Hello everyone!
As the title implies I am having a bit of a problem regarding how to
"comunicate" between multiple widgets.
As an example, I wanted to make a program that has one button and some other
widgets who's properties should change when the button is pressed.
As you know, to do this I must make a special callback function which must be
called inside the "callback" method of a widget.Something like this:
button_1.callback(button_cb,some_data)
The button_cb is a function I have to make myself and in every tutorial it
seems to be like:
void button_cb(Fl_Widget *w, void *v)
Those to input parameters, *w and *v can then be used to "obtain" the widgets
you need by casting.What this theoretically means is that i can only comunicate
between maximum 3 widgets:
-the widget that triggers the callback
-and other 2 widgets via casting from *w and *v
So I made a simple program whit 4 widgets: a Fl_Button and 3 Fl_Input.I chose 4
widgets because 3 seems to be the maximum so I wanted more than 3.This is my
program:
Program 1
#include <FL/Fl.H>
#include <FL/Fl_Double_Window.H>
#include <FL/Fl_Button.H>
#include <FL/Fl_Input.H>
void button_cb(Fl_Widget *w, void *v) // the callback function
{
Fl_Input *my_input1 = (Fl_Input*) w;
Fl_Input *my_input2 = (Fl_Input*) v;
Fl_Input *my_input3 = (Fl_Input*) v; // I used the *v pointer 2 times
my_input1->value("BBB"); // the value should change to "BBB"
}
/*********************************************************************************/
int main()
{
Fl_Double_Window my_win(300,180, "Multiple widgets");
my_win.begin();
Fl_Input input_1(60,20,200,30,"Input 1");
input_1.value("AAA");
Fl_Input input_2(60,60,200,30,"Input 2");
input_2.value("BBB");
Fl_Input input_3(60,100,200,30,"Input 2");
input_3.value("CCC");
Fl_Button button_1(60,140,100,30,"Change");
button_1.callback(button_cb,&input_1);
my_win.end();
my_win.show();
return Fl::run();
}
/*********************************************************************************/
As you can see, I've still passed only 2 arguments to button_1 callback
method,, because that's the maximum, but I thougth about solving this problem
later, after all I want more than one widget to be changed.But untill the I
thougth I should just compile this program and see what happens.The program
does compile, the fact that I've casted more than once isn't an error.But when
I press the button the program crashes.
Where could I have gone wrong???
After all, I casted Fl_Widget *w to an Fl_Input *my_input_1, to "get" the first
widget.And even better I've passed the input_1 object created in main() to the
callback function as well, so the callback would about what widget I was
talking about.And even better, eve if I've casted void *v 2 times for the other
2 widgets, I never tried to change their values, I just casted, that's all.But
still the program crashes.
In my naivety I thougth that maybe Fl_Input does not have a value() method so I
thougth about changing it to something I'm sure Fl_Input has,like color().So I
changed the program a little:
Program 2 , modified callback function
void button_cb(Fl_Widget *w, void *v) // the callback function
{
Fl_Input *my_input1 = (Fl_Input*) w;
Fl_Input *my_input2 = (Fl_Input*) v;
Fl_Input *my_input3 = (Fl_Input*) v; // I used the *v pointer 2 times
my_input1->color(0); // this should make the color of the input black
}
As you can see I've only changed one line of code inside the button_cb
function.The program does compile again, but when I press the "Change" button
guess who's color changes into black???It's the button's color which changes
into black!!!!Incredible!!!!
Even if I casted the Fl_Widget *w to Fl_Input, even if in the callback of
button_1 I've specifically placed an Fl_Input like:
button_1.callback(button_cb,&input_1);
it's the button's color which changes.Why???
This made me understand that in the:
void button_cb(Fl_Widget *w, void *v)
function, the first input, Fl_Widget *w is reserved for the calling object, no
matter how you cast it.In this case the calling object was the button_1 via
it's callback function.So kind of an indirect call, because it doesn't call the
button_cb() directly.
******Anyway, is this true??That he first parameter is reserved somehow for the
calling object???And if so, why???????******
Even better, not only the first parameter Fl_Widget *w seems to be reserved,
but also the second one which doesn't mind casting to other widget types.Like
this:
Program 3 , the second parameter seems to be reserved two
#include <FL/Fl.H>
#include <FL/Fl_Double_Window.H>
#include <FL/Fl_Button.H>
#include <FL/Fl_Input.H>
#include <FL/Fl_Value_Slider.H>
void button_cb(Fl_Widget *w, void *v)
{
Fl_Button *my_button = (Fl_Button*) w; // este defapt butonul
Fl_Input *my_input1 = (Fl_Input*) v;
Fl_Value_Slider *my_input2 = (Fl_Value_Slider*) v;
my_button->color(100000);
my_input1->value("BBB");
my_input2->color(10); // the color should change
}
/*********************************************************************************/
int main()
{
Fl_Double_Window my_win(300,150, "Multiple widgets");
my_win.begin();
Fl_Input input_1(60,20,200,30,"Input 1");
input_1.value("AAA");
Fl_Input input_2(60,60,200,30,"Input 2");
input_2.value("BBB");
Fl_Button button_1(60,100,100,30,"Change");
button_1.callback(button_cb,&input_1);
my_win.end();
my_win.show();
return Fl::run();
}
/*********************************************************************************/
As you can see above, now I'm casting the second parameter, void *v to
different types of widgets, an Fl_Input and an Fl_Value_Slider.But in the
callback function of button_1 I pass an Fl_Input as in:
button_1.callback(button_cb,&input_1);
The program above does compile, and even better when I press the button, not
only that the value of input_1 changes but also it's color, even if in
button_cb i do this:
//
Fl_Value_Slider *my_input2 = (Fl_Value_Slider*) v;
//
my_input2->color(10); // the color should change
I try to change the color of an Fl_Value_Slider, not an Fl_Input, but still the
color of input_1 changes, and that's an Fl_Input.
I suspect that this is because both are Fl_Widgets and thy both have a color
method, but I'm not shore.
Please note that this second problem might be related to my poor understanding
of C++ as well, I am not an expert :)
********Anyway the question is:
Is it true that in the button_cb function the second parameter is reserved for
what you put in the callback method of the object???*******
Like this:
void button_cb(Fl_Widget *w, void *v)
button_1.callback(button_cb,&input_1);
I mean that the *v should be reserved for input_1
Thank for at least reading this long post.As you can see my questions ended up
being about something else than multiple widget callbacks, but the questions
turned out of the need to do multiple widgets callback :)
P.S:
This message was supposed to be larger , but my computer is slow so I will
split it up into 2 or more posts.
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