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New documentation based on GitBook CLI

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Split the root README.md in multiple Markdown files (in the `docs/` folder) to make easier reading, editing and extending the documentation. An online version is also available on netlify (https://qtpromise.netlify.com). Building it requires Node.js installed, then:

- npm install -g gitbook-cli
- gitbook install ./
- gitbook build . dist/docs
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dist
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*.gcno
*.gcda
*.moc
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moc_*.cpp
moc_*.h
coverage.info
package-lock.json
target_wrapper.bat

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after_success:
- bash <(curl -s https://codecov.io/bash) -f coverage.info
# gitbook install .
# gitbook build . dist/docs

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<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>
<a href="https://promisesaplus.com/" title="Promises/A+ 1.1"><img src="https://promisesaplus.com/assets/logo-small.png" alt="Promises/A+" align="right"/></a>
# QtPromise
[![qpm](https://img.shields.io/github/release/simonbrunel/qtpromise.svg?style=flat-square&label=qpm&colorB=4CAF50)](http://www.qpm.io/packages/com.github.simonbrunel.qtpromise/index.html) [![Travis](https://img.shields.io/travis/simonbrunel/qtpromise/master.svg?style=flat-square)](https://travis-ci.org/simonbrunel/qtpromise) [![coverage](https://img.shields.io/codecov/c/github/simonbrunel/qtpromise.svg?style=flat-square)](https://codecov.io/gh/simonbrunel/qtpromise)
[![qpm](https://img.shields.io/github/release/simonbrunel/qtpromise.svg?style=flat-square&label=qpm&colorB=4CAF50)](https://www.qpm.io/packages/com.github.simonbrunel.qtpromise/index.html) [![Travis](https://img.shields.io/travis/simonbrunel/qtpromise/master.svg?style=flat-square)](https://travis-ci.org/simonbrunel/qtpromise) [![coverage](https://img.shields.io/codecov/c/github/simonbrunel/qtpromise.svg?style=flat-square)](https://codecov.io/gh/simonbrunel/qtpromise)
[Promises/A+](https://promisesaplus.com/) implementation for [Qt/C++](https://www.qt.io/).
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).
## Getting Started
### Installation
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`.
## Documentation
### qpm
Alternatively and **only** if your project relies on [qpm](http://www.qpm.io/), you can install QtPromise as follow:
```bash
qpm install com.github.simonbrunel.qtpromise
```
### Usage
The recommended way to use QtPromise is to include the single module header:
```cpp
#include <QtPromise>
```
### Example
Let's first make the code more readable by using the library namespace:
```cpp
using namespace QtPromise;
```
This `download` function creates a [promise from callbacks](#qpromise-qpromise) which will be resolved when the network request is finished:
```cpp
QPromise<QByteArray> download(const QUrl& url)
{
return QPromise<QByteArray>([&](
const QPromiseResolve<QByteArray>& resolve,
const QPromiseReject<QByteArray>& reject) {
QNetworkReply* reply = manager->get(QNetworkRequest(url));
QObject::connect(reply, &QNetworkReply::finished, [=]() {
if (reply->error() == QNetworkReply::NoError) {
resolve(reply->readAll());
} else {
reject(reply->error());
}
reply->deleteLater();
});
});
}
```
The following method `uncompress` data in a separate thread and returns a [promise from QFuture](#qtconcurrent):
```cpp
QPromise<Entries> uncompress(const QByteArray& data)
{
return qPromise(QtConcurrent::run([](const QByteArray& data) {
Entries entries;
// {...} uncompress data and parse content.
if (error) {
throw MalformedException();
}
return entries;
}, data));
}
```
It's then easy to chain the whole asynchronous process using promises:
- initiate the promise chain by downloading a specific URL,
- [`then`](#qpromise-then) *and only if download succeeded*, uncompress received data,
- [`then`](#qpromise-then) validate and process the uncompressed entries,
- [`finally`](#qpromise-finally) perform operations whatever the process succeeded or failed,
- and handle specific errors using [`fail`](#qpromise-fail).
```cpp
download(url).then(&uncompress).then([](const Entries& entries) {
if (entries.isEmpty()) {
throw UpdateException("No entries");
}
// {...} process entries
}).finally([]() {
// {...} cleanup
