[QT] QT multithreading development - four methods to realize multithreading design

Qt— Use Qt Four ways to realize multithreading design

One 、 Write it at the front

​ The content of this article is ： About Qt Designed for multithreaded applications .
stay Qt There are four methods for multithreading programming and design . Reasonably select the corresponding methods to meet the application scenarios in the actual development .

Two 、【 Method 1 】 QThread： Bottom layer with optional event loops API

QThread yes Qt The basis of all threaded controls in , Every QThread An instance represents and controls a thread . Use QThread There are two ways to create a thread ：1）、 You can directly instantiate and create ,2） You can also subclass it for thread creation .

1、 Instantiation QThread： Provides a parallel event loop , Allow calls to... In worker threads QObject Slot function .

2、 Inherit QThread： Allows applications to initialize new threads before starting an event loop , Or run parallel code without event loops .

3、 ... and 、【 Method 2 】 QThreadPool and QRunnable： Reuse threads

​ The cost of creating and destroying threads frequently can be high . To reduce the cost , You can reuse existing threads for new tasks .QThreadPool Is reusable QThreads Set .

To be in QThreadPool Running code in a thread of , It needs to be realized again QRunnable::run() And instantiate subclassed QRunnable.

Use ````QThreadPool::start()``` take QRunnable Put it in QThreadPool In the running queue of . When threads are available ,QRunnable::run() The code in will execute in this thread .

【 remarks 】： Every Qt Applications all have one Global thread pool , Can pass QThreadPool::globalInstance() Visit it . This global thread pool is based on CPU The number of cores in will automatically maintain the optimal number of threads . however , In actual development , You can also explicitly create and manage a separate QThreadPool.

Four 、【 Method 3 】Qt Concurrent ： Use advanced API

​ Qt The concurrency module provides many advanced functions , It is used to deal with some common parallel computing modes . for example ：map、filter and reduce.Qt Concurrency and usage QThread and QRunnable Different , These functions do not need to use the underlying thread primitives , Such as mutual exclusion or semaphores . contrary , They return a QFuture object , This object can be used to automatically retrieve the result of the function when the thread is ready or completed ;QFuture It can also be used to query 、 Calculate progress and pause / recovery / Cancel the calculation . For convenience ,QFutureWatcher It is allowed to communicate with QFutures Interact .

​ Qt Concurrent Parallel computing model ：map、filter and reduce Wait for the algorithm to automatically allocate the calculation to all available processor cores , therefore , The application we write today will continue to be expanded and used when it is deployed to systems with more kernels , It's very convenient .

​ This module also provides QtConcurrent::run() function , It can run any function in another thread . however ,QtConcurrent::run() Only support map、filter and reduce A subset of features available for functions ,QFuture It can be used to retrieve the return value of the function and check whether the thread is running .

​ however , Yes QtConcurrent::run() The call of uses only one thread , It can't be suspended / recovery / Cancel , You cannot query the process .

5、 ... and 、【 Method four 】 WorkerScript：QML Threading in

​ WorkerScriptQML Type allows JavaScript Code and the GUI Threads running in parallel . Every WorkerScript An instance can be attached .js Script . call WorkerScript.sendMessage() when , The script will be in a separate thread ( And individual QML Context ) Run in . When the script finishes running , It can send a reply back GUI Threads , This thread will call WorkerScript.onMessage() Signal processing program .

Use WorkerScript It's similar to using... That has been moved to another thread worker QObject, Data is transmitted between threads through signals .

【 notes 】 This method QML Use in

6、 ... and 、 How to choose the solutions of the above four different multithreading designs

​ As shown above ,Qt It provides different solutions for developing multithreaded applications . The right solution for multithreaded applications depends on ： The purpose of the new thread and the life cycle of the thread . Here is Qt Comparison of several multithreading mechanisms ：

Serial number	characteristic	QThread	QRunnable and QThreadPool	QtConcurrent::run()	Qt Concurrent(Map/Filter/Reduce)	WorkerScript
1	development language	C++	C++	C++	C++	QML
2	Whether thread priority can be specified	Yes	Yes
3	Whether a thread can run an event loop	Yes
4	Threads receive data updates through signals	Yes (received by a worker QObject)				Yes (received by WorkerScript)
5	Whether threads can use signals to control	Yes (received by QThread)			Yes (received by QFutureWatcher)
6	Whether the thread can pass QFuture To monitor			In part	Yes
7	Whether it has built-in capabilities ： Cancel / Pause / recovery				Yes

7、 ... and 、Qt Multithreaded application design example

Thread life cycle	operation	resolvent
One call	Run a new linear function in another thread , You can choose to update the progress during operation .	Qt Provides different solutions ： Put this function in QThread::run() Re implementation of , And start the QThread. Signal to update progress . Put this function in QRunnable::run() Re implementation of , And will QRunnable Add to QThreadPool in . Write thread safe variables to update progress . Use QtConcurrent:: Run() Operation function . Write thread safe variables to update progress .
One call	Run an existing function in another thread and get its return value .	Use QtConcurrent:: Run() Operation function . Give Way QFutureWatcher When the function returns finished() The signal , And call QFutureWatcher::result() To get the return value of the function .
One call	Run an existing function in another thread and get its return value .	Use QtConcurrent:: Run() Operation function . Give Way QFutureWatcher When the function returns finished() The signal , And call QFutureWatcher::result() To get the return value of the function .
One call	Use all available kernel pair containers （Container） All the items of the perform operations . for example , Generate thumbnails from the image list .	Use QtConcurrent Of QtConcurrent::filter() Function to select container elements , Use QtConcurrent::map() Function to apply an operation to each element . To convert the output into a single result , have access to QtConcurrent:: filteredreduce() and QtConcurrent::mappedReduced().
One call / Forever	In pure QML Complete the calculation of growth time in the application , And update when the results are ready GUI.	Put the calculation code in .js Script , And attach it to WorkerScript example . call WorkerScript.sendMessage() Start the calculation in a new thread . Let the script call sendMessage(), Pass the result back to GUI Threads . stay onMessage Process the results in and update them there GUI.
Forever	There is an object in another thread , It can perform different tasks according to the request , also / Or you can receive new data to process .	Subclass a QObject To create a worker. Instantiate this worker Object and a QThread. take worker Move to new thread . Connect to... Through queued signals and slots worker Object to send commands or data .
Forever	Repeat expensive operations in another thread , The thread does not need to receive any signals or events .	Directly in QThread::run() Write infinite loop in the re implementation of . Start the thread without an event loop . Let the thread signal to send data back GUI Threads .