πŸ’ Getting the most of signal/slot connections : Viking Software – Qt Experts

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New: connecting to simple function. The new syntax can even connect to functions, not just QObjects: connect(sender, &Sender::valueChanged.


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This is exactly what connect function does. Signals and slots are the ones needing this connection. The point is if you do one connection, you need to be careful.


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This has a very subtle advantage over a typical mechanism used in standard C or C++ 11 like callbacks with function pointers and lambda functions wrapped in std​.


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Getting the most of signal/slot connections : Viking Software – Qt Experts
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A callback is a pointer to a function, so if you want a processing function to Qt's signals and slots mechanism ensures that if you connect a signal to a slot, the.


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This kind of error usually occurs if you add the Q_OBJECT macro after having written and compiling your class. Rerunning qmake will usually.


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This kind of error usually occurs if you add the Q_OBJECT macro after having written and compiling your class. Rerunning qmake will usually.


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connect function in qt

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This is exactly what connect function does. Signals and slots are the ones needing this connection. The point is if you do one connection, you need to be careful.


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connect function in qt

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This kind of error usually occurs if you add the Q_OBJECT macro after having written and compiling your class. Rerunning qmake will usually.


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connect function in qt

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This has a very subtle advantage over a typical mechanism used in standard C or C++ 11 like callbacks with function pointers and lambda functions wrapped in std​.


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connect function in qt

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A callback is a pointer to a function, so if you want a processing function to Qt's signals and slots mechanism ensures that if you connect a signal to a slot, the.


