MATLAB: Create an empty array of class event.listener

matlab oopoop

Is it possible to create an empty array of the class event.listener?

I want to create event listeners (which calls callbackFcn when event listenEvent occurs for object listenObj) and save the handles to the event listeners in the property hListenerArray of an object obj so that the delete method of obj can delete the event listeners. My idea was to do this:

obj.hListenerArray(end+1) = addlistener(listenObj, listenEvent, callbackFcn);

but it fails

??? The following error occurred converting from event.listener to double: Error using ==> double Conversion to double from event.listener is not possible.

I think this is because the property hListenerArray is initialized as class double.

Best Answer

You can create empty objects with the static empty() method of a class, as documented at http://www.mathworks.com/help/techdoc/matlab_oop/brd4btr.html. In this case using "event.listener.empty" as the initial value for the hListener property should work:

properties
    hListenerArray = event.listener.empty;
end

An additional comment: if you use the event.listener constructor to create the listeners instead of using addlistener then you will not have to write a delete method because you will have the only reference to them in hListenerArray and they will simply go out of scope and delete themselves when obj is deleted. addlistener is adding an extra reference to the object being listened to and you do not want that in this case.

obj.hListenerArray(end+1) = event.listener(listenObj, listenEvent, callbackFcn);

Related Solutions

MATLAB: Can timers trigger a single event that many objects listen to

Sure, that's entirely possible. You can wrap the basic MATLAB timer in an object that broadcasts an event (via the notify method). Then you can use the addlistener method within the "client" class to respond to those notifications.

Here's a basic example. This is the class that defines and broadcasts the events on a regular interval via a timer:

classdef TimerObj < handle
    properties
        theTimer
    end
    events
        TimerEvent
    end
    methods
        function this = TimerObj
            this.theTimer = timer( ...
                'ExecutionMode', 'fixedRate', ...
                'Period', 1, ...
                'TimerFcn', @(src, evt) this.notify( 'TimerEvent' ) );
            start(this.theTimer);
        end
        function delete(this)
            stop(this.theTimer);
        end
    end
end

Here's a class that can listen to those events:

classdef UserObj < handle
    properties
        Name
    end
    methods
        function this = UserObj( aName )
            this.Name = aName;
        end
        function listenToTimer(this, timerobj)
            addlistener( timerobj, 'TimerEvent', @(src, evt) this.onTimerEvent );
        end
        function onTimerEvent(this)
            disp( [this.Name ' responding to TimerEvent!'] );
        end
    end
end

Now they can be used together like this:

>> a = UserObj('Alice');
>> b = UserObj('Bob');
>> t = TimerObj;
>> a.listenToTimer(t);
Alice responding to TimerEvent!
Alice responding to TimerEvent!
>> b.listenToTimer(t)
Bob responding to TimerEvent!
Alice responding to TimerEvent!
Bob responding to TimerEvent!
Alice responding to TimerEvent!
Bob responding to TimerEvent!
Alice responding to TimerEvent!
Bob responding to TimerEvent!
Alice responding to TimerEvent!
>> delete(t)
>>

A lot of this is just a repackaging of this documentation example. So you might want to read through that, too.

MATLAB: Do I get warning messages when I have an event listener in the class and I execute the command: CLEAR CLASSES in MATLAB 7.6 (R2008a)

This is an expected behavior in MATLAB 7.6 (R2008a). What happens here is that the property intializer creates a single handle object that is shared by all instances. For example, defining the following class:

classdef sample
      properties
          rand_no = rand
      end
end

and then creating 2 objects of the sample class like so:

A = sample
B = sample

results in both A and B having the same constant number for each of their rand_no property.

In the example case, the constructor attaches a listener to that single shared handle that is owned by the class definition. In addition, ADDLISTENER adds a hidden property to the object being listened to which therefore creates a circular dependency to the hidden handle. Thus, calling CLEAR CLASSES does not work properly and MATLAB has to be shut down to clear the hidden handle.

To work around this issue, create the enum_deb object in the constructor function instead of in the properties section. For example, create User1.m like so:

classdef User1 < handle
     properties
      a;
     end
     methods
        function obj = User1()
           obj.a = enum_deb;     %(TechSup) create enum_deb object here instead
           lh = addlistener(...
              obj.a,'updated',...
              @(src,event)lstnrCallback(obj,src,event));
           function lstnrCallback(obj,src,event)
              disp('Listener Called');
           end
        end
     end
end

Additional useful information:

The difference between calling ADDLISTENER and EVENT.LISTENER is the following:

- ADDLISTENER automatically adds a hidden property to the object being listened to in order for the listener object to be bound to its lifecycle. This means that the listener object is destroyed once the object it is listening to is cleared from memory.

- On the other hand, the listener object created by EVENT.LISTENER is a local object. This means that deleting the object being listened to will NOT necessarily result in the destruction of the listener object as well.

Best Answer

Related Solutions

MATLAB: Can timers trigger a single event that many objects listen to

MATLAB: Do I get warning messages when I have an event listener in the class and I execute the command: CLEAR CLASSES in MATLAB 7.6 (R2008a)

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