Differential D18758 Diff 58398 kdevplatform/3rdparty/qtpromise/qtpromise-0.5.0/docs/qtpromise/qtconcurrent.md

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kdevplatform/3rdparty/qtpromise/qtpromise-0.5.0/docs/qtpromise/qtconcurrent.md

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		1		# Qt Concurrent
		2
		3		QtPromise integrates with [QtConcurrent](https://doc.qt.io/qt-5/qtconcurrent-index.html) to make easy chaining QFuture with QPromise.
		4
		5		## <a name="qtconcurrent-convert"></a> Convert
		6
		7		Converting `QFuture<T>` to `QPromise<T>` is done using the [`QtPromise::resolve`](helpers/resolve.md) helper:
		8
		9		```cpp
		10		QFuture<int> future = QtConcurrent::run([]() {
		11		// {...}
		12		return 42;
		13		});
		14
		15		QPromise<int> promise = QtPromise::resolve(future);
		16		```
		17
		18		or simply:
		19
		20		```cpp
		21		auto promise = QtPromise::resolve(QtConcurrent::run([]() {
		22		// {...}
		23		}));
		24		```
		25
		26		## Chain
		27
		28		Returning a `QFuture<T>` in [`then`](qpromise/then.md) or [`fail`](qpromise/fail.md) automatically translate to `QPromise<T>`:
		29
		30		```cpp
		31		QPromise<int> input = ...
		32		auto output = input.then([](int res) {
		33		return QtConcurrent::run([]() {
		34		// {...}
		35		return QString("42");
		36		});
		37		});
		38
		39		// output type: QPromise<QString>
		40		output.then([](const QString& res) {
		41		// {...}
		42		});
		43		```
		44
		45		The `output` promise is resolved when the `QFuture` is [finished](https://doc.qt.io/qt-5/qfuture.html#isFinished).
		46
		47		## Error
		48
		49		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 will 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)).
		50
		51		```cpp
		52		QPromise<int> promise = ...
		53		promise.then([](int res) {
		54		return QtConcurrent::run([]() {
		55		// {...}
		56
		57		if (!success) {
		58		throw CustomException();
		59		}
		60
		61		return QString("42");
		62		});
		63		}).fail(const CustomException& err) {
		64		// {...}
		65		});
		66		```