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connect function in qt

But we want to notify the user when the task is done show some message, hide some progress bar, etc. One common use of the above pattern with std::bind is when you have a class implemented through a data pointer private implementation or pimpl idiom. The new syntax allows to call not just a member function declared as slot in the header with public slots: or with protected or private instead of public , but any kind of function more on that in the next section. Since Qt 5. The locks are inside Qt, because QCoreApplication::postEvent the function used to add the event to the queue is thread-safe. The only requirement is that said context object has to be a QObject. We could do the same with a lambda:. Then the context object serves the purpose of automatically breaking the connection when the context is destroyed. Imagine that the button required to close a dialog, or stop some network request, etc. It needs to happen soon but not immediately, so you use a timer with a short timeout. But sometimes you can teach new tricks to an old dog, and QObjects gained a new way to connect between signals and slots in Qt5, plus some extra features to connect to other functions which are not slots. This is not usually a problem, since you can create and ad-hoc QObject instance and even do simple but useful tricks with it. It would mean that you would attempt to call into the UI thread from the thread where the task was run which is a helper thread pool, not the UI thread. If you have some part of the application using Qt classes but no UI tightly related to that, you can surely find a way to leverage those as members of a class not based on QObject. Both sender and receiver are in agreement. But we will leave that topic to another article. This is wrong, and in some cases Qt will nicely warn you that you are using some function from the wrong thread, but if you are not lucky, you will have a mysterious crash. So signals and slots are very safe by default, and in an automatic way. And when there is a free standing function there is a signature, but there is no instance, so the third and the fourth arguments of the first two calls are somewhat merged. Now to the main point that I want you to notice here. If you continue to use this site we will assume that you are happy with it.{/INSERTKEYS}{/PARAGRAPH} That could be QML, for example. It could be that the widget gets shown and immediately closed. But we are missing something more important. But if you did the same typo in the last example, it would be a compile time error, which is clearly much better. But there is one additional important use of it: making the signal be delivered in the thread that you prefer, so you can save from tedious and error prone locking. If you did that in some dialog of an application you would have to navigate to that dialog to confirm that it worked! Bonus point: thread safety by thread affinity The above section is the main goal of this article. This shows a class that derives from QRunnable to reimplement the run function, and that derives from QObject to provide the finished signal. There is still one use case where you would want to declare functions as slots, and that is if you want to make that function usable by any feature that happens at run time. Ported to the new syntax, the above example is:. Again, there is one killer feature of signals and slots that we often ignore because it happens automatically. An instance is created after the user activates a button, and then we show some progress bar and run the task. The previous example shows one way that works across old versions of Qt published so far Qt 1 to 5. If you need a button or a timer to call a member function of the private class that is not going to be a QObject , you can write something like this:. As simple as that. The main problem with the example above is that as you probably knew, or guessed from being all uppercase is that SIGNAL and SLOT are macros, and what those macros do is convert to a string the argument passed. In the above example, the third argument context object might be forgotten, and the code will compile and run, but it would be a serious bug. Now any typo in the name will produce a compile time error. Note that std::bind is actually much more powerful, and can be very useful when the number of arguments differ. That can be expressed in pseudocode like this:. Signals and slots were one of the distinguishing features that made Qt an exciting and innovative tool back in time. Note that the arguments are still checked at compile time: the signal has no arguments, and the lambda has no arguments either. With the previous examples that nice balance of the four arguments is gone. Now it is as if the lambda were a slot in your class, because to the timer, the context of the connection is the same. No need to declare members as slots anymore or almost The new syntax allows to call not just a member function declared as slot in the header with public slots: or with protected or private instead of public , but any kind of function more on that in the next section. Often the alternatives do exist and can be as good, if not better but admittedly this is a matter of taste. For example, say that you want to run a lambda only on the first click:. So since Qt 5. The new versus the old way to use connect The previous example shows one way that works across old versions of Qt published so far Qt 1 to 5. When one QObject instance is the receiver of a signal, its thread affinity is checked, and by default the signal is delivered directly as a function call when is the same thread affinity of the sender. Share this blog post. That warranties the problem mentioned is now gone. For some reason you show some widget and you need to do some last minute update after it has been shown. Just remember that the old is still valid, but the new is preferred in most situations. I am not going to bore you with repeating basic knowledge you already have, but I want you to look at signals and slots from a certain angle, so it will be easier to understand the design of the feature I will cover next. What would happen if the lambda of the previous examples would use an invalid pointer? This assumes you are already moderately familiar with signals and slots. That can be expressed in pseudocode like this: connect oneObject, somethingHappened, otherObject, reactToSomethingHappened ; Notice that the four phrases that are into quotes in the previous paragraph are the four arguments of the function call in the pseudocode. You can easily handle that there are no longer invalid captures on a lambda. You can use any QObject as context object, but the most common case will be to shut down timers, processes, requests, or anything related to what your user interface is doing when some dialog, window or panel closes. Since this is an exciting new feature added to a new major version, which has received some extra polishing during the minor releases, many blog posts from other members of the Qt community have been published about it for example covering implementation details or the issues that could arise when there are arguments involved. That looks in code like the following:. The example using std::bind requires a bit more explanation if you are not familiar with it. Notice that the four phrases that are into quotes in the previous paragraph are the four arguments of the function call in the pseudocode. In this case we have the two objects and the two function pointers, which is to be expected for what is wanted. This is a problem because any typo in what gets passed to those means that the call to connect would fail and return false. If the sender object is destroyed, it obviously can not emit any signal because it is a member function of its class. That works, but has a subtle problem. But if the thread affinity differs, it will be delivered posting an event to the object. The context object can save you crashes, and having to manually disconnect. So what std::bind does here is create a callable object that glues together the particular instance that we want with one member function. One simple thought about the basics I am not going to bore you with repeating basic knowledge you already have, but I want you to look at signals and slots from a certain angle, so it will be easier to understand the design of the feature I will cover next. In case of need, you can force a direct call from different threads, or a queued call from the same thread. And it would be even more annoying if you were connecting to some error handling, and is not that easy to trigger said error. And you write. {PARAGRAPH}{INSERTKEYS}Posted In: Qt. An example. The above section is the main goal of this article. Recently a blog post about porting a tutorial application from Qt 1 to Qt 5. This will delete the ad-hoc QObject guard on the first invocation, and the connection will be automatically broken.