Qt Signals Slots Thread Safety
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Signals and slots is a language construct introduced in Qt for communication between objects[1] which makes it easy to implement the observer pattern while avoiding boilerplate code. The concept is that GUI widgets can send signals containing event information which can be received by other widgets / controls using special functions known as slots. This is similar to C/C++ function pointers, but signal/slot system ensures the type-correctness of callback arguments.[citation needed]
Qt Signal Slot Thread
The signal/slot system fits well with the way graphical user interfaces are designed. Similarly, the signal/slot system can be used for other non-GUI usages, for example asynchronous I/O (including sockets, pipes, serial devices, etc.) event notification or to associate timeout events with appropriate object instances and methods or functions. It is easy to use and no registration/deregistration/invocation code need to be written, because Qt's metaobject compiler (MOC) automatically generates the needed infrastructure.
That's one of the reasons many of the 'data' classes in Qt are implicitly shared, and what's more they have internal thread-safe reference counting. On a side note one of the side effects of using signals and slots is you shouldn't need to use mutexes directly. The signal slot connection (with a few rare exceptions) is safe across thread.
Free downloadable slot games for windows 7 1. A commonly used metaphor is a spreadsheet. A spreadsheet has cells that observe the source cell(s). When the source cell is changed, the dependent cells are updated from the event.
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- Note: Qt's synchronization classes rely on the use of properly aligned pointers. For instance, you cannot use packed classes with MSVC. These synchronization classes can be used to make a method thread safe. However, doing so incurs a performance penalty, which is why most Qt methods are not made thread safe.
- Cross-thread signal-slot connections are implemented by dispatching a QMetaCallEvent to the target object. A QObject instance can be moved to a thread, where it will process its events, such as timer events or slot/method calls. To do work on a thread, first create your own worker class that derives from QObject. Then move it to the thread.
- That's one of the reasons many of the 'data' classes in Qt are implicitly shared, and what's more they have internal thread-safe reference counting. On a side note one of the side effects of using signals and slots is you shouldn't need to use mutexes directly. The signal slot connection (with a few rare exceptions) is safe across thread.
Alternative implementations[edit]
There are some implementations of signal/slot systems based on C++ templates, which don't require the extra metaobject compiler, as used by Qt, such as libsigc++, sigslot, vdk-signals, nano-signal-slot, neosigslot, Signals, boost.signals2, Synapse, Cpp::Events, Platinum and JBroadcaster. Common Language Infrastructure (CLI) languages such as C# also supports a similar construct although with a different terminology and syntax: events play the role of signals, and delegates are the slots. Another implementation of signals exists for ActionScript 3.0, inspired by C# events and signals/slots in Qt. Additionally, a delegate can be a local variable, much like a function pointer, while a slot in Qt must be a class member declared as such. The C based GObject system also provides similar functionality via GSignal.In D it is implemented by std.signals.
See also[edit]
Qt Thread Signal
Libraries[edit]
Java: sig4j - multi-threaded, type-safe, based on the FunctionalInterface annotation introduced in Java 8.
C++: vdk-signals - thread-safe, type-safe, written in C++11 with atomic variables.
References[edit]
- ^'Signals & Slots - QtCore 5.1'. Qt Project. 2013-07-04. Retrieved 2013-07-04.
Qt Signal Slot Connect
