The signal slot mechanism of QT development

First, the signal slot mechanism principle

1. How to declare a signal slot

A simplified version of the QT header file:

Class Example:public Qobject {q_objectsignals:void customsignal (); void customsignal (int i) public slots:void CustomSlot (); void Customslot (int i);};

2. Macro and MoC Source object

Excerpt code:

Qobjectdefs.hi. ... Ii. #define SLOTSIII. #define SIGNALS PUBLICIV. Emitvi. V. #define. ...

Why do we need MOC?

Since C + + native does not provide introspection, and QT's signal slots and attribute lists are based on introspection, it is easy to list the methods and attributes of the object by introspection, and it can be said that the signal slot of QT is the type safety.

For more information, refer to: interpreting the QT introspection mechanism (so-called introspection refers to the ability of object-oriented language to query object information during runtime)

Second, the development and use of signal slots

Qt5 before:

Connect (sender, SIGNAL (customsignal (int i)), receiver, SLOT (customslot (int i)));

Disadvantage: There is no compiler check because the signal and slot functions are processed into strings, the compiler cannot check for errors at compile time, and all checks are done at run time. This may occur if the compilation passes through the slot but is not called.

You can only view the error message in the console.

QT5 New wording:

Connect (sender, &sender::customsignal,receiver, &receiver::customslot);

Like the previous syntax, the new signal slot features:

• The new syntax
• Compiler check
• Friendly Error Hints
• Automatic parameter type conversion
• Allows any function to be connected (using a function pointer to call the function directly, so the slot is not processed by the MOC.) The signal still needs to be processed by the MOC, which needs to be declared in a specific area

• C++lambda expression (can be willful written in the following format)

Connect (this, &example::customsignal,[=] (int i) {cout << "resule:" << i;}

Common signal Slot usage:

Qtdesigner Create void example::on_pushbutton_clicked () {...} Connect (sender, SIGNAL (customsignal (int i)), receiver, SLOT (customslot (int i))); Connect (sender, &sender:: Customsignal,receiver, &receiver::customslot); Connect (sender, &sender::customsignal,custonslot); Connect (this, &example::customsignal,[=] (int i) {cout << "resule:" << i;}

Third, QT components and signal connection

Simple adder:

The UI interface is designed to

Right click pushbutton Select Go to slot Select clicked Method, will automatically generate and edit code in Summator, run

The signal slot mechanism of QT development