Trigger deletion of handle object when one of its IMROI properties is deleted

I have a handle class one of whose properties is an imroi object hande (e.g., as generated by imellipse),

      classdef myclass < handle

       properties

           prop=1;
           H=imellipse;

       end

       methods

         function delete(obj)
            ... 
         end

       end

      end

I would like to make it so that when the imellipse object (pointed to by obj.H) is deleted from the figure it resides in, the myclass delete() method is triggered on obj. I imagine there is some way to do it with listener objects, but I am new to the subject of listeners and could use some guidance. For one thing, I cannot see how to customize the imroi class' delete() method to issue the notify() command.

Answer 1

[per isakson]

Here is an example. Since I don't want to modify the Matlab functions, I have subclassed imellipse.

Step 1. Execute this script

    figure, imshow( 'cameraman.tif' );
    mye = my_ellipse( gca, [10 10 100 100] );
    ear = my_ear( mye );

Step 2. Delete circle interactively. The following line will dispay in the command window

    I hear you load and clear!
    >>

where

    classdef    my_ellipse < imellipse
        properties
        end
        events
           ImDeleted
        end
        methods
            function this = my_ellipse( varargin )
                this = this@imellipse( varargin{:} );
            end
            function    delete( this )
               notify( this, 'ImDeleted' ) 
            end
        end
    end

and

    classdef    my_ear < handle
        properties
            ear
        end
        methods
            function this = my_ear( sender )
               this.ear = addlistener( sender, 'ImDeleted', @this.gotIt );
            end
            function    gotIt( this, varargin )
                disp( 'I hear you load and clear!' )
            end
        end
    end

The delete methods of the ancestors are executed automagically. (I don't find the line describing this magic in the documentation. And there is a "strong recommendation" not to cause run time errors in the delete methods.)

.

Continuation:

Run

    figure, imshow( 'cameraman.tif' );
    axh = gca;
    pos = [10 10 100 100];
    myc = my_class( axh, pos );

and delete the circle object, which will output a line to the command window

    my_class.delete is executed
    >>

where

    classdef    my_class < handle
        properties
            H    
            ear
        end
        methods
            function    this = my_class( axh, pos )
                this.H   = my_ellipse( axh, pos );
                this.ear = addlistener( this.H, 'ImDeleted', @this.delete );
            end
            function delete( this, varargin )
                disp( 'my_class.delete is executed' )
            end
        end
    end

.

The documentation says:

    [The delete method] Must have [exactly] one scalar input argument 
    that is an object of the class.

Thus, function delete( this, varargin ) is wrong. However, I wonder whether varargin is an exception.

[Matt J]

Thanks, per. Actually, though, I discovered that you don't need to subclass imellipse. As a handle subclass itself, imellipse inherits an event called 'ObjectBeingDestroyed', which can be used as below.

       classdef myclass < handle

         properties
             prop=1;
             H
         end

         methods

           function this=myclass

              H=imellipse;
              this.H=H;
              addlistener(H,'ObjectBeingDestroyed',@this.delete);

           end

           function delete(this,varargin)

            if isvalid(this.H) 
             %delete() was triggered by "delete(this)", not the listener

               delete(this.H);

            end

             ... 

           end

         end
        end

[per isakson]

Yes, of course. When you mention it, I recall reading about ObjectBeingDestroyed. The language is growing large.