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https://github.com/simonbrunel/qtpromise.git
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b47ca0569e
This method returns a promise that will be fulfilled with the same value as the `input` promise and after at least `msec` milliseconds. If the `input` promise is rejected, the `output` promise is immediately rejected with the same reason.
432 lines
15 KiB
Markdown
432 lines
15 KiB
Markdown
<a href="https://promisesaplus.com/" title="Promises/A+ 1.1"><img src="http://promisesaplus.com/assets/logo-small.png" alt="Promises/A+" align="right"/></a>
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# QtPromise
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[](http://www.qpm.io/packages/com.github.simonbrunel.qtpromise/index.html) [](https://travis-ci.org/simonbrunel/qtpromise) [](https://codecov.io/gh/simonbrunel/qtpromise)
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[Promises/A+](https://promisesaplus.com/) implementation for [Qt/C++](https://www.qt.io/).
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Requires [Qt 5.4](https://www.qt.io/download/) (or later) with [C++11 support enabled](https://wiki.qt.io/How_to_use_C++11_in_your_Qt_Projects).
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## Getting Started
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### Installation
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QtPromise is a [header-only](https://en.wikipedia.org/wiki/Header-only) library, simply download the [latest release](https://github.com/simonbrunel/qtpromise/releases/latest) (or [`git submodule`](https://git-scm.com/docs/git-submodule])) and include `qtpromise.pri` from your project `.pro`.
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### qpm
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Alternatively and **only** if your project relies on [qpm](http://www.qpm.io/), you can install QtPromise as follow:
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```bash
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qpm install com.github.simonbrunel.qtpromise
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```
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### Usage
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The recommended way to use QtPromise is to include the single module header:
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```cpp
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#include <QtPromise>
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```
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### Example
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Let's first make the code more readable by using the library namespace:
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```cpp
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using namespace QtPromise;
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```
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This `download` function creates a [promise from callbacks](#qpromise-qpromise) which will be resolved when the network request is finished:
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```cpp
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QPromise<QByteArray> download(const QUrl& url)
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{
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return QPromise<QByteArray>([&](
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const QPromiseResolve<QByteArray>& resolve,
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const QPromiseReject<QByteArray>& reject) {
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QNetworkReply* reply = manager->get(QNetworkRequest(url));
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QObject::connect(reply, &QNetworkReply::finished, [=]() {
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if (reply->error() == QNetworkReply::NoError) {
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resolve(reply->readAll());
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} else {
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reject(reply->error());
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}
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reply->deleteLater();
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});
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});
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}
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```
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The following method `uncompress` data in a separate thread and returns a [promise from QFuture](#qtconcurrent):
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```cpp
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QPromise<Entries> uncompress(const QByteArray& data)
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{
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return qPromise(QtConcurrent::run([](const QByteArray& data) {
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Entries entries;
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// {...} uncompress data and parse content.
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if (error) {
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throw MalformedException();
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}
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return entries;
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}, data));
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}
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```
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It's then easy to chain the whole asynchronous process using promises:
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- initiate the promise chain by downloading a specific URL,
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- [`then`](#qpromise-then) *and only if download succeeded*, uncompress received data,
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- [`then`](#qpromise-then) validate and process the uncompressed entries,
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- [`finally`](#qpromise-finally) perform operations whatever the process succeeded or failed,
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- and handle specific errors using [`fail`](#qpromise-fail).
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```cpp
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download(url).then(&uncompress).then([](const Entries& entries) {
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if (entries.isEmpty()) {
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throw UpdateException("No entries");
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}
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// {...} process entries
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}).finally([]() {
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// {...} cleanup
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}).fail([](QNetworkReply::NetworkError err) {
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// {...} handle network error
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}).fail([](const UpdateException& err) {
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// {...} handle update error
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}).fail([]() {
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// {...} catch all
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});
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```
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## QtConcurrent
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QtPromise integrates with [QtConcurrent](http://doc.qt.io/qt-5/qtconcurrent-index.html) to make easy chaining QFuture with QPromise.
