Qt Connect Signal Slot Another Class



Introduction

  1. How Qt Signal And Slots Works
  2. Qt Connect Signal Slot Another Classifieds
  3. Qt Connect Slot

Remember old X-Window call-back system? Generally it isn't type safe and flexible. There are many problems with them. Qt offer new event-handling system - signal-slot connections. Imagine alarm clock. When alarm is ringing, signal is sending (emitting). And you're handling it as a slot.

  1. Every QObject class may have as many signals of slots as you want.
  2. You can emit signal only from that class, where signal is.
  3. You can connect signal with another signal (make chains of signals);
  4. Every signal and slot can have unlimited count of connections with other.
  5. ATTENTION! You can't set default value in slot attributes. e.g.void mySlot(int i = 0);
Connect

Connection

You can connect signal with this template:QObject::connect ( const QObject * sender, const char * signal, const QObject * receiver, const char * method);You have to wrap const char * signal and const char * method into SIGNAL () and SLOT() macros.

My signal was declared with the namespace alias. To reproduce this, just create a namespace around a class. Create an alias for the namespace. Declare a signal in another class with a parameter of type alias::class. Then connect the signal to a slot. It will compile, but when you run it, you will get a message that the signal or slot does not. Hello, I am attempting to connect a signal from one class instance to another but am having trouble. Here is the scenario. I have 3 classes, ClassA, ClassB and ClassC. ClassA inherits ClassB, ClassB inherits QObject, and ClassC inherits QObject.

Qt Connect Signal Slot Another Class

How Qt Signal And Slots Works

And you also can disconnect signal-slot:QObject::disconnect ( const QObject * sender, const char * signal, const QObject * receiver, const char * method);

Qt Connect Signal Slot Another Classifieds

Deeper

Widgets emit signals when events occur. For example, a button will emit a 'clicked' signal when it is clicked. A developer can choose to connect to a signal by creating a function (a 'slot') and calling the connect() function to relate the signal to the slot. Qt's signals and slots mechanism does not require classes to have knowledge of each other, which makes it much easier to develop highly reusable classes. Since signals and slots are type-safe, type errors are reported as warnings and do not cause crashes to occur.

For example, if a Quit button's clicked() signal is connected to the application's quit() slot, a user's click on Quit makes the application terminate. In code, this is written as

Qt Connect Signal Slot Another Class

connect(button, SIGNAL (clicked()), qApp, SLOT (quit()));

Connections can be added or removed at any time during the execution of a Qt application, they can be set up so that they are executed when a signal is emitted or queued for later execution, and they can be made between objects in different threads.

The signals and slots mechanism is implemented in standard C++. The implementation uses the C++ preprocessor and moc, the Meta-Object Compiler, included with Qt. Code generation is performed automatically by Qt's build system. Developers never have to edit or even look at the generated code.

Qt Connect Slot

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