Emitting signal from callback

Question

I am using RtMidi library to handle midi message in my Qt application and I am facing problem with slot trigger:

My PhMidiInput object is emiting signal from the RtMidi callback upon specific midi message but the slots are not always triggered.

Here is a part of the PhMidiInput class:

class PhMidiInput : QObject
{
    Q_OBJECT
public:
    void PhMidiInput() {}
signals:
    void quarterFrame(unsigned char data);
private:
    static void callback(double, std::vector< unsigned char > *message, void *userData ) {
        PhMidiInput *midiInput = (PhMidiInput*)userData;
        if(midiInput)
            midiInput->onMessage(message);
    }

    void onMessage(std::vector<unsigned char> *message) {
        ...
        emit quarterFrame(data);
        ...
    }

}

Connecting to a lambda functor works:

PhMidiInput midiIn;
int quarterFrameCount;

connect(&midiIn, &PhMidiInput::quarterFrame, [&](unsigned char data) {
    quarterFrameCount++;
});

Connecting to my application window works to:

// MyWindow inherits from QMainWindow
connect(_midiIn, &PhMidiInput::quarterFrame, this, &MyWindow::onQuarterFrame);

When trying to connect to a custom class (MidiTest) inheriting from QObject it does'nt trigger:

connect(_midiIn, &PhMidiInput::quarterFrame, this, &MidiTest::onQuarterFrame);

I was wondering if there was something around QObject::moveToThread() but since I don't create the thread myself (the signal is sent from a callback) I don't know if I need to use it or not.

ratchet freak ratchet freak · Accepted Answer · 2014-08-13T09:46:37

It is as simple as calling emit obj->quarterFrame(data); from the callback. If the connection type is default then this will be perfectly thread safe.

Though you should create a QByteArray from data to pass around as data will likely not be valid by the time the slots get called.

void callback(..., void* user){
    //user is the standard void* in most callbacks passed as reinterpret_cast<void*>(this)
    unsigned char* data = ...;
    QByteArray bytes(data);
    emit reinterpret_cast<PhMidiInput>(user)->quarterFrame(bytes);//calling the signal which will behave as you'd expect

}

Emitting signal from callback

2 Answers