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### <a name="qtconcurrent-convert"></a> Convert
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Converting `QFuture<T>` to `QPromise<T>` is done using the [`qPromise`](#helpers-qpromise) helper:
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```cpp
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QFuture<int> future = QtConcurrent::run([]() {
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// {...}
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return 42;
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});
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QPromise<int> promise = qPromise(future);
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```
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or simply:
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```cpp
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auto promise = qPromise(QtConcurrent::run([]() {
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// {...}
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}));
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```
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### Chain
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Returning a `QFuture<T>` in [`then`](#qpromise-then) or [`fail`](#qpromise-fail) automatically translate to `QPromise<T>`:
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```cpp
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QPromise<int> input = ...
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auto output = input.then([](int res) {
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return QtConcurrent::run([]() {
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// {...}
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return QString("42");
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});
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});
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// output type: QPromise<QString>
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output.then([](const QString& res) {
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// {...}
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});
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```
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The `output` promise is resolved when the `QFuture` is [finished](http://doc.qt.io/qt-5/qfuture.html#isFinished).
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### Error
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Exceptions thrown from a QtConcurrent thread reject the associated promise with the exception as the reason. Note that if you throw an exception that is not a subclass of `QException`, the promise with be rejected with [`QUnhandledException`](http://doc.qt.io/qt-5/qunhandledexception.html#details) (this restriction only applies to exceptions thrown from a QtConcurrent thread, [read more](http://doc.qt.io/qt-5/qexception.html#details)).
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```cpp
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QPromise<int> promise = ...
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promise.then([](int res) {
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return QtConcurrent::run([]() {
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// {...}
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if (!success) {
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throw CustomException();
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}
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return QString("42");
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});
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}).fail(const CustomException& err) {
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// {...}
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});
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```
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## Thread-Safety
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QPromise is thread-safe and can be copied and accessed across different threads. QPromise relies on [explicitly data sharing](http://doc.qt.io/qt-5/qexplicitlyshareddatapointer.html#details) and thus `auto p2 = p1` represents the same promise: when `p1` resolves, handlers registered on `p1` and `p2` are called, the fulfilled value being shared between both instances.
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> **Note:** while it's safe to access the resolved value from different threads using [`then`](#qpromise-then), QPromise provides no guarantee about the object being pointed to. Thread-safety and reentrancy rules for that object still apply.
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## QPromise
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### <a name="qpromise-qpromise"></a> `QPromise<T>::QPromise(resolver)`
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Creates a new promise that will be fulfilled or rejected by the given `resolver` lambda:
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```cpp
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QPromise<int> promise([](const QPromiseResolve<int>& resolve, const QPromiseReject<int>& reject) {
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async_method([=](bool success, int result) {
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if (success) {
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resolve(result);
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} else {
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reject(customException());
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}
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});
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});
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```
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> **Note:** `QPromise<void>` is specialized to not contain any value, meaning that the `resolve` callback takes no argument.
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**C++14**
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```cpp
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QPromise<int> promise([](const auto& resolve, const auto& reject) {
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// {...}
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});
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```
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### <a name="qpromise-then"></a> `QPromise<T>::then(onFulfilled, onRejected) -> QPromise<R>`
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See [Promises/A+ `.then`](https://promisesaplus.com/#the-then-method) for details.
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```cpp
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QPromise<int> input = ...
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auto output = input.then([](int res) {
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// called with the 'input' result if fulfilled
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}, [](const ReasonType& reason) {
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// called with the 'input' reason if rejected
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// see QPromise<T>::fail for details
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});
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```
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> **Note**: `onRejected` handler is optional, `output` will be rejected with the same reason as `input`.
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> **Note**: it's recommended to use the [`fail`](#qpromise-fail) shorthand to handle errors.
