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Dependent property is shown in the variable editor even before its get method

Asked by Zoltán Csáti on 31 Jan 2016
Latest activity Commented on by Zoltán Csáti on 7 Mar 2019
I have a class with a dependent property. But even before I invoke its get method, its value is shown in the Variable Editor. How can this happen, isn't a dependent property calculated on demand (i.e. calling its get method)? For an actual code, see e.g. the documentation .

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1 Answer

Answer by per isakson
on 31 Jan 2016
Edited by per isakson
on 6 Mar 2019
 Accepted Answer

"before I invoke its get method, its value is shown" Yes, that's is obviously the case, but I fail to find this behaviour described in the documentation. However, I think it's useful.
IIRC: The dependent property is calculated "in the background" when displayed in a tooltip or the Variable Editor. Several releases back, the dependent property was calculated by an explicit call to the get-function when required for a tooltip. That caused a lot of pain when there was a bug in the get-function.
Try the following steps
>> cdp = class_dep_prop();
  • set a breakpoint in the foo-method
>> cdp.foo
  • point at this to see a tooltip. Note that a value is displayed for x, but not for y.
  • replace b=(1:3); by b=(1:2); and repeat the steps. Now, a value is displayed for y.
This behaviour is better than the old one, which throw an error in the get-function. (Maybe, it would be even better to show something like y=-error- in the tooltip when there is a bug in the get-function.)
where
classdef class_dep_prop < handle
properties
x = 1;
end
properties ( Dependent = true )
y
end
methods
function val = get.y( this )
a = (1:2);
b = (1:3);
c = a .* b;
val = c * this.x;
end
function val = foo( this ) %#ok<MANU>
val = 17;
end
end
end

  3 Comments

Thank you. So basically it only calculates the value if a tooltip or the variable editor asks for it and if there is an error in the getter method, it will not show up the result (however, the variable editor will show the dependent variable).
Yes, the value of a dependent property is calculated on-the-fly when needed. The results in not saved in the object. Exactly how this is done is not documented.
The behavior when there is bug in the get method seems to vary between releases. With R2013a no property names or values are displayed in the variable editor. The pane is blank.
Thanks for the updated answer. I tried it and this is a good example. If MATLAB had an issue tracker (like in Github), we could send recommendations and feature requests to Mathworks.

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