The QCoro library provides set of tools to make use of C++20 coroutines with Qt.
Take a look at the example below to see what an amazing thing coroutines are:
QNetworkAccessManager networkAccessManager;
// co_await the reply - the coroutine is suspended until the QNetworkReply is finished.
// While the coroutine is suspended, *the Qt event loop runs as usual*.
const QNetworkReply *reply = co_await networkAccessManager.get(url);
// Once the reply is finished, your code resumes here as if nothing amazing has just happened ;-)
const auto data = reply->readAll();It requires a compiler with support for the couroutines TS.
👉 📘 Documentation
QCoro provides QCoro::Task<T> which can be used as a coroutine return type and allows the coroutine
to be awaited by its caller. Additionally, it provides qCoro() wrapper function, which wraps an
object of a supported Qt type to a thin, coroutine-friendly wrapper that allows co_awaiting asynchronous
operations on this type. Finally, it allows to directly co_await default asynchronous operations on
certain Qt types. Below is a list of a few supported types, you can find the full list in the
documentation.
QCoro can wait for an asynchronous D-Bus call to finish. There's no need to use QDBusPendingCallWatcher
with QCoro - just co_await the result instead. While co_awaiting, the Qt event loop runs as usual.
QDBusInterface remoteServiceInterface{serviceName, objectPath, interface};
const QDBusReply<bool> isReady = co_await remoteServiceInterface.asyncCall(QStringLiteral("isReady"));QFuture represents a result of an asynchronous task. Normally you have to use QFutureWatcher to get
notified when the future is ready. With QCoro, you can just co_await it!
const QFuture<int> task1 = QtConcurrent::run(....);
const QFuture<int> task2 = QtConcurrent::run(....);
const int a = co_await task1;
const int b = co_await task2;
co_return a + b;Doing network requests with Qt can be tedious - the signal/slot approach breaks the flow
of your code. Chaining requests and error handling quickly become mess and your code is
broken into numerous functions. But not with QCoro, where you can simply co_await the
QNetworkReply to finish:
QNetworkReply qnam;
QNetworkReply *reply = qnam.get(QStringLiteral("https://github.com/danvratil/qcoro"));
const auto contents = co_await reply;
reply->deleteLater();
if (reply->error() != QNetworkReply::NoError) {
co_return handleError(reply);
}
const auto link = findLinkInReturnedHtmlCode(contents);
reply = qnam.get(link);
const auto data = co_await reply;
reply->deleteLater();
if (reply->error() != QNetworkReply::NoError) {
co_return handleError(reply);
}
...Maybe you want to delay executing your code for a second, maybe you want to execute some
code in repeated interval. This becomes super-trivial with co_await:
QTimer timer;
timer.setInterval(1s);
timer.start();
for (int i = 1; i <= 100; ++i) {
co_await timer;
qDebug() << "Waiting for " << i << " seconds...";
}
qDebug() << "Done!";QIODevice is a base-class for many classes in Qt that allow data to be asynchronously
written and read. How do you find out that there are data ready to be read? You could
connect to QIODevice::readyRead() singal, or you could use QCoro and co_await the object:
socket->write("PING");
// Waiting for "pong"
const auto data = co_await socket;
co_return calculateLatency(data);Go check the full documentation to learn more.
Sometimes it's not possible to use co_await to handle result of a coroutine - usually
when interfacing with a 3rd party code that does not support coroutines. In those
scenarios it's possible to chain a continuation callback to the coroutine which will
get invoked asynchronously when the coroutine finishes.
void regularFunction() {
someCoroutineReturningInt().then([](int result) {
// handle result
});
}The continuation callback can also be a coroutine and the result of the entire
expression is Task where T is the return type of the continuation. Thanks to
that it's possible to co_await the entire chain, or chain multiple .then()
continuations.
Generator is a coroutine that lazily produces multiple values. While there's nothing Qt-specific, QCoro provides the necessary tools for users to create custom generators in their Qt applications.
QCoro provides API for both synchronous generators (QCoro::Generator<T>)
and asynchronous generators (QCoro::AsyncGenerator<T>). Both generators provide
container-like API: begin() and end() member functions that return iterator-like
objects, which is well-known and established API and makes generators compatible
with existing algorithms.
QCoro::Generator<int> fibonacci() {
quint64 a = 0, b = 0;
Q_FOREVER {
co_yield b;
const auto tmp = b;
a = b;
b += tmp;
}
}
void printFib(quint64 max) {
for (auto fib : fibonacci()) {
if (fib > max) {
break;
}
std::cout << fib << std::endl;
}
}MIT License
Copyright (c) 2022 Daniel Vrátil <dvratil@kde.org>
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