QT signal and slot connection (loose coupling)

    GUI In the user interface , When a user operates a widget , Need other widgets to respond , The traditional way often uses callback（ Callback mechanism ） To achieve . The so-called callback is to pass the function pointer as a parameter to another function in advance , Then call the callback function in the appropriate place in the process of function processing. The callback mechanism has two defects ： The type is not safe , There is no guarantee that the correct parameters will be used in calling the function ; Strong coupling , The handler must know which callback function to call .Qt The signal and slot mechanism is type safe , loose coupling , More flexible , More convenient .

    Signal and slot （Signal & Slot） yes  Qt  Fundamentals of programming , It's also Qt A big innovation of . Because of the programming mechanism of signal and slot , stay Qt It becomes more intuitive and simple when dealing with the interactive operation of various components of the interface . 

    The signal （Signal） It's an event launched in a particular situation , for example PushButton The most common signal is emitted when the mouse clicks clicked() The signal . Use of transmitted signal Qt Of emit keyword .QT Of signals Keyword indicates entering the signal declaration area , Then you can declare your signal . 

Put three controls on the dialog window , When clicking the corresponding control, the DeBug Output “XXX come on.  ” word .

    Slot （Slot） Is a function of the response to the signal . Slot is a function , With general C++ The function is the same , Can be declared in any part of the class （public、private or protected）, Can have any parameter , It can also be called directly . The difference between slot function and general function is ： Slot functions can be associated with a signal , When the signal is transmitted , The associated slot function is automatically executed . 

     We can add slot methods to the control by right clicking . You can also press on_ Object name _ Manually add slot method in the format of event name , The slot method added in this format can be automatically connected with Emitted when clicked clicked() The signal Connect .

private slots:
    void on_whBtn_clicked();
    void on_zgBtn_clicked();
    void on_sjBtn_clicked();    // Auto connect  on_ Object name _ Signal name 

keyword signals Define a signal , When the slot method clicked by the control executes, we let it send this signal .

signals:
    void Commeon(QString& str); // Signals cannot and need not be realized , And the return value is void

Use emit Keywords send signals .

void MainWindow::on_whBtn_clicked()
{
    QString str=" Come on, Wuhan ";
    emit Commeon(str);
}

void MainWindow::on_zgBtn_clicked()
{
    QString str=" Go China ";
    emit Commeon(str);
}

void MainWindow::on_sjBtn_clicked()
{
    QString str=" Let the world go ";
    emit Commeon(str);
}

Let's take another slot method , Used to correlate with the above signal ; The slot method will be a CString stay DeBug Print out

private slots:
    void CommeonGuys(QString& str);

void MainWindow::CommeonGuys(QString& str)
{
    qDebug()<<str;
}

Bind the signal to the slot method newly defined above in the constructor

connect(this,SIGNAL(Commeon(QString&)),this,SLOT(CommeonGuys(QString&)));

The signal is associated with the slot by QObject::connect() Function implementation ：

QMetaObject::Connection QObject::connect(
const QObject *sender, // Signal sender 
const char *signal, // Transmitted signal 
const QObject *receiver, // The receiver of the signal 
const char *method, // A string that represents the way the signal is connected , It can be a slot or a signal 
Qt::ConnectionType type = Qt::AutoConnection // How to connect , Default auto connect 

)

Common formats ：connect(sender, SIGNAL(signal()), receiver, SLOT(slot()));

Signal usage SIGNAL, For slots, use SLOT, Use them to surround the prototype of the function . Be careful connect The method is SIGNAL() And SLOT() when , there Function prototypes can only write types , Cannot have any parameter name , Otherwise, the connection will fail .

At the same time, the signal is connected with the slot ：

1. Qt::AutoConnection：( Default connection mode ) Automatic mode , The system automatically selects the connection mode .
2. Qt::DirectConnection： Direct way , When the signal is transmitted , Call slot now .
3. Qt::QueuedConnection： Queue mode , When the signal is transmitted, an event is generated and enters the queue , The slot can only call... When the event is processed .
4. Qt::BlockQueuedConnection： Blocking queue mode , When the signal is transmitted, an event is generated and enters the queue , Then the current thread enters the blocking state , Until the event is handled , If the receiver is in the thread sending the signal , Then the program will deadlock , Therefore, this connection method is only used for multithreading .

You can also not use SIGNAL and SLOT, Take the address directly .

connect(this,&MainWindow::Common,this,&MainWindow::CommonGuys);

The signal can be regarded as a special function , Need to be in parentheses , With parameters , The signal does not need to be realized and cannot be realized . Slot function requires parentheses , When there are parameters, you also need to specify the parameters . When the signal and slot function have parameters , stay connect() In the function , Specify the type of parameter . The parameters of the signal shall be consistent with the parameter list of the slot , More than slot parameters are allowed . If there are no matches or too few parameters , There will be compilation errors or running errors . In classes that use signals and slots , You must add macros to the class definition Q_OBJECT. When a signal is transmitted , The slot function associated with it is usually executed immediately , Just like calling a function normally . Only when all slot functions associated with the signal are executed , Will execute the code behind the transmitting signal .