Add a event callback across all instances of class??

Question

Song 2015-7-28

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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/231428-add-a-event-callback-across-all-instances-of-class

评论： Steven Lord 2015-7-28

The idea is as follows:

If a property of an instance of class is changed, then all the instances of this class should take this property and do some update. The problem is I don't know how many instances of class exist, so I thought using event and listener could be a solution. Here is my code:

classdef myclass < handle
    %EVENTTAKER Summary of this class goes here
    %   Detailed explanation goes here
      properties
          hf 
          sharedProp
      end
      events
         sharedPropChanged
      end    
      methods
         function obj = myclass() 
             obj.sharedProp = rand(100, 2); % define prop as random data
             obj.hf = figure;
             plot(obj.sharedProp);
             addlistener(obj,'sharedPropChanged',@obj.listenerCallback);
         end
         function setSharedProp(obj, newProp)
             obj.sharedProp = newProp;
             % broadcast event to all objects of class
             notify(obj, 'sharedPropChanged');
         end
         function listenerCallback(obj, srcObj, ~)
             obj.sharedProp = srcObj.sharedProp;
             obj.update();
         end    
         function update(obj)
             plot(obj.sharedProp);
         end
      end
end

To test:

o1 = myclass; o2 = myclass;
o1.setSharedProp(rand(10, 2))

The result is that only the first instance get update, the second not.

Is this a plausible solution, or there is any way to implement this?

Thnks

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Answer 1

Guillaume 2015-7-28

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https://ww2.mathworks.cn/matlabcentral/answers/231428-add-a-event-callback-across-all-instances-of-class#answer_187429

编辑：Guillaume 2015-7-28

在 MATLAB Online 中打开

What you're trying to create is a static property (belongs to the class not the instances). Matlab does not have that, but you could possibly emulate it with a constant property holding a reference to a handle class. Either make a lightweight handle class to wrap your static property or simply use a containers.Map with only one key:

classdef classwithstaticprop < handle
  properties (Constant, GetAccess = private)
      staticprop = containers.Map; %or just make a lightweight handle class
  end
    properties (Dependent)
        sharedprop
    end
    methods
        function this = classwithstaticprop() %constructor
            if length(this.staticprop) == 0 %#ok<ISMT> isempty not defined for map
                %first instance, initialise;
                m = this.staticprop; %you have to use a local variable, see the doc
                m('prop') = 1; %#ok<NASGU> %initialise to default value
            end
        end
        function set.sharedprop(this, value)
            m = this.staticprop;
            m('prop') = value; %#ok<NASGU> %it's a handle
        end
        function value = get.sharedprop(this)
            value = this.staticprop('prop');
        end
    end
  end

See this page for why you have to use a local variable when changing the value of the handle class referred by the constant property.

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Guillaume 2015-7-28

在 MATLAB Online 中打开

Sorry, I missed that you also wanted to run some update code on property update.

The problem with your code is that the listener you create only listen to changes to its own owner, not to changes to all the other instances. With your design you would need to create, in each object, a listener per other object instances. And every time you'd create a new object, you'd have to go through all the previous one to add a new listener.

Instead, what you can do is still use a static property that is in fact a handle to an auxiliary class that actually contain the shared value. Each object can then just listen to the change of that shared object:

This is the main class:

classdef classwithstaticprop < handle
  properties (Constant, GetAccess = private)
      propwithupdate = staticprophelper
  end
    properties (Dependent)
        sharedprop;
    end
    properties (Access = private)
        lh; %listener handle
    end
    properties (SetAccess = private)
        count;
    end
    methods
        function this = classwithstaticprop() %constructor
            persistent count;
            if isempty(count), count = 1; else count = count+1; end
            this.count = count;
            %do not use addlistener, otherwise you can never clear classes
            %the syntax for event.proplistener is a bit obscure
            mc = ?staticprophelper;
            this.lh = event.proplistener(this.propwithupdate, mc.PropertyList, 'PostSet', @this.doupdate); 
        end
        function set.sharedprop(this, value)
            this.propwithupdate.value = value;
        end
        function value = get.sharedprop(this)
            value = this.propwithupdate.value;
        end
    end
    methods (Access = private)
        function doupdate(this, ~, ~)
            fprintf('updating instance %d\n', this.count);
        end
    end                    
  end

And this is the helper class, a singleton object is shared by all instances:

classdef staticprophelper < handle %has to be handle class   
  properties (Access = ?classwithstaticprop, SetObservable)
      value = 0;
  end
end

Guillaume 2015-7-28

Nor can you put classes in private folders, which would be another way of limiting the visibility of the utility class.

Steven Lord 2015-7-28

You cannot place multiple classes in the same file.

You cannot place class files in private folders.

You CAN place class files in package directories, however.

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