accessing timer UserData fields

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Joris Lambrecht
Joris Lambrecht 2016-9-13
评论： Walter Roberson 2016-9-13
t=timer;
t.UserData = struct('field1', 'a', 'field2, 'b');
t.UserData.field1
% returns an error:
%inconsistently placed '.' in subscript expression
get(t.UserData, 'field1')
%returns an error:
%Conversion from double to struct not possible
get(get(t, 'UserData'), 'field1')
% returns same error
u = t.UserData;
u.field1 %works fine
Thus I have a workaround but it's kind of clunky: I set a variable to u=t.UserData at the beginning of my timer callback function and then set t.UserData=u at the end of my timercallback function
Why can't these fields be accessed directly?

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采纳的回答

Walter Roberson
Walter Roberson 2016-9-13
Interesting, I had never seen that message before.
The work-around is
getfield(t.UserData, 'field1')
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Joris Lambrecht
Joris Lambrecht 2016-9-13
编辑：Joris Lambrecht 2016-9-13
Unfortunately, the counter to this does not seem to work. If I want to set the value of the field:
setfield(t.UserData, 'field1', 'c')
returns field: 'c' as expected, but a follow up call to
getfield(t.UserData, 'field1')
returns 'a'
Walter Roberson
Walter Roberson 2016-9-13
t.UserData = setfield(t.UserData, 'field1', 'c')
setfield does not assign to a variable: setfield returns the modified value.
As to why it happens:
MATLAB objects can have properties that are represented as fields in a struct that is marked as being of the appropriate class. If any given property happens to be a struct and the property is directly represented as a struct field, then it can be sub-referenced in the way that would seem natural.
But MATLAB objects can also have properties that are calculated rather than being directly represented, "Dependent" properties. Nearly everything about timers are implemented as Dependent properties, with the code that handles the implementation accessing a java object that is directly stored in a struct. Dependent properties cannot be sub-referenced in the way that would seem natural. When I look at the implementation of timer.m it appears to me that the Dependent properties might be created as dynamic properties; I do not know the backing implementation for those.

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