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The type `<R>` of the `output` promise depends on the return type of the `onFulfilled` handler:
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```cpp
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QPromise<int> input = {...}
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auto output = input.then([](int res) {
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return QString::number(res); // -> QPromise<QString>
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});
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// output type: QPromise<QString>
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output.then([](const QString& res) {
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// {...}
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});
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```
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> **Note**: only `onFulfilled` can change the promise type, `onRejected` **must** return the same type as `onFulfilled`. That also means if `onFulfilled` is `nullptr`, `onRejected` must return the same type as the `input` promise.
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```cpp
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QPromise<int> input = ...
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auto output = input.then([](int res) {
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return res + 4;
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}, [](const ReasonType& reason) {
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return -1;
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});
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```
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If `onFulfilled` doesn't return any value, the `output` type is `QPromise<void>`:
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```cpp
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QPromise<int> input = ...
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auto output = input.then([](int res) {
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// {...}
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});
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// output type: QPromise<void>
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output.then([]() {
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// `QPromise<void>` `onFulfilled` handler has no argument
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});
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```
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You can also decide to skip the promise result by omitting the handler argument:
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```cpp
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QPromise<int> input = {...}
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auto output = input.then([]( /* skip int result */ ) {
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// {...}
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});
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```
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The `output` promise can be *rejected* by throwing an exception in either `onFulfilled` or `onRejected`:
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```cpp
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QPromise<int> input = {...}
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auto output = input.then([](int res) {
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if (res == -1) {
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throw ReasonType();
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} else {
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return res;
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}
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});
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// output.isRejected() is true
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```
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If an handler returns a promise (or QFuture), the `output` promise is delayed and will be resolved by the returned promise.
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### <a name="qpromise-fail"></a> `QPromise<T>::fail(onRejected) -> QPromise<T>`
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Shorthand to `promise.then(nullptr, onRejected)`, similar to the [`catch` statement](http://en.cppreference.com/w/cpp/language/try_catch):
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```cpp
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promise.fail([](const MyException&) {
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// {...}
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}).fail(const QException&) {
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// {...}
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}).fail(const std::exception&) {
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// {...}
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}).fail() {
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// {...} catch-all
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});
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```
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### <a name="qpromise-finally"></a> `QPromise<T>::finally(handler) -> QPromise<T>`
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This `handler` is **always** called, without any argument and whatever the `input` promise state (fulfilled or rejected). The `output` promise has the same type as the `input` one but also the same value or error. The finally `handler` **can not modify the fulfilled value** (the returned value is ignored), however, if `handler` throws, `output` is rejected with the new exception.
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```cpp
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auto output = input.finally([]() {
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// {...}
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});
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```
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If `handler` returns a promise (or QFuture), the `output` promise is delayed until the returned promise is resolved and under the same conditions: the delayed value is ignored, the error transmitted to the `output` promise.
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### <a name="qpromise-tap"></a> `QPromise<T>::tap(handler) -> QPromise<T>`
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This `handler` allows to observe the value of the `input` promise, without changing the propagated value. The `output` promise will be resolved with the same value as the `input` promise (the `handler` returned value will be ignored). However, if `handler` throws, `output` is rejected with the new exception. Unlike [`finally`](#qpromise-finally), this handler is **not** called for rejections.
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```cpp
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QPromise<int> input = {...}
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auto output = input.tap([](int res) {
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log(res);
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}).then([](int res) {
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// {...}
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});
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```
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If `handler` returns a promise (or QFuture), the `output` promise is delayed until the returned promise is resolved and under the same conditions: the delayed value is ignored, the error transmitted to the `output` promise.
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### <a name="qpromise-delay"></a> `QPromise<T>::delay(handler) -> QPromise<T>`
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This method returns a promise that will be fulfilled with the same value as the `input` promise and after at least `msec` milliseconds. If the `input` promise is rejected, the `output` promise is immediately rejected with the same reason.
