The slot is called after it is disconnected or blocked.

extream

// .h - file
class QtClipboard : public QMainWindow
{
    Q_OBJECT
public:
    QtClipboard(QWidget *parent = Q_NULLPTR);
 
private:
    Ui::QtClipboardClass ui;
    QClipboard* pcb;
// ...
private slots:
    void clipboardChanged();
//...
}
 
// .сpp - file
QtClipboard::QtClipboard(QWidget *parent)
    : QMainWindow(parent)
{
ui.setupUi(this);
//...
pcb = QApplication::clipboard();
connect(pcb, &QClipboard::dataChanged, this, &QtClipboard::clipboardChanged);
}
 
//...
 
void QtClipboard::slotCopyToClipboard()
{       
    const QListWidgetItem *item = listHistory->selectedItems().at(0);
    QString itemText = item->text();
    
    
    //pcb->disconnect(SIGNAL(dataChanged()), 0, 0); 
    //pcb->blockSignals(true);
    bool ret = pcb->disconnect(this); // returned true
    pcb->setText(itemText);   // This method continues to send a signal to the slot
    connect(pcb, &QClipboard::dataChanged, this, &QtClipboard::clipboardChanged); // if to comment this line slot will be disconnected successful!
    //pcb->blockSignals(false);
    //connect(pcb, &QClipboard::dataChanged, this, &QtClipboard::clipboardChanged, Qt::DirectConnection);
}

jsulm

@extream said in The slot is called after it is disconnected or blocked.:

connect(pcb, &QClipboard::dataChanged, this, &QtClipboard::clipboardChanged);

This is your second connect for that signal/slot.
First one is in constructor.

extream

Yes, first time i call connect(pcb, &QClipboard::dataChanged, this, &QtClipboard::clipboardChanged) in the constructor

jsulm

@extream But why do you connect it twice?

JonB

@extream
As @jsulm has said, you should not be connecting (same signal/slot) twice. Note also from the docs:

By default, a signal is emitted for every connection you make; 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 an invalid QMetaObject::Connection.

VRonin

You just need to wait 1 event loop cycle before reconnecting:
add a private member to the class QMetaObject::Connection clipConn;

QtClipboard::QtClipboard(QWidget *parent)
    : QMainWindow(parent)
{
ui.setupUi(this);
//...
clipConn = QObject::connect(QApplication::clipboard(), &QClipboard::dataChanged, this, &QtClipboard::clipboardChanged);
}

void QtClipboard::slotCopyToClipboard()
{       
    const QString itemText = listHistory->selectedItems().at(0)->text();
    QObject::disconnect(clipConn);
    QApplication::clipboard()->setText(itemText);
    QTimer::singleShot(0,this,[this](){
        if(!clipConn)
            clipConn = QObject::connect(QApplication::clipboard(), &QClipboard::dataChanged, this, &QtClipboard::clipboardChanged);
    });
}

extream

@VRonin said in The slot is called after it is disconnected or blocked.:

QTimer::singleShot(0,this,this{
if(!clipConn)
clipConn = QObject::connect(QApplication::clipboard(), &QClipboard::dataChanged, this, &QtClipboard::clipboardChanged);
});

Tahnks! It work!