Connecting Built-In PySide/PyQt Signals

Create a Qt app and a window. Slots can be used for receiving signals, but they are also normal member functions. A template based solution was introduced in Qt 5. For example, if a user clicks a Close button, we probably want the window's close function to be called. The current state of affairs is still:

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MOC, the Meta Object Compiler

In GUI programming, when we change one widget, we often want another widget to be notified. More generally, we want objects of any kind to be able to communicate with one another. For example, if a user clicks a Close button, we probably want the window's close function to be called. Older toolkits achieve this kind of communication using callbacks.

A callback is a pointer to a function, so if you want a processing function to notify you about some event you pass a pointer to another function the callback to the processing function. The processing function then calls the callback when appropriate.

Callbacks have two fundamental flaws: Firstly, they are not type-safe. We can never be certain that the processing function will call the callback with the correct arguments. Secondly, the callback is strongly coupled to the processing function since the processing function must know which callback to call.

In Qt, we have an alternative to the callback technique: We use signals and slots. A signal is emitted when a particular event occurs. Qt's widgets have many predefined signals, but we can always subclass widgets to add our own signals to them. I know I'm missing the 'receiver widget' from the connect statement, although it does work without it in the main window class and I don't get any errors when compiling.

Let me know if I need to include my header files as well. It's not the connection that is the problem, the problem is that your object vanishes before you can do anything useful with it. Below is a self-contained example that demonstrates this problem in both Qt 4 and 5. As soon as you click "Delete Other", the Test Button doesn't work. After your objects are initialized, you should connected them to each other. Explicitly naming whose slots are whose and whose signals are whose, will help a lot.

Sometimes I'll make a helper function void Widget:: You can see these errors in the application output when you run it in Debug mode. By clicking "Post Your Answer", you acknowledge that you have read our updated terms of service , privacy policy and cookie policy , and that your continued use of the website is subject to these policies.

Qt connect signal to slot Ask Question. Now you understand why posting code is very important. The situation is slightly different when using queued connections ; in such a case, the code following the emit keyword will continue immediately, and the slots will be executed later.

If several slots are connected to one signal, the slots will be executed one after the other, in the order they have been connected, when the signal is emitted. Signals are automatically generated by the moc and must not be implemented in the. They can never have return types i. A note about arguments: Our experience shows that signals and slots are more reusable if they do not use special types.

Range, it could only be connected to slots designed specifically for QScrollBar. Connecting different input widgets together would be impossible. A slot is called when a signal connected to it is emitted. However, as slots, they can be invoked by any component, regardless of its access level, via a signal-slot connection. This means that a signal emitted from an instance of an arbitrary class can cause a private slot to be invoked in an instance of an unrelated class.

Compared to callbacks, signals and slots are slightly slower because of the increased flexibility they provide, although the difference for real applications is insignificant. In general, emitting a signal that is connected to some slots, is approximately ten times slower than calling the receivers directly, with non-virtual function calls.

This is the overhead required to locate the connection object, to safely iterate over all connections i. While ten non-virtual function calls may sound like a lot, it's much less overhead than any new or delete operation, for example. As soon as you perform a string, vector or list operation that behind the scene requires new or delete , the signals and slots overhead is only responsible for a very small proportion of the complete function call costs. The same is true whenever you do a system call in a slot; or indirectly call more than ten functions.

The simplicity and flexibility of the signals and slots mechanism is well worth the overhead, which your users won't even notice. Note that other libraries that define variables called signals or slots may cause compiler warnings and errors when compiled alongside a Qt-based application.

To solve this problem, undef the offending preprocessor symbol. The QObject -based version has the same internal state, and provides public methods to access the state, but in addition it has support for component programming using signals and slots. This class can tell the outside world that its state has changed by emitting a signal, valueChanged , and it has a slot which other objects can send signals to. They must also derive directly or indirectly from QObject.

Slots are implemented by the application programmer. Here is a possible implementation of the Counter:: The emit line emits the signal valueChanged from the object, with the new value as argument.

In the following code snippet, we create two Counter objects and connect the first object's valueChanged signal to the second object's setValue slot using QObject:: Then b emits the same valueChanged signal, but since no slot has been connected to b 's valueChanged signal, the signal is ignored.

Note that the setValue function sets the value and emits the signal only if value! This prevents infinite looping in the case of cyclic connections e.

By default, for every connection you make, a signal is emitted; two signals are emitted for duplicate connections. You can break all of these connections with a single disconnect call.

If you pass the Qt:: UniqueConnection type , the connection will only be made if it is not a duplicate. If there is already a duplicate exact same signal to the exact same slot on the same objects , the connection will fail and connect will return false.

This example illustrates that objects can work together without needing to know any information about each other. To enable this, the objects only need to be connected together, and this can be achieved with some simple QObject:: The following is an example of the header of a simple widget class without member functions.

The purpose is to show how you can utilize signals and slots in your own applications. After the class constructor and public members, we declare the class signals. The LcdNumber class emits a signal, overflow , when it is asked to show an impossible value.

Subclassing QDialog