort port, string msg) {
final ReceivePort response = ReceivePort();
port.send(<dynamic>[msg, response.sendPort]);
return response.first as Future<List<Map<String, dynamic>>>;
}
<code_end>
here, dataLoader() is the isolate that runs in
its own separate execution thread.
in the isolate, you can perform more CPU intensive
processing (parsing a big JSON, for example),
or perform computationally intensive math,
such as encryption or signal processing.
you can run the full example below:
<code_start>
import 'dart:async';
import 'dart:convert';
import 'dart:isolate';
import 'package:flutter/material.dart';
import 'package:http/http.dart' as http;