set class property only once

I am having multiples classes which needs to read a shared common variable called 'MasterData'.

This MasterData variable is getting its data from an api call. In order to avoid to make this api call in each of my different classes that need it, I wanted to define MasterData as a propery in a handle class (MasterDataConfiguration class) and then make other classes inherit from that superclass.

classdef MasterDataConfiguration < handle
    
    
    properties
        MasterData
    end
    
    methods
        function obj = MasterDataConfiguration2(~)
            
            % Get MasterData from CyPET4
            % Read data form config
            ini = IniConfig();
            ini.ReadFile('MPS_AutoJSON_ConfigFile.ini');
            
            % API info
            APIbaseurl = ini.GetValues('Web App API', 'PreformFilesrequestURI_Prod');
            APIparameter = 'admin/masterdata';
            
            % API call
            options = weboptions('Timeout',120);
            obj.MasterData = webread([APIbaseurl,APIparameter],options);
            disp('MasterData API was queried');
        end
        

For the sub clases which would inherit from the above class:

classdef SubClassExample < MasterDataConfiguration
    
    properties
        ColdHalfDataStruct
    end
    
    methods
        function obj = SubClassExample(OtherObj)
            %ColdHalfData Construct an instance of ColdHalfData from a PreformDataObj of
            %class PreformData
            
            % work done on MasterData
            obj.MasterData
            
        end
    end
end
    

My difficulty here is that I get the API requested each time I create a subclass object SubClassExample.

What I would like is that the attribute MasterData from class MasterDataConfiguration gets set once and for all and the other subclasses simply refers to it without re generating it.

I tried using Properties (constant) but I can't make an API call in such way.

I read some documentation here :

https://nl.mathworks.com/help/matlab/matlab_oop/properties-containing-objects.html#bu008rz-1

But could not understand whet is meant by the following :

"MATLAB® evaluates property default values only once when loading the class. MATLAB does not reevaluate the assignment each time you create an object of that class. If you assign an object as a default property value in the class definition, MATLAB calls the constructor for that object only once when loading the class."

In my case I see the API is always called.

Thanks in advance for any hint on how to proceed.

Answer 1

[Guillaume]

It sounds like you want a singleton property. This is how I'd designed it. As commented in Steven's answer, this doesn't and should not involve inheritence:

classdef MasterConfiguration < handle  %handle require so it can modify its own properties
    properties (SetAccess = private)
        MasterData
    end
    methods
        function this = MasterConfiguration() %constructor
            this.Initialise();  %Initialise 
        end
        function Initialise(this)  %Actual initialisation code. Decoupled from the constructor, so it can be reinitialised as needed
            %... initialisation code
        end
    end
end

classdef MasterDataUser  %can be handle or value class depending on needs
    properties (Constant)
        MasterDataConfiguration = MasterConfiguration;  %singleton object shared by all instances
    end
    methods
        %...
        function ReinitialiseMasterData(this)  %in case MasterData need to be reread
            this.MasterDataConfiguration.Initialise();  %all instances of MasterDataUser will see the newly initialised MasterConfiguration
        end
    end
end

If several classes need access to the same shared MasterData, then they must derive from MasterDataUser not from MasterConfiguration.

[Marc Youcef]

It now gets much more clear. For other classes that would use same shared MasterData, could I do same implementation for them as for class MasterDataUser ? I want to avoid those other classes from inheriting from MasterDataUser because they are really different classes and have different properties except their common need to use MasterData.

[Guillaume]

No, they will all have to inherit from this class holding the singleton. Call it MasterDataInterface if you wish.

The only other option is to do the sharing with a global variable with all the pitfalls that entails.

[Marc Youcef]

That is perfectly doing the job. Thanks to you both for detailed guidance.

Answer 2

[Steven Lord]

It sounds like you want an immutable property as described here or perhaps to have your classes that want to use the data to store a MasterDataConfiguration handle object in their properties. In that latter case, you may want to assign the MasterDataConfiguration handle object in the property definition for the objects that need it rather than in their constructors. See the "Expression Evaluation in Handle and Value Classes" section on this documentation page for more information about why that may be more appropriate for your class hierarchy.

[Marc Youcef]

Thanks for the support.

I probably misunderstood your suggesion as it did not work as expected for me.

