How To listen to a list of properties and call a method when a property of that list changes value?

Hi,

I would like to automatically call a method when some class properties do change.

I have been able to do it for in single property:

    mySetup.rwConfigChangeListener = event.proplistener(mySetup,{'rwConfig'},'PostSet', @mySetup.ResetRwAoaToMin);

But now I need a method to be called anytime one of 50 prop do change.

Documentation seems to indicate I should be able to input a list of properties, but I have not been able to do so:

    lh = event.proplistener(Hobj,Properties,'PropEvent',@CallbackFunction) creates a property listener object for one or more properties on the specified object.

    Hobj — handle of object whose property or properties are to be listened to. If Hobj is an array, the listener responds to the named event on all objects in the array.
    Properties — an object array or a cell array of meta.property object handles representing the properties to which you want to listen.
    PropEvent — must be one of the strings: PreSet, PostSet, PreGet, PostGet
    @CallbackFunction — function handle to the callback function that executes when the event occurs.

Also maybe I should use a different kind of listener?

Thanks for your help.

Answer 1

[Sean de Wolski]

You have to build the meta property list from the metaclass:

p = IHaveProps;
mc = metaclass(p);
metaprops = [mc(:).Properties];
listener = event.proplistener(p, metaprops, 'PostSet', @(~,evt)disp(evt))
p.Property1 = pi
p.Property2 = exp(1)

For the class:

classdef IHaveProps < handle

      properties (SetObservable)
          Property1
          Property2
          PropertyTuesday
      end

end

[Laurent Davenne]

Thank you very much Sean!,

I managed to make it work following your example despite I do not know what a meta property or class is... I need to read about that.

One more question:

How can I explicitly make a list of the properties I want to listen to?

Because At the moment I had to select everything even dependant properties....

[Sean de Wolski]

Hi Laurent,

Yes you can. I'd recommend you start playing with metaclass. It contains all of this information and you can filter however you want.

>>doc metaclass

And just a quick example with weboptions:

mc = metaclass(weboptions)
mc.PropertyList

mc = class with properties:

                     Name: 'weboptions'
              Description: ''
      DetailedDescription: ''
                   Hidden: 0
                   Sealed: 0
                 Abstract: 0
              Enumeration: 0
          ConstructOnLoad: 0
         HandleCompatible: 0
          InferiorClasses: {0×1 cell}
        ContainingPackage: [0×0 meta.package]
     RestrictsSubclassing: 0
             PropertyList: [16×1 meta.property]
               MethodList: [20×1 meta.method]
                EventList: [0×1 meta.event]
    EnumerationMemberList: [0×1 meta.EnumeratedValue]
           SuperclassList: [1×1 meta.class]
>> mc.PropertyList
ans = 
  16×1 property array with properties:

    Name
    Description
    DetailedDescription
    GetAccess
    SetAccess
    Dependent
    Constant
    Abstract
    Transient
    Hidden
    GetObservable
    SetObservable
    AbortSet
    NonCopyable
    GetMethod
    SetMethod
    HasDefault
    DefaultValue
    Validation
    DefiningClass

So you can grab just the properties you care about based on name or any other feature.

[Laurent Davenne]

Hi Sean,

I managed to index into the meta prop list to reduce it to the properties I want to listen to.

Is there a more "elegant" way to create a list of the meta property names than using a for loop?

                  for i=1:numel(metaPropsAll)
                      metaPropsAll_Names{i}=mc.PropertyList(i).Name;
                  end

[Sean de Wolski]

You can use comma-separated list expansion like I did above

metaprops = [mc(:).Properties];

or

[mc.PropertyList(:).Dependent]

https://www.mathworks.com/help/matlab/matlab_prog/comma-separated-lists.html

[Laurent Davenne]

Am I really obliged to go through meta property list of the metaclass to listen to a list of properties?

This is a bit too complex for me to work with meta class... doc metaclass is not very helpful...

I manage to call the callback function but then the input I pass into it is a meta property. I would need it to be the object of the class instead.

Here the property "model" is a class of the class "setup". I want to listen to some properties of "setup" to update the property "model" anytime a property of "setup" changes.

"setup" is an input for the Constructor of "model".

   mc = metaclass(mySetup);
   metaPropsAll = [mc(:).Properties];

   metaPropsAll_Names=cell(numel(metaPropsAll),1);

   for i=1:numel(metaPropsAll)
          metaPropsAll_Names{i}=mc.PropertyList(i).Name;
   end

   idx=ismember(metaPropsAll_Names, listeningPropList);
                 metaPropsAll(~idx)=[]; % Filetered Property list

   mySetup.listener = event.proplistener(mySetup, metaPropsAll, 'PostSet', @mySetup.CallBackFunction);

    function CallBackFunction(obj,~,~)             
               obj.model=model(obj);  
    end

[Laurent Davenne]

It's maybe because I did not manage to use properly the last input of your code:

 @(~,evt)disp(evt)

Can you please give me some explanations about this bit.

[Laurent Davenne]

I reply myself.

This does what I need:

@(~,evt)mySetup.CallBackFunction(mySetup)

Thanks for your help Sean!

[Guillaume]

@(~,evt)mySetup.CallBackFunction(mySetup)

This is a very odd construct, the second mySetup is redundant, note that the above is equivalent (ignoring the way matlab search for functions) to:

@(~, evt) CallBackFunction(mySetup, mySetup)

As you can see you're passing the same argument twice. Since you've defined the CallBackFunction as

function CallBackFunction(obj,~,~)

the second mySetup is ignored.

The two way you could define the callback would be

@mySetup.CallbackFunction  %preferred
@(~, ~) mySetup.CallbackFunction

Now for some explanation about Sean's callback. An event callback function always receive two inputs: The source generating the event, and some event specific arguments. This is actually fairly standard across many languages. Therefore when Sean defined:

 @(~, evt) ...

He defined a function with two inputs as required. That function ignores the first input (the source) and uses the event arguments, which he displays.

[Laurent Davenne]

@mySetup.ResetRwAoaToMin

Does not work for me because it passes a meta property to the callbackfunction.