CODEWITHSUNDEEP
c++qtstaticcallbacksignals-slots
user99545
user99545

Reputation: 1173

Sending signal from static class method in Qt

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

Upvotes: 14

Views: 20075

Answers (4)

Louis Xu
Louis Xu

Reputation: 41

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

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

Upvotes: 3

BЈовић
BЈовић

Reputation: 64223

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.

Upvotes: 13

max
max

Reputation: 146

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);
   }

Upvotes: 4

pnezis
pnezis

Reputation: 12321

You must provide a pointer to the class instance in order to make it work.

Notice however that in general it is not advised to emit signals from static functions. Static functions can be called without creating an instance of the class, which means that (if you do not provide as argument the sender and use it explicitely), you will not have a "sender" object for the emited signals.

For these cases as Lol4t0 suggests I also prefer callbacks.

Upvotes: 0

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