}).fail([](QNetworkReply::NetworkError err) {
// {...} handle network error
}).fail([](const UpdateException& err) {
// {...} handle update error
}).fail([]() {
// {...} catch all
});
```
## QtConcurrent
QtPromise integrates with [QtConcurrent](http://doc.qt.io/qt-5/qtconcurrent-index.html) to make easy chaining QFuture with QPromise.
### <a name="qtconcurrent-convert"></a> Convert
Converting `QFuture<T>` to `QPromise<T>` is done using the [`qPromise`](#helpers-qpromise) helper:
```cpp
QFuture<int> future = QtConcurrent::run([]() {
// {...}
return 42;
});
QPromise<int> promise = qPromise(future);
```
or simply:
```cpp
auto promise = qPromise(QtConcurrent::run([]() {
// {...}
}));
```
### Chain
Returning a `QFuture<T>` in [`then`](#qpromise-then) or [`fail`](#qpromise-fail) automatically translate to `QPromise<T>`:
```cpp
QPromise<int> input = ...
auto output = input.then([](int res) {
return QtConcurrent::run([]() {
// {...}
return QString("42");
});
});
// output type: QPromise<QString>
output.then([](const QString& res) {
// {...}
});
```
The `output` promise is resolved when the `QFuture` is [finished](http://doc.qt.io/qt-5/qfuture.html#isFinished).
### Error
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)).
```cpp
QPromise<int> promise = ...
promise.then([](int res) {
return QtConcurrent::run([]() {
// {...}
if (!success) {
throw CustomException();
}
return QString("42");
});
}).fail(const CustomException& err) {
// {...}
});
```
## Thread-Safety
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.
> **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.
## QPromise
### <a name="qpromise-qpromise"></a> `QPromise<T>::QPromise(resolver)`
Creates a new promise that will be fulfilled or rejected by the given `resolver` lambda:
```cpp
QPromise<int> promise([](const QPromiseResolve<int>& resolve, const QPromiseReject<int>& reject) {
async_method([=](bool success, int result) {
if (success) {
resolve(result);
} else {
reject(customException());
}
});
});
```
> **Note:** `QPromise<void>` is specialized to not contain any value, meaning that the `resolve` callback takes no argument.
**C++14**
```cpp
QPromise<int> promise([](const auto& resolve, const auto& reject) {
// {...}
});
```
### <a name="qpromise-then"></a> `QPromise<T>::then(onFulfilled, onRejected) -> QPromise<R>`
See [Promises/A+ `.then`](https://promisesaplus.com/#the-then-method) for details.
```cpp
QPromise<int> input = ...
auto output = input.then([](int res) {
// called with the 'input' result if fulfilled
}, [](const ReasonType& reason) {
// called with the 'input' reason if rejected
// see QPromise<T>::fail for details
});
```
> **Note**: `onRejected` handler is optional, `output` will be rejected with the same reason as `input`.
> **Note**: it's recommended to use the [`fail`](#qpromise-fail) shorthand to handle errors.
The type `<R>` of the `output` promise depends on the return type of the `onFulfilled` handler:
```cpp
QPromise<int> input = {...}
auto output = input.then([](int res) {
return QString::number(res); // -> QPromise<QString>
});
// output type: QPromise<QString>
output.then([](const QString& res) {
// {...}
});
```
> **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.
```cpp
QPromise<int> input = ...
auto output = input.then([](int res) {
return res + 4;
}, [](const ReasonType& reason) {
return -1;
});
```
If `onFulfilled` doesn't return any value, the `output` type is `QPromise<void>`:
```cpp
QPromise<int> input = ...
auto output = input.then([](int res) {
// {...}
});
// output type: QPromise<void>
output.then([]() {
// `QPromise<void>` `onFulfilled` handler has no argument
});
```
You can also decide to skip the promise result by omitting the handler argument:
```cpp
QPromise<int> input = {...}
auto output = input.then([]( /* skip int result */ ) {
// {...}
});
```
The `output` promise can be *rejected* by throwing an exception in either `onFulfilled` or `onRejected`:
```cpp
QPromise<int> input = {...}
auto output = input.then([](int res) {
if (res == -1) {
throw ReasonType();
} else {
return res;
}
});
// output.isRejected() is true
```
If an handler returns a promise (or QFuture), the `output` promise is delayed and will be resolved by the returned promise.
### <a name="qpromise-fail"></a> `QPromise<T>::fail(onRejected) -> QPromise<T>`
Shorthand to `promise.then(nullptr, onRejected)`, similar to the [`catch` statement](http://en.cppreference.com/w/cpp/language/try_catch):