This is what I did :

classdef MasterDataConfiguration < handle

    properties
        MasterData2
    end

    methods
        function obj = MasterDataConfiguration2(~)
            % Get MasterData 
            % Read data form config
            ini = IniConfig();
            ini.ReadFile('MPS_AutoJSON_ConfigFile.ini');            

            % API info
            APIbaseurl = ini.GetValues('Web App API', 'PreformFilesrequestURI_Prod');
            APIparameter = 'admin/masterdata';
            
            % API call
            options = weboptions('Timeout',120);
            obj.MasterData = webread([APIbaseurl,APIparameter],options);
            disp('MasterData API was queried');
        end
       

And for the sub class:

classdef SubClassExample < MasterDataConfiguration
    properties (Constant)
        MasterData = MasterDataConfiguration
    end
    properties
        ColdHalfDataStruct
    end

    methods
        function obj = SubClassExample(OtherObj)
            %ColdHalfData Construct an instance of ColdHalfData from a PreformDataObj of

            %class PreformData

            % work done on MasterData
            obj.MasterData
        end
    end
end        

Each time I construct SubClassExample, I get the API being called while I would like it to be called once and just refer to MasterData only.

Can you give more details on you solution with classes?

I also tried the following and still API was called each time (which is logic as it is in the constructor but I do not see how to do that in properties):

classdef MasterDataConfiguration2 < handle
    
    
    properties (SetAccess = immutable)
        %         MasterData = load("MasterData.mat");
        MasterData
    end
    
    methods
        function obj = MasterDataConfiguration2(~)
            % Read data form config
            ini = IniConfig();
            ini.ReadFile('MPS_AutoJSON_ConfigFile.ini');
            
            % API info
            APIbaseurl = ini.GetValues('Web App API', 'PreformFilesrequestURI_Prod');
            APIparameter = 'admin/masterdata';
            
            % API call
            options = weboptions('Timeout',120,'Debug',true);
            obj.MasterData = webread([APIbaseurl,APIparameter],options);
            
        end
    end
end

[Guillaume]

I'm very confused about your design. If MasterData in subClassExample is a property of type MasterDataConfiguration, why does subClassExample derive from MasterDataConfiguration? It sounds like there should be no inheritance from MasterDataConfiguration.

I'm a bit unclear on the desired lifetime for MasterData. What the bit about constant properties say, is that if the property is constant, the initialisation function is called only once, when the class is loaded which is typically the firsts time an object is created (documentation is purposefully unclear, this may depends on the jit optimisation). It is then never called again, regardless of the number of instances of the class. The only way to get it to be called again would be to restart matlab, use clear classes or possibly edit the class code.

[Marc Youcef]

Indeed I should have removed that inheritance. I can't try it out now. Will do that tomorrow.

Your description is exactly what I would be looking for. Meaning that I would like subClassExample first instance to create only once the MasterData property. In return, to build MasterData property, it will construct MasterDataConfiguration.

Then when another instance of subClassExample is generated, I am hoping that it would not construct again MasterDataConfiguration. Removing the inheritance should solve that.

I try out and let you know.

Thanks.

Answer 3

[per isakson]

Here is an minimal working example of the design, which I think OP tried to implement. That is, use the fact that "MATLAB® evaluates property default values only once when loading the class.".

"I tried using Properties (constant) but I can't make an API call in such way." The trick is to put the code, which creates the value of MaterData, in a static method (or an ordinary function) and call that in the property block.

First, I create three instancies of SubClassExample(). The method, configureMasterData(), is called only for the first instance. (Instead of calling the API, I call randi() to create a vector of three random whole numbers.) Next, I show that the values of the property, MasterData, is identical for the three instances.

>> sce_1 = SubClassExample();
MasterData API was queried
>> sce_2 = SubClassExample();
>> sce_3 = SubClassExample();
>> sce_1.MasterData
ans =
     8     9     2
>> sce_2.MasterData
ans =
     8     9     2
>> sce_3.MasterData
ans =
     8     9     2

where

classdef    MasterDataConfiguration < handle
    properties ( SetAccess = private )
        MasterData = MasterDataConfiguration.configureMasterData;
    end
    methods ( Static, Access = private )
        function    master_data = configureMasterData(); 
            master_data = randi( [1,9], 1,3 ); 
            disp('MasterData API was queried');
        end
    end
end

and

classdef SubClassExample < MasterDataConfiguration
    properties
        ColdHalfDataStruct
    end
end