Weird issue with SIGNAL/SLOT

leinad

Hi,
I've been using signal and slots mostly between processes running under multiple threads but now I'm try instantiate two classes under a signal thread and using signal/slot to communicate between them. But for some reason the slot from one object is not being received from the other. Here is an example.

I'm creating a thread in a class:

#include "PluginWorker.h"

void StartThread::startThread()
{
    std::cout << "STARTING THREAD" << std::endl;

    QThread* pluginWorkerThread = new QThread;
    pluginWorkerThread->setObjectName("pluginWorkerThread");
    PluginWorker *pluginWorker = new PluginWorker;
    pluginWorker->moveToThread(pluginWorkerThread);
    
    connect(pluginWorker, SIGNAL(finished()), pluginWorkerThread, SLOT(quit()));
    connect(pluginWorker, SIGNAL(finished()), pluginWorker, SLOT(deleteLater()));
    connect(pluginWorkerThread, SIGNAL(finished()), pluginWorkerThread, SLOT(deleteLater()));
    connect(this, SIGNAL(sendExit()), pluginWorker, SLOT(receiveExit()), Qt::DirectConnection);

    pluginWorkerThread->start();
}

Once the thread is started using this call from another class

startThread_ = new StartThread();
startThread_->startThread();

The thread does indeed start so far it is ok.
In the pluginWorker class which runs under the above thread, I instantiate 2 classes as follows:

PluginWorker::PluginWorker() :
   selectdatabase_(nullptr)
  ,databaseProcessing_(nullptr)
{
    std::cout << "STARTING PLUGIN WORKER" << std::endl;

    databaseProcessing_ = new DatabaseProcessing(); 
    selectdatabase_ = new SelectDatabase();

    //setup communication between selectdatabase_ and databaseProcessing_
    connect(selectdatabase_, SIGNAL(sendDatabaseName(QString)), databaseProcessing_, SLOT(getDatabaseName(QString)));

}

However when I emit from selectDatabase::sendDatabaseName to the slot in databaseProcessing::getDatabaseName nothing is received.

I'm using Q_Object in both classes for selectDatabase_ and databaseProcessing_

Below is a include example, both are similar:

#ifndef DATBASEPROCESSING_H
#define DATBASEPROCESSING_H

#include <QObject>
#include <QMainWindow>

class DatabaseProcessing : public QObject
{
    Q_OBJECT

public:
    DatabaseProcessing();
    virtual ~DatabaseProcessing();


public Q_SLOTS:
    void getDatabaseName(QString databaseName);
};

Would anyone have any idea why the slot is not being called? I really would appreciate any feedback.

Thanks!

leinad

@JonB
I figured out what is happening. Seems the Emit is being called before the connect statement had a chance to be called. I'll have to rearrange how statements get executed.

Thanks!

JonB

@leinad
Hi. I see you have posted quite a lot in this forum. Would you care to start marking your code blocks inside the forum's Code tags? It's the icon which looks like </>, or put a line of ``` (3 backticks) above and below your blocks. You could edit your post. For my part it would make it a lot easier to see what's going on in your post.

However when I emit from selectDatabase::sendDatabaseName to the slot in databaseProcessing::getDatabaseName nothing is received.

Why don't you start by connecting a lambda to the signal so that you know that it's being emitted?

leinad

@JonB Sorry. I fixed using the back tics. Can you explain how I can connect a lambda to the signal?

Atr0p0s

@leinad Here is a good explanation.

JonB

@leinad
That's much more readable, thank you :)

I didn't want to bring this up, because it likely has nothing to do with your issue, but SIGNAL/SLOT() macros are the old style of Qt connect()s. Unfortunately a lot of examples on the web still use them. The new style was introduced a decade ago as C++ supported it. It is type-safe and gives compile time errors if the slot is not right for the signal.

connect(signalObject, &SignalClass::method, slotObject, &SlotClass::method);

So for your connect(selectdatabase_, SIGNAL(sendDatabaseName(QString)), databaseProcessing_, SLOT(getDatabaseName(QString)));:

connect(selectdatabase_, &SelectDatabase::sendDatabaseName, databaseProcessing_, &DatabaseProcessing::getDatabaseName);

Doesn't that look tidier? :) I really suggest you change to this syntax from now onwards.

You need new-style to use C++ lambdas for the slot. I suggest you try something like:

connect(selectdatabase_, &SelectDatabase::sendDatabaseName,
        databaseProcessing_, [](QString databaseName) { qDebug() << "sendDatabaseName" << databaseName; } );

Now we know whether signal is emitted and passes database name. Does this work?

leinad

@JonB
Thanks, I see. I will give it a try.

leinad

@JonB said in Weird issue with SIGNAL/SLOT:

connect(signalObject, &SignalClass::method, slotObject, &SlotClass::method);

Funny, I use QtCreator with TIDY and it never called it out as a bad style. Perhaps the default options disable it?

leinad

@JonB

connect(selectdatabase_, &SelectDatabase::sendDatabaseName,
        databaseProcessing_, [](QString databaseName) { qDebug() << "sendDatabaseName" << databaseName; } );

I'm afraid I can't use this lambda function because I don't have access to databaseName, it is a QString which is sent from another object outside of the connect class.

leinad

@JonB
Sorry never mind. I can indeed use it.

leinad

@leinad
So it appears that the lambda does indeed show the correct value passed but yet the SLOT does not show it :(

JonB

@leinad
This was just the first step. Now we know the signal does get emitted and the connect() attaches to it.

Now:

connect(selectdatabase_, &SelectDatabase::sendDatabaseName, databaseProcessing_, &DatabaseProcessing::getDatabaseName)

(Put a qDebug() << databaseName; as first statement in slot.) I presume you will say the slot does not get called?

If not, are selectdatabase_ and databaseProcessing_ in different threads? If so, do you allow the event loop to run (signals across threads are queued)?

leinad

@JonB
I figured out what is happening. Seems the Emit is being called before the connect statement had a chance to be called. I'll have to rearrange how statements get executed.

Thanks!