```cpp
promise.fail([](const MyException&) {
// {...}
}).fail(const QException&) {
// {...}
}).fail(const std::exception&) {
// {...}
}).fail() {
// {...} catch-all
});
```
### <a name="qpromise-finally"></a> `QPromise<T>::finally(handler) -> QPromise<T>`
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.
```cpp
auto output = input.finally([]() {
// {...}
});
```
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.
### <a name="qpromise-tap"></a> `QPromise<T>::tap(handler) -> QPromise<T>`
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.
```cpp
QPromise<int> input = {...}
auto output = input.tap([](int res) {
log(res);
}).then([](int res) {
// {...}
});
```
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.
### <a name="qpromise-delay"></a> `QPromise<T>::delay(handler) -> QPromise<T>`
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.
```cpp
QPromise<int> input = {...}
auto output = input.delay(2000).then([](int res) {
// called 2 seconds after `input` is fulfilled
});
```
### <a name="qpromise-wait"></a> `QPromise<T>::wait() -> QPromise<T>`
This method holds the execution of the remaining code **without** blocking the event loop of the current thread:
```cpp
int result = -1;
QPromise<int> input = qPromise(QtConcurrent::run([]() { return 42; }));
auto output = input.then([&](int res) {
result = res;
});
// output.isPending() is true && result is -1
output.wait();
// output.isPending() is false && result is 42
```
### `QPromise<T>::isPending() -> bool`
Returns `true` if the promise is pending (not fulfilled or rejected), otherwise returns `false`.
### `QPromise<T>::isFulfilled() -> bool`
Returns `true` if the promise is fulfilled, otherwise returns `false`.
### `QPromise<T>::isRejected() -> bool`
Returns `true` if the promise is rejected, otherwise returns `false`.
## QPromise (statics)
### <a name="qpromise-resolve"></a> `[static] QPromise<T>::resolve(value) -> QPromise<T>`
Creates a `QPromise<T>` that is fulfilled with the given `value` of type `T`:
```cpp
QPromise<int> compute(const QString& type)
{
if (type == "magic") {
return QPromise<int>::resolve(42);
}
return QPromise<int>([](const QPromiseResolve<int>& resolve) {
// {...}
});
}
```
See also: [`qPromise`](#helpers-qpromise)
### <a name="qpromise-reject"></a> `[static] QPromise<T>::reject(reason) -> QPromise<T>`
Creates a `QPromise<T>` that is rejected with the given `reason` of *whatever type*:
```cpp
QPromise<int> compute(const QString& type)
{
if (type == "foobar") {
return QPromise<int>::reject(QString("Unknown type: %1").arg(type));
}
return QPromise<int>([](const QPromiseResolve<int>& resolve) {
// {...}
});
}
```
### <a name="qpromise-all"></a> `[static] QPromise<T>::all(Sequence<QPromise<T>>) -> QPromise<QVector<T>>`
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.
`Sequence` is any STL compatible container (eg. `QVector`, `QList`, `std::vector`, etc.)
```cpp
QVector<QPromise<QByteArray> > promises{
download(QUrl("http://a...")),
download(QUrl("http://b...")),
download(QUrl("http://c..."))
};
auto output = QPromise<QByteArray>::all(promises);
// output type: QPromise<QVector<QByteArray>>
output.then([](const QVector<QByteArray>& res) {
// {...}
});
```
See also: [`qPromiseAll`](#helpers-qpromiseall)
## Helpers
### <a name="helpers-qpromise"></a> `qPromise(T value) -> QPromise<R>`
Similar to the `QPromise<T>::resolve` static method, creates a promise resolved from a given `value` without the extra typing:
```cpp
auto promise = qPromise(); // QPromise<void>
auto promise = qPromise(42); // QPromise<int>
auto promise = qPromise(QString("foo")); // QPromise<QString>
```
This method also allows to convert `QFuture<T>` to `QPromise<T>` delayed until the `QFuture` is finished ([read more](#qtconcurrent-convert)).
### <a name="helpers-qpromiseall"></a> `qPromiseAll(Sequence<QPromise<T> promises) -> QPromise<QVector<T>>`
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:
```cpp
QVector<QPromise<QByteArray> > promises{...}
auto output = qPromiseAll(promises);
// eq. QPromise<QByteArray>::all(promises)
```
* [Getting Started](https://qtpromise.netlify.com/qtpromise/getting-started)
* [Thread-Safety](https://qtpromise.netlify.com/qtpromise/thread-safety)
* [QtConcurrent](https://qtpromise.netlify.com/qtpromise/qtconcurrent)
* [API Reference](https://qtpromise.netlify.com/qtpromise/api-reference)
## License
QtPromise is available under the [MIT license](LICENSE).

