How to efficiently use dependent properties if dependence is computational costly?

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Michael Baumgart 2014-5-8

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https://ww2.mathworks.cn/matlabcentral/answers/128905-how-to-efficiently-use-dependent-properties-if-dependence-is-computational-costly

评论： Jan Kappen 2024-3-5

Hello!

I want to design a class where some properties are dependent on others. I understand the general concept of Matlab OOP for dependent properties: the property is recalculated each time it is accessed.

But if these calculations/dependencies are costly and the dependent properties are often accessed by methods within the class, what are you suggesting to do. Basically, it would suffice to recalculated the property each time its parents properties are updated.

If I do so, I get a warning that accessing another property from the set method of an independent property is not recommended. What else can I do?

Cheers Michael

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Answer 1

Jacob Halbrooks 2014-5-8

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https://ww2.mathworks.cn/matlabcentral/answers/128905-how-to-efficiently-use-dependent-properties-if-dependence-is-computational-costly#answer_136129

编辑：Jacob Halbrooks 2014-5-8

在 MATLAB Online 中打开

I would recommend you add a method to compute and update the value of your dependent property and then call that method from the setters for each property that updates it. This single-sources the computation of that property, caches it to avoid unnecessary re-computation, and satisfies MATLAB Code Analyzer.

Here is an example, where Area of a Rectangle depends on Width and Height and is updated when each of those is set:

   classdef Rectangle < handle
      properties
          Width = 10;
          Height = 20;
      end
      properties(SetAccess = private)
          Area = 200;
      end
      methods        
          function set.Width(obj, v)
              obj.Width = v;
              obj.updateArea;
          end
          function set.Height(obj, v)
              obj.Height = v;
              obj.updateArea;
          end
      end
      methods(Access = private)
          function updateArea(obj)
              obj.Area = obj.Width * obj.Height;
          end
      end
   end

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Chris 2017-4-22

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I just want to thank you for this example. The

properties(SetAccess = private)

bit is exactly what I was looking for. I think there is a lot of confusion around Dependent properties because people tend to begin learning about classes "on the job" and have an application in mind. Dependent can mean a lot of things, and I think many approach Dependent properties as if they simply mean "Derived from some function of other properties". In fact, it means "Always computed on the fly". You could use an active variable to keep an always-accurate list of files in a directory, for example, which need not interact with any other properties. Once I started thinking about Dependent properties as "Active", and internalized the significance that set methods cannot be used in conjunction with them, it all became more clear.

Most examples utilize a private "mirror" of the critical property that is not dependent, and then have the get method associated with public-facing dependent variable reference that private mirror (e.g., this similar thread). It is kind of a mess, and gives the false impression that a Dependent property is essential for the use case where you want a variable to update every time some other properties are set. In other words, every other example I have seen has forced Dependent properties into the solution, but they are not needed.

So, if you want a property to update whenever another property is set, and prevent it being directly changed by the user, the example provided above is a great template. If, on the other hand, you want a property to never store a value, but to instead essentially store a method that will generate a value on the fly when referenced, then you should use a Dependent property. These imply are quite different use cases, both involving dependency... which comes around to my main point that "Dependent" is kind of an unfortunate bit of language here.

Steven Lord 2023-10-22

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Often dependent properties can be useful when you define property getters and setters. Suppose you had a Circle class with two properties, a normal one Radius and a Dependent property named Area. The property get method for Area would use the current value of Radius. The property set method for Area would determine the Radius for a Circle with that area.

classdef Circle
    properties
        Radius
    end
    properties(Dependent)
        Area
    end
    methods
        function obj = Circle(r)
            if nargin == 0
                obj.Radius = 0;
            elseif isa(r, 'Circle')
                obj = r;
            else
                obj.Radius = r;
            end
        end
        function a = get.Area(obj)
            a = pi*obj.Radius.^2;
        end
        function obj = set.Area(obj, a)
            r = sqrt(a./pi);
            obj.Radius = r;
        end
    end
end

Try creating a Circle with Radius 1. Then change the Radius of that object to 2 and note the automatic change to the Area. Finally set the Area of the Circle to pi and note that the Radius updates back to 1.

If neither property was Dependent you could set incompatible Radius and Area properties on the object.

You could have multiple properties that are both Dependent on the same concrete property, like a Perimeter or a Circumference property of the Circle object.

Trym Gabrielsen 2023-10-25

在 MATLAB Online 中打开

Ok, thanks! I think this makes a little clearer.

One thing that bothers me is that the dependent variable is recomputed every time you get its value. Though, upon trying some workarounds that don't use dependent variables, I think I understand that they are convenient for their purpose.

What is a good way to structure your class (best practice) such that a variable that is dependent on another variable can be stored and not recomputed all the time, but still remain valid with respect to the other variable?

That is, f.ex., if you use the value of the area very often during computations, but once in a while it is updated because the radius changed.

I would naturally think that I could do this:

classdef Circle
    properties
        Radius
        Area
    end
    methods
        function obj = Circle(r)
            obj.Radius = r;
        end
        
        function obj = set.Radius(obj, r)
            obj.Radius = r;
            obj.Area = pi*r.^2;
        end
    end
end

But now I loose the ability to set the area directly, and have the radius computed. I could add a "set.Area" function to set the radius, but that causes a reccursion problem. Is there a way to get both of these properties? (1. store the area as number for quick access, 2. have both the area and radius updated when the other is changed)

Trym Gabrielsen 2023-10-26

Ok, thank you so much!

Jan Kappen 2024-3-5

@Jacob Halbrooks,

I'm using that pattern in handle classes too, but how to implement it in value classes? Updating dependent properties in the update Method does not work because the setter & getter do not return the object.

Thanks :)

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