Property listener with OOP

Hello all,

I'm not exactly sure what I'm supposed to be asking here, so forgive me if I'm way off base. I have an object created in matlab with three properties. The class is called an nUnit, and the properties are value, unit, and unitVec. Both the unit and unitVec are heavily related to one another. So, if the user modifies the .unit, I wish for the .unitVec to be automatically updated and vice versa.

I have the conversion functions written, but how do I trigger them to run? I've read a lot about listeners and the like, but I can't seem to find a proper example for a function that would get triggered if a property is modified. Below is my class definition file. Thank you for your time!

Trevor

    classdef nUnit
        %NUNIT Create numerical unit class
        %
        %   A = NUNIT(VALUE, UNITSTRING)
        %   A = NUNIT(VALUE, UNITVEC)
        %   Inputs:
        %      VALUE:      Numeric scalar or array
        %      UNITSTRING: String of accepted unit strings
        %      UNITVEC:    1x9 numeric array of base unit powers
        %

        properties
            value
            unit
            unitVec
        end

        methods
            function nU = nUnit(value,unit)
                % ==============================================================================
                % Constructor - Create an nUnit
                % ==============================================================================
                if ~isnumeric(value) || isempty(value); error('input VALUE must be a non-empty numeric'); end

                % If value is 1-dimensional, force vertical concatenation
                if any(1 == size(value))
                    value = value(:);
                end

                if ~(ischar(unit) ||...
                        (isnumeric(unit) &&...
                        all(size(unit) == [1, 9]))); error('input UNIT must be a string or unitVector'); end %#ok<ALIGN>

                if ischar(unit) % Unit string is inputed
                    [nU.unitVec, valueMult] = unit2vec(unit);
                    nU.unit    = vec2unit(nU.unitVec);
                    nU.value   = value * valueMult;
                else % Unit vector is inputed - Base units only
                    nU.unitVec = unit;
                    nU.unit    = vec2unit(unit);
                    nU.value   = value;
                end

            end

            % ==============================================================================
            % Functions that only modify value property
            % ==============================================================================

            function nUnitA = diff(varargin);   nUnitA = genValueFun(@diff,   varargin{:}); end
            function nUnitA = unique(varargin); nUnitA = genValueFun(@unique, varargin{:}); end

            function nUnitX = genValueFun(functionH, nUnitX, varargin)
                nUnitX.value = feval(functionH, nUnitX.value, varargin{:});
            end

            % ==============================================================================
            % Overload Numel for O-O compatability
            % Source: http://www.mathworks.com/matlabcentral/answers/101955-why-do-i-receive-errors-when-overloading-subsref-for-types-and-for-matlab-classes
            % ==============================================================================
            function n = numel(varargin)
                  n = 1;
            end

            % ==============================================================================
            % Functions that query the value property
            % ==============================================================================

            function [varargout] = size(varargin)
                varargout = genValueQueryFun(@size, varargin{:});
                varargout = {varargout};
            end
            function [varargout] = length(varargin)
                varargout = genValueQueryFun(@length, varargin{:});
                varargout = {varargout};
            end

            function [varargout] = genValueQueryFun(functionH, nUnitX, varargin)
                varargout = feval(functionH, nUnitX.value, varargin{:});
                varargout = {varargout};
            end

        end
    end

Answer 1

[Adam]

I use dependent properties for this, e.g.

    properties
       value
    end

    properties( Dependent )
       unit
       unitVec
    end

    properties( Access = private )
       unit_;
       unitVec_;
    end

    methods
       function obj = set.unit( obj, unit )
           unit_ = unit;
           updateUnitVecFromUnit( obj );
       end

       function unit = get.unit( obj )
           unit = obj.unit_;
       end

       function obj = set.unitVec( obj, unitVec )
           unitVec_ = unitVec;
           updateUnitFromUnitVec( obj );
       end

       function unitVec = get.unitVec( obj )
           unitVec = obj.unitVec_;
       end
    end

    methods( Access = private )
       function updateUnitFromUnitVec( obj )
           obj.unit_ = f( obj.unitVec_ );
       end

       function updateUnitVecFromUnit( obj )
           obj.unitVec_ = f2( obj.unit_ );
       end
    end

There may be some errors in there as I did it off the top of my head, but the general idea is that only a dependent property is allowed to (or rather should do) alter other properties in its set function so you use a double property solution with a private property for each that actually stores the value and a dependent property which is publicly visible and whose set function will then update the other property also.

It is very important though that you update unit_ and unitVec_ in the functions as shown above otherwise you will have a circular set of calls that will lead you to infinite recursion! My code above just uses placeholder functions f and f2 instead of whatever it is you have already written to update one property from the other.

You could use a property listener. I do in many many cases, but not usually if I am doing something like this.

[Trevor Harris]

Thanks so much, that's perfect! Very helpful.