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```cpp
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QPromise<int> input = {...}
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auto output = input.delay(2000).then([](int res) {
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// called 2 seconds after `input` is fulfilled
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});
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```
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### <a name="qpromise-wait"></a> `QPromise<T>::wait() -> QPromise<T>`
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This method holds the execution of the remaining code **without** blocking the event loop of the current thread:
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```cpp
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int result = -1;
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QPromise<int> input = qPromise(QtConcurrent::run([]() { return 42; }));
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auto output = input.then([&](int res) {
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result = res;
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});
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// output.isPending() is true && result is -1
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output.wait();
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// output.isPending() is false && result is 42
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```
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### `QPromise<T>::isPending() -> bool`
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Returns `true` if the promise is pending (not fulfilled or rejected), otherwise returns `false`.
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### `QPromise<T>::isFulfilled() -> bool`
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Returns `true` if the promise is fulfilled, otherwise returns `false`.
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### `QPromise<T>::isRejected() -> bool`
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Returns `true` if the promise is rejected, otherwise returns `false`.
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## QPromise (statics)
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### <a name="qpromise-resolve"></a> `[static] QPromise<T>::resolve(value) -> QPromise<T>`
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Creates a `QPromise<T>` that is fulfilled with the given `value` of type `T`:
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```cpp
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QPromise<int> compute(const QString& type)
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{
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if (type == "magic") {
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return QPromise<int>::resolve(42);
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}
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return QPromise<int>([](const QPromiseResolve<int>& resolve) {
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// {...}
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});
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}
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```
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See also: [`qPromise`](#helpers-qpromise)
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### <a name="qpromise-reject"></a> `[static] QPromise<T>::reject(reason) -> QPromise<T>`
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Creates a `QPromise<T>` that is rejected with the given `reason` of *whatever type*:
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```cpp
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QPromise<int> compute(const QString& type)
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{
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if (type == "foobar") {
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return QPromise<int>::reject(QString("Unknown type: %1").arg(type));
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}
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return QPromise<int>([](const QPromiseResolve<int>& resolve) {
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// {...}
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});
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}
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```
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### <a name="qpromise-all"></a> `[static] QPromise<T>::all(QVector<QPromise<T>>) -> QPromise<QVector<T>>`
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Returns a `QPromise<QVector<T>>` that fulfills when **all** `promises` of (the same) type `T` have been fulfilled. The `output` value is a vector containing **all** the values of `promises`, in the same order. If any of the given `promises` fail, `output` immediately rejects with the error of the promise that rejected, whether or not the other promises are resolved.
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```cpp
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QVector<QPromise<QByteArray> > promises{
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download(QUrl("http://a...")),
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download(QUrl("http://b...")),
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download(QUrl("http://c..."))
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};
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auto output = QPromise<QByteArray>::all(promises);
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// output type: QPromise<QVector<QByteArray>>
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output.then([](const QVector<QByteArray>& res) {
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// {...}
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});
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```
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See also: [`qPromiseAll`](#helpers-qpromiseall)
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## Helpers
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### <a name="helpers-qpromise"></a> `qPromise(T value) -> QPromise<R>`
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Similar to the `QPromise<T>::resolve` static method, creates a promise resolved from a given `value` without the extra typing:
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```cpp
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auto promise = qPromise(); // QPromise<void>
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auto promise = qPromise(42); // QPromise<int>
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auto promise = qPromise(QString("foo")); // QPromise<QString>
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```
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This method also allows to convert `QFuture<T>` to `QPromise<T>` delayed until the `QFuture` is finished ([read more](#qtconcurrent-convert)).
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### <a name="helpers-qpromiseall"></a> `qPromiseAll(QVector<QPromise<T> promises) -> QPromise<QVector<T>>`
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This method simply calls the appropriated [`QPromise<T>::all`](#qpromise-all) static method based on the given `QVector` type. In some cases, this method is more convenient than the static one since it avoid some extra typing:
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```cpp
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QVector<QPromise<QByteArray> > promises{...}
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auto output = qPromiseAll(promises);
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// eq. QPromise<QByteArray>::all(promises)
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```
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## License
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QtPromise is available under the [MIT license](LICENSE).
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