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// Copyright (C) 2020 The Qt Company Ltd.
// SPDX-License-Identifier: LicenseRef-Qt-Commercial OR GFDL-1.3-no-invariants-only
/*!
\page qtconcurrenttask.html
\title Concurrent Task
\ingroup thread
QtConcurrent::task provides an alternative interface for running a
task in a separate thread. The return value of the function is made
available through the QFuture API.
If you want to just run a function in a separate thread without adjusting
any parameters, use QtConcurrent::run as that lets you write less code.
The QtConcurrent::task is designed for cases where you need to perform
extra configurations steps.
This function is a part of the \l {Qt Concurrent} framework.
\section1 Fluent interface
The QtConcurrent::task returns an instance of an auxiliary class called
QtConcurrent::QTaskBuilder. Normally, you don't need to create an instance
of this class manually. The QtConcurrent::QTaskBuilder provides an interface
to adjust different task parameters in a chain-like manner. This approach
is known as a
\l {https://en.wikipedia.org/wiki/Fluent_interface}{fluent interface}.
You can just set the parameters you need and then kick a task off.
In order to finalize the configuration of a task you must invoke
QtConcurrent::QTaskBuilder::spawn. This function is non-blocking (i.e.
returns a future object immediately), but it's not guaranteed that the
task starts immediately. You can use the QFuture and QFutureWatcher classes
to monitor the status of the task.
See more examples and explanations below.
\section1 Running a task in a separate thread
To run a function in another thread, use QtConcurrent::QTaskBuilder::spawn:
\snippet code/src_concurrent_qtconcurrenttask.cpp 0
This will run a lambda function in a separate thread obtained from
the default QThreadPool.
\section1 Passing arguments to the task
Invoking a function with arguments is done by passing them to
QtConcurrent::QTaskBuilder::withArguments:
\snippet code/src_concurrent_qtconcurrenttask.cpp 1
A copy of each argument is made at the point where
QtConcurrent::QTaskBuilder::withArguments is called, and these values
are passed to the thread when it begins executing the task. Changes made
to the arguments after calling QtConcurrent::QTaskBuilder::withArguments
are not visible to the thread.
If you want to run a function that accepts arguments by reference, you
should use \l {https://en.cppreference.com/w/cpp/utility/functional/ref}
{std::ref/cref} auxiliary functions. These functions create thin wrappers
around passed arguments:
\snippet code/src_concurrent_qtconcurrenttask.cpp 2
Make sure that all wrapped objects live long enough. It is possible to
get undefined behavior if a task outlives the object wrapped by
std::ref/cref.
\section1 Returning values from the task
You can obtain the result of a task with the QFuture API:
\snippet code/src_concurrent_qtconcurrenttask.cpp 3
Note that QFuture::result() is a blocking call, it waits for the
result to become available. Use QFutureWatcher to get a notification
when the task has finished execution and the result is available.
In case you want to pass a result to another asynchronous task, you can
use QFuture::then() to create a chain of dependent tasks. See the QFuture
documentation for more details.
\section1 Additional API features
\section2 Using different types of callable objects
Strictly speaking, you can use any type of tasks and arguments that
satisfy the following condition:
\snippet code/src_concurrent_qtconcurrenttask.cpp 4
You can use a free function:
\snippet code/src_concurrent_qtconcurrenttask.cpp 5
You can use a member function:
\snippet code/src_concurrent_qtconcurrenttask.cpp 6
You can use a callable object with an operator():
\snippet code/src_concurrent_qtconcurrenttask.cpp 7
If you want to use an existing callable object, you need to either
copy/move it to QtConcurrent::task or wrap it with std::ref/cref:
\snippet code/src_concurrent_qtconcurrenttask.cpp 8
\section2 Using custom thread pool
You can specify a custom thread pool:
\snippet code/src_concurrent_qtconcurrenttask.cpp 9
\section2 Setting priority for a task
You can set the priority for a task:
\snippet code/src_concurrent_qtconcurrenttask.cpp 10
If you don't need a future object, you can call
QtConcurrent::QTaskBuilder::spawn(QtConcurrent::FutureResult::Ignore):
\snippet code/src_concurrent_qtconcurrenttask.cpp 11
You can access the promise object associated with the task by defining an
additional argument of \c {QPromise<T> &} type inside the function.
This additional argument must be the first argument passed to the function, and
like in \l {Concurrent Run With Promise} mode, the function is expected to return void type.
Result reporting is done through QPromise API:
\snippet code/src_concurrent_qtconcurrenttask.cpp 12
*/
/*!
\fn template <typename Task> [[nodiscard]] QTaskBuilder<Task> QtConcurrent::task(Task &&task);
\since 6.0
Creates an instance of QtConcurrent::QTaskBuilder. This object can be used
to adjust some parameters and run \a task in a separate thread.
\sa {Concurrent Task}, QtConcurrent::QTaskBuilder
*/
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