Qt 中的多线程技术

Qt 提供用于处理线程的很多类和函数。Qt 程序员可以使用以下 4 种不同方式以实现多线程应用程序。

QThread：具有可选事件循环的低级 API

QThread 是 Qt 中所有线程控制的基础。每个 QThread 实例表示并控制一线程。

QThread 可以被直接实例化或子类化。实例化 QThread 提供并行事件循环，允许 QObject 槽在第 2 线程中被援引。子类化 QThread 允许应用程序初始化新线程在开始其事件循环之前，或运行没有事件循环的并行代码。

QThreadPool 和 QRunnable：重用线程

Creating and destroying threads frequently can be expensive. To reduce this overhead, existing threads can be reused for new tasks. QThreadPool is a collection of reuseable QThreads.

To run code in one of a QThreadPool 's threads, reimplement QRunnable::run () and instantiate the subclassed QRunnable 。使用 QThreadPool::start () to put the QRunnable 在 QThreadPool 's run queue. When a thread becomes available, the code within QRunnable::run () will execute in that thread.

Each Qt application has a global thread pool, which is accessible through QThreadPool::globalInstance (). This global thread pool automatically maintains an optimal number of threads based on the number of cores in the CPU. However, a separate QThreadPool can be created and managed explicitly.

Qt Concurrent：使用高级 API

The Qt Concurrent module provides high-level functions that deal with some common parallel computation patterns: map, filter, and reduce. Unlike using QThread and QRunnable , these functions never require the use of low-level threading primitives such as mutexes or semaphores. Instead, they return a QFuture object which can be used to retrieve the functions' results when they are ready. QFuture can also be used to query computation progress and to pause/resume/cancel the computation. For convenience, QFutureWatcher enables interactions with QFuture s via signals and slots.

Qt Concurrent 's map, filter and reduce algorithms automatically distribute computation across all available processor cores, so applications written today will continue to scale when deployed later on a system with more cores.

This module also provides the QtConcurrent::run () function, which can run any function in another thread. However, QtConcurrent::run () only supports a subset of features available to the map, filter and reduce functions. The QFuture can be used to retrieve the function's return value and to check if the thread is running. However, a call to QtConcurrent::run () uses one thread only, cannot be paused/resumed/canceled, and cannot be queried for progress.

见 Qt Concurrent module documentation for details on the individual functions.

WorkerScript：QML 中的线程

The WorkerScript QML type lets JavaScript code run in parallel with the GUI thread.

每个 WorkerScript instance can have one .js script attached to it. When WorkerScript::sendMessage () is called, the script will run in a separate thread (and a separate QML context ). When the script finishes running, it can send a reply back to the GUI thread which will invoke the WorkerScript::onMessage() signal handler.

Using a WorkerScript is similar to using a worker QObject that has been moved to another thread. Data is transferred between threads via signals.

见 WorkerScript documentation for details on how to implement the script, and for a list of data types that can be passed between threads.

选择适当方式

As demonstrated above, Qt provides different solutions for developing threaded applications. The right solution for a given application depends on the purpose of the new thread and the thread's lifetime. Below is a comparison of Qt's threading technologies, followed by recommended solutions for some example use cases.

解决方案比较

特征	QThread	QRunnable and QThreadPool	QtConcurrent::run ()	Qt Concurrent (Map, Filter, Reduce)	WorkerScript
语言	C++	C++	C++	C++	QML
Thread priority can be specified	Yes	Yes
Thread can run an event loop	Yes
Thread can receive data updates through signals	Yes (received by a worker QObject )				Yes (received by WorkerScript )
Thread can be controlled using signals	Yes (received by QThread )			Yes (received by QFutureWatcher )
Thread can be monitored through a QFuture			Partially	Yes
Built-in ability to pause/resume/cancel				Yes

范例使用案例

线程寿命	Operation	Solution
One call	Run a new linear function within another thread, optionally with progress updates during the run.	Qt 提供不同解决方案： Place the function in a reimplementation of QThread::run () and start the QThread . Emit signals to update progress. OR Place the function in a reimplementation of QRunnable::run () and add the QRunnable 到 QThreadPool . Write to a thread-safe variable to update progress. OR Run the function using QtConcurrent::run (). Write to a thread-safe variable to update progress.
One call	Run an existing function within another thread and get its return value.	Run the function using QtConcurrent::run (). Have a QFutureWatcher emit the finished() signal when the function has returned, and call QFutureWatcher::result () to get the function's return value.
One call	Perform an operation on all items of a container, using all available cores. For example, producing thumbnails from a list of images.	Use Qt Concurrent's QtConcurrent::filter () function to select container elements, and the QtConcurrent::map () function to apply an operation to each element. To fold the output into a single result, use QtConcurrent::filteredReduced () 和 QtConcurrent::mappedReduced () 代替。
One call/Permanent	Perfrom a long computation in a pure QML application, and update the GUI when the results are ready.	Place the computation code in a `.js` script and attach it to a WorkerScript instance. Call sendMessage() to start the computation in a new thread. Let the script call WorkerScript::sendMessage () too, to pass the result back to the GUI thread. Handle the result in `onMessage` and update the GUI there.
Permanent	Have an object living in another thread that can perform different tasks upon request and/or can receive new data to work with.	Subclass a QObject to create a worker. Instantiate this worker object and a QThread . Move the worker to the new thread. Send commands or data to the worker object over queued signal-slot connections.
Permanent	Repeatedly perform an expensive operation in another thread, where the thread does not need to receive any signals or events.	Write the infinite loop directly within a reimplementation of QThread::run (). Start the thread without an event loop. Let the thread emit signals to send data back to the GUI thread.