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<a href="https://promisesaplus.com/" title="Promises/A+ 1.1"><img src="https://promisesaplus.com/assets/logo-small.png" alt="Promises/A+" align="right"/></a>
# QtPromise
[Promises/A+](https://promisesaplus.com/) implementation for [Qt/C++](https://www.qt.io/).
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).
## QtPromise for C++
* [Getting Started](qtpromise/getting-started.md)
* [Thread-Safety](qtpromise/thread-safety.md)
* [QtConcurrent](qtpromise/qtconcurrent.md)
* [API Reference](qtpromise/api-reference.md)
## License
QtPromise is available under the [MIT license](https://github.com/simonbrunel/qtpromise/blob/master/LICENSE).

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### QtPromise for C++
* [Getting Started](qtpromise/getting-started.md)
* [QtConcurrent](qtpromise/qtconcurrent.md)
* [Thread-Safety](qtpromise/thread-safety.md)
* [API Reference](qtpromise/api-reference.md)
* [QPromise](qtpromise/qpromise/constructor.md)
* [.delay](qtpromise/qpromise/delay.md)
* [.fail](qtpromise/qpromise/fail.md)
* [.finally](qtpromise/qpromise/finally.md)
* [.isFulfilled](qtpromise/qpromise/isfulfilled.md)
* [.isPending](qtpromise/qpromise/ispending.md)
* [.isRejected](qtpromise/qpromise/isrejected.md)
* [.tap](qtpromise/qpromise/tap.md)
* [.then](qtpromise/qpromise/then.md)
* [.wait](qtpromise/qpromise/wait.md)
* [::all (static)](qtpromise/qpromise/all.md)
* [::reject (static)](qtpromise/qpromise/reject.md)
* [::resolve (static)](qtpromise/qpromise/resolve.md)
* [qPromise](qtpromise/helpers/qpromise.md)
* [qPromiseAll](qtpromise/helpers/qpromiseall.md)

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## QPromise
### Public Members
* [`QPromise<T>::QPromise`](qpromise/constructor.md)
* [`QPromise<T>::delay`](qpromise/delay.md)
* [`QPromise<T>::fail`](qpromise/fail.md)
* [`QPromise<T>::finally`](qpromise/finally.md)
* [`QPromise<T>::isFulfilled`](qpromise/isfulfilled.md)
* [`QPromise<T>::isPending`](qpromise/ispending.md)
* [`QPromise<T>::isRejected`](qpromise/isrejected.md)
* [`QPromise<T>::tap`](qpromise/tap.md)
* [`QPromise<T>::then`](qpromise/then.md)
* [`QPromise<T>::wait`](qpromise/wait.md)
### Public Static Members
* [`[static] QPromise<T>::all`](qpromise/all.md)
* [`[static] QPromise<T>::reject`](qpromise/reject.md)
* [`[static] QPromise<T>::resolve`](qpromise/resolve.md)
## Helpers
* [`qPromise`](helpers/qpromise.md)
* [`qPromiseAll`](helpers/qpromiseall.md)

