How to disconnect a signal with a slot temporarily in Qt?

Question

I connect a slot with a signal. But now I want to disconnect them temporarily.

Here is part of my class declaration:

class frmMain : public QWidget
{
           


        

Answer 1

Disconnect/reconnect syntax

There are many ways to call disconnect, depending on exactly what you want disconnected. See the QObject documentation page for an explanation of how they work.

Here's an example using 0 to mean "disconnect all slots."

void frmMain::on_btnDownload_clicked()
{
    // disconnect everything connected to myReadTimer's timeout
    disconnect(myReadTimer, SIGNAL(timeout()), 0, 0);    

    ...//the code that I want myReadTimer to leave me alone

    // restore the connection
    connect(myReadTimer,SIGNAL(timeout()),this,SLOT(ReadMyCom()));
}

Or you can specify the exact signal-slot pair to disconnect by copying your 'connect' syntax, like this:

disconnect(myReadTimer,SIGNAL(timeout()),this,SLOT(ReadMyCom()));

Stopping the timer

Since you're working with a timer, this may be simpler:

void frmMain::on_btnDownload_clicked()
{
    // stop the timer (so you won't get any timeout signals)
    myReadTimer->stop();    

    ...//the code that I want myReadTimer to leave me alone

    // restart the timer (using whatever interval was set previously)
    myReadTimer->start();
}

Differences from your original approach:

• Since you're stopping and restarting the timer, the next time it fires will be interval after your slot function finishes.

Do you need to do anything special at all?

In a single-threaded Qt application, if you're already handling a signal, another signal won't "jump in the middle" of that code. Instead it'll be queued up as an even to handle immediately after the current slot returns.

So perhaps you don't need to stop or disconnect your timer at all.

Differences from your original approach:

• If on_btnDownload_clicked takes a while to execute, you might have multiple ReadMyCom events queued up after on_btnDownload_clicked completes. (Note that at this point you'd have an operation that basically "locks up" your GUI for a while anyway; it may make more sense to refactor the function or give it its own thread.)