Re: [Scilab-users] bug when setting gcbo field in a callback function ?

[Stéphane Mottelet]

Hello Antoine,

Never trust the life time of gcbo, and first copy the value of gcbo like 
this:

function  funcb1()
    h=gcbo
    disp(typeof(h));
    h.callback_type=-1;
    disp(typeof(h));
    h.callback_type=0;
endfunction

With this trick the type of h is always a handle. However, I cannot 
explain why the second syntax using set() always works...

S.

Le 28/09/2018 à 21:00, antoine monmayrant a écrit :
Hi all,

In a callback function, gcbo is a handle to the calling uicontrol.
There are 2 ways to set one of the fields of this handle (let's take 
"callback_type" as an example):
    (1)    gcbo.callback_type=-1;
or equivalently
    (2)    set(gcbo, "callback_type",-1);

In theory, both are perfectly equivalent.
In practice however, (1) leads to recurrent issues where gcbo is no 
longer a handle, but a structure!

See below  an example script showing the difference:
- two slides with equivalent callback functions funcb1 and funcb2, but 
for the different syntactic sugar (1) or (2).
- if you play with the top slider in the figure, everything's fine, 
gcbo remains a handle (see output on the console).
- if you play with the bottom slider in the figure, gcbo turns into a 
struct (see output on the console).

Needless to say, this kind of issue made my GUI debugging session 
really fun.
To make things even more fun, if you "pause" then execute "funcb1()" 
step by step, you don't necessarily trigger the bug, it's quite random 
and fairly unfrequent.

Can you try to reproduce this bug on your computer?

Cheers,

Antoine


// BUG_gcbo_set.sce //
h=scf();

hs1=uicontrol(h, ...
    "style", "slider", ...
    "tag", "slide1", ...
    "backgroundcolor", [1 1 1], ...
    "value", 0, ...
    "min", -100, ...
    "max", 100, ...
    "sliderstep", [1, 10], ...
    "position", [20,40,200,20], ...
    "constraints", createConstraints("border", "top"), ...
    "callback", "funcb1");

function  funcb1()
    disp(typeof(gcbo));
    gcbo.callback_type=-1;
    disp(typeof(gcbo));
    gcbo.callback_type=0;
endfunction


hs2=uicontrol(h, ...
    "style", "slider", ...
    "tag", "slide1", ...
    "backgroundcolor", [1 1 1], ...
    "value", 0, ...
    "min", -100, ...
    "max", 100, ...
    "sliderstep", [1, 10], ...
    "position", [20,80,200,20], ...
    "constraints", createConstraints("border", "top"), ...
    "callback", "funcb2");

function  funcb2()
    disp(typeof(gcbo));
    set(gcbo, "callback_type",-1);
    disp(typeof(gcbo));
    set(gcbo, "callback_type",0)
endfunction

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