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## Installation
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`.
## qpm
Alternatively and **only** if your project relies on [qpm](https://www.qpm.io/), you can install QtPromise as follow:
```bash
qpm install com.github.simonbrunel.qtpromise
```
## Usage
The recommended way to use QtPromise is to include the single module header:
```cpp
#include <QtPromise>
```
## Example
Let's first make the code more readable by using the library namespace:
```cpp
using namespace QtPromise;
```
This `download` function creates a [promise from callbacks](qpromise/constructor.md) which will be resolved when the network request is finished:
```cpp
QPromise<QByteArray> download(const QUrl& url)
{
return QPromise<QByteArray>([&](
const QPromiseResolve<QByteArray>& resolve,
const QPromiseReject<QByteArray>& reject) {
QNetworkReply* reply = manager->get(QNetworkRequest(url));
QObject::connect(reply, &QNetworkReply::finished, [=]() {
if (reply->error() == QNetworkReply::NoError) {
resolve(reply->readAll());
} else {
reject(reply->error());
}
reply->deleteLater();
});
});
}
```
The following method `uncompress` data in a separate thread and returns a [promise from QFuture](qtconcurrent.md):
```cpp
QPromise<Entries> uncompress(const QByteArray& data)
{
return qPromise(QtConcurrent::run([](const QByteArray& data) {
Entries entries;
// {...} uncompress data and parse content.
if (error) {
throw MalformedException();
}
return entries;
}, data));
}
```
It's then easy to chain the whole asynchronous process using promises:
- initiate the promise chain by downloading a specific URL,
- [`then`](qpromise/then.md) *and only if download succeeded*, uncompress received data,
- [`then`](qpromise/then.md) validate and process the uncompressed entries,
- [`finally`](qpromise/finally.md) perform operations whatever the process succeeded or failed,
- and handle specific errors using [`fail`](qpromise/fail.md).
```cpp
download(url).then(&uncompress).then([](const Entries& entries) {
if (entries.isEmpty()) {
throw UpdateException("No entries");
}
// {...} process entries
}).finally([]() {
// {...} cleanup
}).fail([](QNetworkReply::NetworkError err) {
// {...} handle network error
}).fail([](const UpdateException& err) {
// {...} handle update error
}).fail([]() {
// {...} catch all
});
```

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## `qPromise`
```
qPromise(T value) -> QPromise<R>
```
Similar to the [`QPromise<T>::resolve`](../qpromise/resolve.md) static method, creates a promise resolved from a given `value` without the extra typing:
```cpp
auto promise = qPromise(); // QPromise<void>
auto promise = qPromise(42); // QPromise<int>
auto promise = qPromise(QString("foo")); // QPromise<QString>
```
This method also allows to convert `QFuture<T>` to `QPromise<T>` delayed until the `QFuture` is finished ([read more](../qtconcurrent.md#convert)).

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## `qPromiseAll`
```
qPromiseAll(Sequence<QPromise<T>> promises) -> QPromise<QVector<T>>
qPromiseAll(Sequence<QPromise<void>> promises) -> QPromise<void>
```
This method simply calls the appropriated [`QPromise<T>::all`](../qpromise/all.md) 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:
```cpp
QVector<QPromise<QByteArray> > promises{...}
auto output = qPromiseAll(promises);
// eq. QPromise<QByteArray>::all(promises)
```

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## `[static] QPromise<T>::all`
```
[static] QPromise<T>::all(Sequence<QPromise<T>> promises) -> QPromise<QVector<T>>
```
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.
`Sequence` is any STL compatible container (eg. `QVector`, `QList`, `std::vector`, etc.)
```cpp
QVector<QPromise<QByteArray> > promises{
download(QUrl("http://a...")),
download(QUrl("http://b...")),
download(QUrl("http://c..."))
};
auto output = QPromise<QByteArray>::all(promises);
// output type: QPromise<QVector<QByteArray>>
output.then([](const QVector<QByteArray>& res) {
// {...}
});
```
See also: [`qPromiseAll`](../helpers/qpromiseall.md)

