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Please review the following ** information to ensure the GNU General Public License requirements will ** be met: https://www.gnu.org/licenses/gpl-2.0.html and ** https://www.gnu.org/licenses/gpl-3.0.html. ** ** $QT_END_LICENSE$ ** ****************************************************************************/ /*! \page qtconcurrentrun.html \title Concurrent Run \ingroup thread The QtConcurrent::run() function runs a function in a separate thread. The return value of the function is made available through the QFuture API. QtConcurrent::run() is an overloaded method. You can think of these overloads as slightly different \e modes. In \l {Concurrent Run (basic mode)} {basic mode}, the function passed to QtConcurrent::run() is able to report merely a single computation result to its caller. In \l {Concurrent Run With Promise} {run with promise mode}, the function passed to QtConcurrent::run() can make use of the additional QPromise API, which enables multiple result reporting, progress reporting, suspending the computation when requested by the caller, or stopping the computation on the caller's demand. This function is a part of the Qt Concurrent framework. \section1 Concurrent Run (basic mode) The function passed to QtConcurrent::run() may report the result through its return value. \section2 Running a Function in a Separate Thread To run a function in another thread, use QtConcurrent::run(): \snippet code/src_concurrent_qtconcurrentrun.cpp 0 This will run \c aFunction in a separate thread obtained from the default QThreadPool. You can use the QFuture and QFutureWatcher classes to monitor the status of the function. To use a dedicated thread pool, you can pass the QThreadPool as the first argument: \snippet code/src_concurrent_qtconcurrentrun.cpp explicit-pool-0 \section2 Passing Arguments to the Function Passing arguments to the function is done by adding them to the QtConcurrent::run() call immediately after the function name. For example: \snippet code/src_concurrent_qtconcurrentrun.cpp 1 A copy of each argument is made at the point where QtConcurrent::run() is called, and these values are passed to the thread when it begins executing the function. Changes made to the arguments after calling QtConcurrent::run() are \e not visible to the thread. Note that QtConcurrent::run does not support calling overloaded functions directly. For example, the code below won't compile: //! [run-with-overload-calls] \snippet code/src_concurrent_qtconcurrentrun.cpp 15 The easiest workaround is to call the overloaded function through lambda: \snippet code/src_concurrent_qtconcurrentrun.cpp 16 Or you can tell the compiler which overload to choose by using a \c static_cast: \snippet code/src_concurrent_qtconcurrentrun.cpp 17 Or qOverload: \snippet code/src_concurrent_qtconcurrentrun.cpp 18 //! [run-with-overload-calls] \section2 Returning Values from the Function Any return value from the function is available via QFuture: \snippet code/src_concurrent_qtconcurrentrun.cpp 2 As documented above, passing arguments is done like this: \snippet code/src_concurrent_qtconcurrentrun.cpp 3 Note that the QFuture::result() function blocks and waits for the result to become available. Use QFutureWatcher to get notification when the function has finished execution and the result is available. \section2 Additional API Features \section3 Using Member Functions QtConcurrent::run() also accepts pointers to member functions. The first argument must be either a const reference or a pointer to an instance of the class. Passing by const reference is useful when calling const member functions; passing by pointer is useful for calling non-const member functions that modify the instance. For example, calling QByteArray::split() (a const member function) in a separate thread is done like this: \snippet code/src_concurrent_qtconcurrentrun.cpp 4 Calling a non-const member function is done like this: \snippet code/src_concurrent_qtconcurrentrun.cpp 5 \section3 Using Lambda Functions Calling a lambda function is done like this: \snippet code/src_concurrent_qtconcurrentrun.cpp 6 Calling a function modifies an object passed by reference is done like this: \snippet code/src_concurrent_qtconcurrentrun.cpp 7 Using callable object is done like this: \snippet code/src_concurrent_qtconcurrentrun.cpp 8 \section1 Concurrent Run With Promise The \e {Run With Promise} mode enables more control for the running task compared to \e basic mode of QtConcurrent::run(). It allows progress reporting of the running task, reporting multiple results, suspending the execution if it was requested, or canceling the task on caller's demand. \section2 The mandatory QPromise argument The function passed to QtConcurrent::run() in \e {Run With Promise} mode is expected to have an additional argument of \c {QPromise &} type, where T is the type of the computation result (it should match the type T of QFuture returned by the QtConcurrent::runWithPromise()), like e.g.: \snippet code/src_concurrent_qtconcurrentrun.cpp 9 The \c promise argument is instantiated inside the QtConcurrent::run() function, and its reference is passed to the invoked \c aFunction, so the user doesn't need to instantiate it by himself, nor pass it explicitly when calling QtConcurrent::runWithPromise(). The additional argument of QPromise type always needs to appear as a first argument on function's arguments list, like: \snippet code/src_concurrent_qtconcurrentrun.cpp 10 \section2 Reporting results In contrast to \e basic mode of QtConcurrent::run(), the function passed to QtConcurrent::run() in \e {Run With Promise} mode is expected to always return void type. Result reporting is done through the additional argument of QPromise type. It also enables multiple result reporting, like: \snippet code/src_concurrent_qtconcurrentrun.cpp 11 \section2 Suspending and canceling the execution The QPromise API also enables suspending and canceling the computation, if requested: \snippet code/src_concurrent_qtconcurrentrun.cpp 12 The call to \c future.suspend() requests the running task to hold its execution. After calling this method, the running task will suspend after the next call to \c promise.suspendIfRequested() in its iteration loop. In this case the running task will block on a call to \c promise.suspendIfRequested(). The blocked call will unblock after the \c future.resume() is called. Note, that internally suspendIfRequested() uses wait condition in order to unblock, so the running thread goes into an idle state instead of wasting its resources when blocked in order to periodically check if the resume request came from the caller's thread. The call to \c future.cancel() from the last line causes that the next call to \c promise.isCanceled() will return \c true and \c aFunction will return immediately without any further result reporting. \section2 Progress reporting It's also possible to report the progress of a task independently of result reporting, like: \snippet code/src_concurrent_qtconcurrentrun.cpp 13 The caller installs the \c QFutureWatcher for the \c QFuture returned by QtConcurrent::run() in order to connect to its \c progressValueChanged() signal and update e.g. the graphical user interface accordingly. \section2 Invoking functions with overloaded operator()() By default, QtConcurrent::run() doesn't support functors with overloaded operator()() in \e {Run With Promise} mode. In case of overloaded functors the user needs to explicitly specify the result type as a template parameter passed to QtConcurrent::run(), like: \snippet code/src_concurrent_qtconcurrentrun.cpp 14 */ /*! \typedef Function \internal This typedef is a dummy required to make the \c Function type name known so that clang doesn't reject functions that use it. */ /*! \fn QFuture QtConcurrent::run(Function function, ...); Equivalent to \code QtConcurrent::run(QThreadPool::globalInstance(), function, ...); \endcode Runs \a function in a separate thread. The thread is taken from the global QThreadPool. Note that \a function may not run immediately; \a function will only be run once a thread becomes available. //! [run-description] In \l {Concurrent Run (basic mode)} {basic mode} T is the same type as the return value of \a function. Non-void return values can be accessed via the QFuture::result() function. In \l {Concurrent Run (basic mode)} {basic mode} the QFuture returned can only be used to query for the running/finished status and the return value of the function. In particular, canceling or pausing can be issued only if the computations behind the future has not been started. In \l {Concurrent Run With Promise} {run with promise mode}, the \a function is expected to return void and must take an additional argument of \c {QPromise &} type, placed as a first argument in function's argument list. T is the result type and it is the same for the returned \c QFuture. In \l {Concurrent Run With Promise} {run with promise mode}, similar to \e basic mode, the QFuture returned can be used to query for the running/finished status and the value reported by the function. In addition, it may be used for suspending or canceling the running task, fetching multiple results from the called \a function or monitoring progress reported by the \a function. \sa {Concurrent Run (basic mode)}, {Concurrent Run With Promise} //! [run-description] */ /*! \since 5.4 \fn QFuture QtConcurrent::run(QThreadPool *pool, Function function, ...); Runs \a function in a separate thread. The thread is taken from the QThreadPool \a pool. Note that \a function may not run immediately; \a function will only be run once a thread becomes available. \include qtconcurrentrun.cpp run-description */