Sending signal from static class method in Qt

Question

I am trying to code a static callback function that is called frequently from another static function within the same class. My callback function needs to emit a signal but for some reason it simply fails to do so. I have put it under a debugger and the slot never gets called. However when I place the code I used to emit the data in a non-static function it works. Is there a reason I cannot emit a signal from a static function? I have tried declaring a new instance of the class and calling the emit function but with no luck.

class Foo
{
signals:
    emitFunction(int);
private:
    static int callback(int val)
    {
        /* Called multiple times (100+) */
        Foo *foo = new Foo;
        foo.emitFunction(val);
    }
    void run()
    {
        callback(percentdownloaded);
    }
};

I have posted some basic code that demonstrates what I am attempting to do. I will post full code upon request.

Edit: I am posting the full code since this is kind of an odd scenario. http://pastebin.com/6J2D2hnM

Answer 1

That is not going to work, because you are creating a new Foo every time you enter that static function, and you do not connect a signal to a slot.

So, the fix would be to pass the object to that function :

class Foo
{
signals:
    emitFunction(int);
private:
    static int callback(int val, Foo &foo)
    {
        /* Called multiple times (100+) */
        foo.emitFunction(val);
    }
    void run()
    {
        callback(percentdownloaded, *this);
    }
};

Another option is to use postEvent, but I wouldn't recommend it.

---

Since you can not modify callback's signature, you can do it like this :

class Foo
{
signals:
    emitFunction(int);
private:
    static int callback(int val)
    {
        /* Called multiple times (100+) */
        theFoo->emitFunction(val);
    }
    static Foo *theFoo;
    void run()
    {
        callback(percentdownloaded, *this);
    }
};

but you'll have to initialize that static variable somewhere.

Answer 2

in case someone still finding the solution, here is what I did, it works in my project. 1. make your class to be a singleton 2. in static cb function , load emitFunction from the your singleton class

    static int callback(int val)
   {
   /* Called multiple times (100+) */
   MYClass::getInstance()->emitFunction(val);
   }

Answer 3

There is an elegant solution. You can emit a signal in a static member function like this:

emit (instance().newMessage(message));