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## `QPromise<T>::QPromise`
```
QPromise<T>::QPromise(Function resolver)
```
Creates a new promise that will be fulfilled or rejected by the given `resolver` lambda:
```cpp
QPromise<int> promise([](const QPromiseResolve<int>& resolve, const QPromiseReject<int>& reject) {
async_method([=](bool success, int result) {
if (success) {
resolve(result);
} else {
reject(customException());
}
});
});
```
> **Note:** `QPromise<void>` is specialized to not contain any value, meaning that the `resolve` callback takes no argument.
**C++14**
```cpp
QPromise<int> promise([](const auto& resolve, const auto& reject) {
// {...}
});
```

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## `QPromise<T>::delay`
```
QPromise<T>::delay(int msec) -> QPromise<T>
```
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.
```cpp
QPromise<int> input = {...}
auto output = input.delay(2000).then([](int res) {
// called 2 seconds after `input` is fulfilled
});
```

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## `QPromise<T>::fail`
```
QPromise<T>::fail(Function onRejected) -> QPromise<T>
```
Shorthand to `promise.then(nullptr, onRejected)`, similar to the [`catch` statement](http://en.cppreference.com/w/cpp/language/try_catch):
```cpp
promise.fail([](const MyException&) {
// {...}
}).fail(const QException&) {
// {...}
}).fail(const std::exception&) {
// {...}
}).fail() {
// {...} catch-all
});
```

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## `QPromise<T>::finally`
```
QPromise<T>::finally(Function handler) -> QPromise<T>
```
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.
```cpp
auto output = input.finally([]() {
// {...}
});
```
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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# `QPromise<T>::isFulfilled`
```
QPromise<T>::isFulfilled() -> bool
```
Returns `true` if the promise is fulfilled, otherwise returns `false`.

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# `QPromise<T>::isPending`
```
QPromise<T>::isPending() -> bool
```
Returns `true` if the promise is pending (not fulfilled or rejected), otherwise returns `false`.

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# `QPromise<T>::isRejected`
```
QPromise<T>::isRejected() -> bool
```
Returns `true` if the promise is rejected, otherwise returns `false`.

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## `[static] QPromise<T>::reject`
```
[static] QPromise<T>::reject(any reason) -> QPromise<T>
```
Creates a `QPromise<T>` that is rejected with the given `reason` of *whatever type*:
```cpp
QPromise<int> compute(const QString& type)
{
if (type == "foobar") {
return QPromise<int>::reject(QString("Unknown type: %1").arg(type));
}
return QPromise<int>([](const QPromiseResolve<int>& resolve) {
// {...}
});
}
```

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## `[static] QPromise<T>::resolve`
```
[static] QPromise<T>::resolve(T value) -> QPromise<T>
```
Creates a `QPromise<T>` that is fulfilled with the given `value` of type `T`:
```cpp
QPromise<int> compute(const QString& type)
{
if (type == "magic") {
return QPromise<int>::resolve(42);
}
return QPromise<int>([](const QPromiseResolve<int>& resolve) {
// {...}
});
}
```
See also: [`qPromise`](../helpers/qpromise.md)

