FLTK value change and redraw waits until after callback: any way around this? - callback

Question

I am new to GUIs and am learning FLTK in order to produce a multi platform application. In the application a user clicks a button and invokes a time consuming function through the callback. As it takes a long time I would like to update an output widget with the progress of the function. As such I pass the pointer to an Fl_Ouput widget and update using widget->value(x) and widget->redraw(). However, the widget only redraws when the entire callback function is completed making the whole thing pointless as it doesn't update the user in real time. The code is of the form:
void calculate(Fl_Widget* widget,Fl_Output* op){
/*function call to actual code but contains inside a loop:*/
stringstream progress;
progress<<p; //where p is some percentage calculated on the fly in the function
op->value(progress.str().c_str());
op->redraw();
}
int main(){
/*main and GUI code*/
Fl_Output* op = new Fl_Output(100,50,100,10,0);
Fl_Button* go = new Fl_Button(100,100,10,10,"Go");
go->callback((Fl_Callback*) calculate,op);
/*rest of GUI code and main*/
}
My problem is that the widget pointed to by op is not redrawing in sequence when I want it to, when op->redraw() is called, but only after the rest of the code in calculate has executed or, as I suspect, when control leaves the callback function when it is finished executing.
My question is: is there any way to update such a widget in real time when its value is changed and before executing any more code (which may be whilst control is passed to a function)? Or is there a better way?
Thanks,
R.

Answer 1

I assume you did not show us the actual C++ code where the problem you have actually appears, so my first guess is that you are doing a long-running processing in a GUI loop, which is always a bad idea.
To see an almost idiomatic FLTK solution, check the threads example in the test directory (you need to gram FLTK source code - it is good idea to go through all the FLTK examples there - that is how I learned FLTK nearly 2 decades ago).
FLTK Manual touches the topic of multi-threading on this page: http://www.fltk.org/doc-1.3/advanced.html .