Qt slot thread
Apr 21, · Is it thread safe to exchange data between threads using signals and slots, such that the signal is emitted in the sending thread and the slot is executed in the receiving thread, i.e. using queued connections? QThreads general usage. From Qt Wiki. different ways to use threads in Qt, cases which involve event-driven programming and signals + slots across threads. Aug 05, · "How to use QThread in the right way (Part 1)" But when SLOTS and Qt event loop are used in the worker thread, Connect the timeout signal to the slot of Thread;.
C++ GUI Programming with Qt4: Multithreading
Threads Events QObjects
Conventional GUI applications have one thread of execution and perform one operation at a time. If the user invokes a time-consuming operation from the user interface, the interface typically freezes while the operation is in progress.
Chapter 7 presents some solutions to this problem. Multithreading is another solution. In a multithreaded application, the GUI runs in its own thread and additional processing takes place in one or more other threads.
This results in applications that have responsive GUIs even during intensive processing. When runnning on a single processor, multithreaded applications may run slower than a single-threaded equivalent due to the overhead of having multiple threads. But on multiprocessor systems, which are becoming increasingly common, multithreaded applications can execute several threads simultaneously on different processors, resulting in better overall performance.
Finally, we round off with a review of which Qt classes can be used in secondary threads and which cannot. Multithreading is a large topic with many books devoted to the subject—for example, Threads Primer: Here it is assumed that you already understand the fundamentals of multithreaded programming, so the focus is on explaining how to develop multithreaded Qt applications rather than on the subject of threading itself.
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One of the most popular topics on the " qt IRC channel": Nine times out of ten, a quick inspection of their code shows that the biggest problem is the very fact they're using threads in the first place, and they're falling in one of the endless pitfalls of parallel programming.
Moreover, threading support in Qt is a double-edged sword: The purpose of this document is not to teach you how to use threads, do proper locking, exploit parallelism, nor write scalable programs; there are many good books about these topics; for instance, take a look to the recommended reading list on this page. Instead, this small article is meant to be a guide to introduce users to threading in Qt 4, in order to avoid the most common pitfalls and help them to develop code that is at the same time more robust and with a better structure.
Not being a general-purpose introduction to threads programming, we expect you to have some previous knowledge about:. In this document we'll follow the Qt naming conventions , which are:. Being an event-driven toolkit, events and event delivery play a central role in Qt architecture. In this article we'll not give a comprehensive coverage about this topic; we'll instead focus on some thread-related key concepts see here and here for more information about the Qt event system.
An event in Qt is an object which represents something interesting that happened; the main difference between an event and a signal is that events are targeted to a specific object in our application which decides what to do with that event , while signals are emitted "in the wild".
From a code point of view, all events are instances of some subclass of QEvent , and all QObject-derived classes can override the QObject:: The important thing about events is that they're not delivered as soon as they're generated; they're instead queued up in an event queue and sent sometime later. The dispatcher itself loops around the event queue and sends queued events to their target objects, and therefore it is called the event loop.
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