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## `QPromise<T>::tap`
```
QPromise<T>::tap(Function handler) -> QPromise<T>
```
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`](finally.md), this handler is **not** called for rejections.
```cpp
QPromise<int> input = {...}
auto output = input.tap([](int res) {
log(res);
}).then([](int res) {
// {...}
});
```
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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## `QPromise<T>::then`
```
QPromise<T>::then(Function onFulfilled, Function onRejected) -> QPromise<R>
QPromise<T>::then(Function onFulfilled) -> QPromise<R>
```
See [Promises/A+ `.then`](https://promisesaplus.com/#the-then-method) for details.
```cpp
QPromise<int> input = ...
auto output = input.then([](int res) {
// called with the 'input' result if fulfilled
}, [](const ReasonType& reason) {
// called with the 'input' reason if rejected
// see QPromise<T>::fail for details
});
```
> **Note**: `onRejected` handler is optional, `output` will be rejected with the same reason as `input`.
> **Note**: it's recommended to use the [`fail`](fail.md) shorthand to handle errors.
The type `<R>` of the `output` promise depends on the return type of the `onFulfilled` handler:
```cpp
QPromise<int> input = {...}
auto output = input.then([](int res) {
return QString::number(res); // -> QPromise<QString>
});
// output type: QPromise<QString>
output.then([](const QString& res) {
// {...}
});
```
> **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.
```cpp
QPromise<int> input = ...
auto output = input.then([](int res) {
return res + 4;
}, [](const ReasonType& reason) {
return -1;
});
```
If `onFulfilled` doesn't return any value, the `output` type is `QPromise<void>`:
```cpp
QPromise<int> input = ...
auto output = input.then([](int res) {
// {...}
});
// output type: QPromise<void>
output.then([]() {
// `QPromise<void>` `onFulfilled` handler has no argument
});
```
You can also decide to skip the promise result by omitting the handler argument:
```cpp
QPromise<int> input = {...}
auto output = input.then([]( /* skip int result */ ) {
// {...}
});
```
The `output` promise can be *rejected* by throwing an exception in either `onFulfilled` or `onRejected`:
```cpp
QPromise<int> input = {...}
auto output = input.then([](int res) {
if (res == -1) {
throw ReasonType();
} else {
return res;
}
});
// output.isRejected() is true
```
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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## `QPromise<T>::wait`
```
QPromise<T>::wait() -> QPromise<T>
```
This method holds the execution of the remaining code **without** blocking the event loop of the current thread:
```cpp
int result = -1;
QPromise<int> input = qPromise(QtConcurrent::run([]() { return 42; }));
auto output = input.then([&](int res) {
result = res;
});
// output.isPending() is true && result is -1
output.wait();
// output.isPending() is false && result is 42
```

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## QtConcurrent
QtPromise integrates with [QtConcurrent](https://doc.qt.io/qt-5/qtconcurrent-index.html) to make easy chaining QFuture with QPromise.
## <a name="qtconcurrent-convert"></a> Convert
Converting `QFuture<T>` to `QPromise<T>` is done using the [`qPromise`](helpers/qpromise.md) helper:
```cpp
QFuture<int> future = QtConcurrent::run([]() {
// {...}
return 42;
});
QPromise<int> promise = qPromise(future);
```
or simply:
```cpp
auto promise = qPromise(QtConcurrent::run([]() {
// {...}
}));
```
## Chain
Returning a `QFuture<T>` in [`then`](qpromise/then.md) or [`fail`](qpromise/fail.md) automatically translate to `QPromise<T>`:
```cpp
QPromise<int> input = ...
auto output = input.then([](int res) {
return QtConcurrent::run([]() {
// {...}
return QString("42");
});
});
// output type: QPromise<QString>
output.then([](const QString& res) {
// {...}
});
```
The `output` promise is resolved when the `QFuture` is [finished](https://doc.qt.io/qt-5/qfuture.html#isFinished).
## Error
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`](https://doc.qt.io/qt-5/qunhandledexception.html#details) (this restriction only applies to exceptions thrown from a QtConcurrent thread, [read more](https://doc.qt.io/qt-5/qexception.html#details)).
```cpp
QPromise<int> promise = ...
promise.then([](int res) {
return QtConcurrent::run([]() {
// {...}
if (!success) {
throw CustomException();
}
return QString("42");
});
}).fail(const CustomException& err) {
// {...}
});
```

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## Thread-Safety
QPromise is thread-safe and can be copied and accessed across different threads. QPromise relies on [explicitly data sharing](https://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.
> **Note:** while it's safe to access the resolved value from different threads using [`then`](qpromise/then.md), QPromise provides no guarantee about the object being pointed to. Thread-safety and reentrancy rules for that object still apply.