OOP Dynamic Properties ordering is very strange...

I have a class defintion which makes use of OOP dynamicprops and I've noticed some odd behaivor in accessing the properties.

    classdef myClass < dynamicprops

        properties
            x
        end

        methods
            function this = myClass(x)
               this.x = x;
            end   
        end
    end

If I add new properties to the class in a script which looks something like this

   clear all
   clear classes

   hObj = myClass(10);
   hObj.addprop('a');
   hObj.addprop('b');
   hObj.addprop('c');
   hObj.addprop('d');
   hObj.addprop('e');
   fn = fieldnames(hObj)'

I was expecting the properties to be added to hObj in the same order that they are scripted in, but I find that the properties added to hObj do not get added in any consistent way. EVERY TIME I run the script, hObj is returned with a different order in the properties ?! Can anyone explain why these dynamic properties behave in such a stranage way? Is there no way to control the order in which these properties appear in the object?

It would be nice if I could control the order so that a function call like fieldnames(hObj) would return values in a predictable way. As a secondary but also irksome issue there appears to be no way to REMOVE a dynamic property once it has been added?!

... here is the output from 5 runs of the script I show above in R2010b:

fn = 

      'x'    'b'    'a'    'c'    'd'    'e'

fn = 

      'x'    'd'    'e'    'a'    'c'    'b'

fn = 

      'x'    'd'    'a'    'b'    'e'    'c'

fn = 

      'x'    'b'    'd'    'a'    'c'    'e'

fn = 

      'x'    'b'    'a'    'd'    'c'    'e'

Answer 1

[Ellis King]

I did come up with a workaround for this which solves the immediate issue. It involves much more sophistication than I was originally hoping for with these classes, but it appears to do the trick. For the application I am working on, it would be counter-intuitive to have the dynamic properties appear to the user in an order that did not match the order they were inserted in. In general the property names are not inserted in alphabetical order. So here is an outline of the solution:

• add a private, hidden property "dynPropOrder" for the class.
• restrict access to addprop() by making it private for the class.
• add a public addDynamicProperty(propertyName) method for the class, which inserts adds the dynamic property using addprop(), but also appends the property name to the hidden dynPropOrder property in a cell string array.
• overload the display(), properties() and fieldnames() methods to return the properties in the expected order using the hidden dynPropOrder attribute.
• add a removeDynamicProperty(propertyName) which does reverse operations for the specified property.

Does this seem overly complicated to anyone else? Like I said it works, but I am disappointed there did not appear to be a more straightforward way. I welcome any suggestions...

Answer 2

[William Warriner]

I know this is a tremendously old question, but thought I would add that in (at latest) R2018b one can do the following, and it seems to work. Hopefully this helps future solution seekers like myself.

Add public overrides for properties and fieldnames like below. Technically properties() isn't a method of handle dynamicprops, but this should still work because methods are called before built-ins (https://www.mathworks.com/help/matlab/matlab_prog/function-precedence-order.html).

function value = properties( obj )
    if nargout == 0
        disp( builtin( "properties", obj ) );
    else
        value = sort( builtin( "properties", obj ) );
    end
end

function value = fieldnames( obj )
    value = sort( builtin( "fieldnames", obj ) );
end

Add a matlab.mixin.CustomDisplay superclass and add protected override like below.

        
function group = getPropertyGroups( obj )
    props = properties( obj );
    group = matlab.mixin.util.PropertyGroup( props );
end

The CustomDisplay mixin then displays only the group returned by getPropertyGroups() when disp() and display() are called. Since the properties() function called in getPropertyGroups() is the new "override", the group is displayed in sorted order.

Hope this helps someone!

Edit: added a public gist at github. https://gist.github.com/wwarriner/f189373bf40cc741f6d945165a85e115

[Alexandru Bitca]

Thank you so much for this! It works like a treat.

[William Warriner]

Happy to help! Adding a public gist on github.com as well.

[Benjamin Gaudin]

You can also override

function value = fields( obj )
    value = sort( builtin( "fields", obj ) );
end

Answer 3

[Daniel Shub]

Why not just overload fieldnames to return sort(fieldnames)?

[Ellis King]

Daniel, I did essentially something like this for the classes I've been working with. It was a bit more complicated than what you describe because the first property 'x' should not have been included in the list of dynamic properties, and in general the dynamic property names are not inserted alphabetical order. (At the time of writing this I was primarily interested in getting the properties to appear in the order the dynamic properties were inserted). Creating a new display() method with some of the logic I describe above does do the trick. However what I was really hoping for is someone to comment on WHY the dynamic properties behave in such a non-deterministic way... ?

Answer 4

[Andrew Newell]

According to the documentation, you could add a property using a command like

Pa = hObj.addprop('a');

and later delete the property using

delete(Pa);

As for the order of names - even easier than overloading fieldnames would be to sort afterwards:

fn = sort(properties(hObj))'

[Ellis King]

Andrew, you are correct about using that method to delete the dynamic properties -- I had missed that in the documentation. See my comment to Daniel above about using the sort() function.

[Brett]

Incidentally, if you forgot to save the handle Pa, it is still possible to delete the property 'a' using

delete(hObj.findprop('a'));

Answer 5

[Steven Lord]

I would be very wary of overloading properties or methods for your object.

There may be an easier way to do it, but you can query the metadata about a property and ask if it is "fixed" (defined in a properties block in the classdef file) or dynamic. When you ask for information about a property using findprop information about a fixed property is returned in a meta.Property object while information about a dynamic property is returned in a meta.DynamicProperty object. There's probably a more sophisticated way to create the property groups, but I wanted to get a quick example written before lunch. See the attached object.

y = example9777(5) % no dynamic properties yet

setDynamicProperty(y, 'Aardvark', 42)
y % one dynamic property

setDynamicProperty(y, 'Zebra', 999)
y % Zebra comes after Aardvark alphabetically

setDynamicProperty(y, 'Mouse', 'Good dog!')
y % Mouse was created after Zebra but comes before it alphabetically
  % Yes, I am a fan of the Dresden Files

[Benjamin Gaudin]

Excellent. Thank you for this answer. However here, properties( y ) or fieldnames( y ) will still return properties in a random order.