Define an Interface Superclass

Interfaces

The properties and methods defined by a class form the interface that determines how class users interact with objects of the class. When creating a group of related classes, interfaces define a common interface to all these classes. The actual implementations of the interface can differ from one class to another.

Consider a set of classes designed to represent various types of graphs. All classes must implement a Data property to contain the data used to generate the graph. However, the form of the data can differ considerably from one type of graph to another. Each class can implement the Data property differently.

The same differences apply to methods. All classes can have a draw method that creates the graph, but the implementation of this method changes with the type of graph.

The basic idea of an interface class is to specify the properties and methods that each subclass must implement without defining the actual implementation. This approach enables you to enforce a consistent interface to a group of related objects. As you add more classes in the future, the interface remains the same.

Interface Class Implementing Graphs

This example creates an interface for classes used to represent specialized graphs. The interface is an abstract class that defines properties and methods that the subclasses must implement, but does not specify how to implement these components.

This approach enforces the use of a consistent interface while providing the necessary flexibility to implement the internal workings of each specialized subclass differently.

In this example, a namespace folder contains the interface, derived subclasses, and a utility function:

+graphics/GraphInterface.m  % abstract interface class
+graphics/LineGraph.m       % concrete subclass

Interface Properties and Methods

The GraphInterface class specifies the following properties, which the subclasses must define:

Primitive — Handle of the graphics object used to implement the specialized graph. The class user has no need to access these objects directly so this property has protected SetAccess and GetAccess.
AxesHandle — Handle of the axes used for the graph. The specialized graph objects can set axes object properties. This property has protected SetAccess and GetAccess.
Data — All subclasses of the GraphInterface class must store data. The type of data varies and each subclass defines the storage mechanism. Subclass users can change the data values so this property has public access rights.

The GraphInterface class names three abstract methods that subclasses must implement. The GraphInterface class also suggests in comments that each subclass constructor must accept the plot data and property name/property value pairs for all class properties.

Subclass constructor — Accept data and P/V pairs and return an object.
draw — Used to create a drawing primitive and render a graph of the data according to the type of graph implemented by the subclass.
zoom — Implementation of a zoom method by changing the axes CameraViewAngle property. The interface suggests the use of the camzoom function for consistency among subclasses. The zoom buttons created by the addButtons static method use this method as a callback.
updateGraph — Method called by the set.Data method to update the plotted data whenever the Data property changes.

Interface Guides Class Design

The namespace of classes that derive from the GraphInterface abstract class implement the following behaviors:

Creating an instance of a specialized GraphInterface object (subclass object) without rendering the plot
Specifying any or none of the object properties when you create a specialized GraphInterface object
Changing any object property automatically updates the currently displayed plot
Allowing each specialized GraphInterface object to implement whatever additional properties it requires to give class users control over those characteristics.

Define the Interface

The GraphInterface class is an abstract class that defines the methods and properties used by the subclasses. Comments in the abstract class describe the intended implementation:

classdef GraphInterface < handle
   % Abstract class for creating data graphs
   % Subclass constructor should accept
   % the data that is to be plotted and
   % property name/property value pairs
   properties (SetAccess = protected, GetAccess = protected)
      Primitive
      AxesHandle
   end
   properties
      Data
   end
   methods (Abstract)
      draw(obj)
      % Use a line, surface,
      % or patch graphics primitive
      zoom(obj,factor)
      % Change the CameraViewAngle
      % for 2D and 3D views
      % use camzoom for consistency
      updateGraph(obj)
      % Update the Data property and
      % update the drawing primitive
   end
   
   methods
      function set.Data(obj,newdata)
         obj.Data = newdata;
         updateGraph(obj)
      end
      function addButtons(gobj)
         hfig = get(gobj.AxesHandle,'Parent');
         uicontrol(hfig,'Style','pushbutton','String','Zoom Out',...
            'Callback',@(src,evnt)zoom(gobj,.5));
         uicontrol(hfig,'Style','pushbutton','String','Zoom In',...
            'Callback',@(src,evnt)zoom(gobj,2),...
            'Position',[100 20 60 20]);
      end
   end
end

The GraphInterface class implements the property set method (set.Data) to monitor changes to the Data property. An alternative is to define the Data property as Abstract and enable the subclasses to determine whether to implement a set access method for this property. The GraphInterface class defines a set access method that calls an abstract method (updateGraph, which each subclass must implement). The GraphInterface interface imposes a specific design on the whole namespace of classes, without limiting flexibility.

Method to Work with All Subclasses

The addButtons method adds push buttons for the zoom methods, which each subclass must implement. Using a method instead of an ordinary function enables addButtons to access the protected class data (the axes handle). Use the object zoom method as the push-button callback.

function addButtons(gobj)
   hfig = get(gobj.AxesHandle,'Parent');
   uicontrol(hfig,'Style','pushbutton',...
      'String','Zoom Out',...
      'Callback',@(src,evnt)zoom(gobj,.5));
   uicontrol(hfig,'Style','pushbutton',...
      'String','Zoom In',...
      'Callback',@(src,evnt)zoom(gobj,2),...
      'Position',[100 20 60 20]);
end

Derive a Concrete Class — LineGraph

This example defines only a single subclass used to represent a simple line graph. It derives from GraphInterface, but provides implementations for the abstract methods draw, zoom, updateGraph, and its own constructor. The base class GraphInterface and subclass are all contained in a namespace (graphics), which you must use to reference the class name:

classdef LineGraph < graphics.GraphInterface

Add Properties

The LineGraph class implements the interface defined in the GraphInterface class and adds two additional properties—LineColor and LineType. This class defines initial values for each property, so specifying property values in the constructor is optional. You can create a LineGraph object with no data, but you cannot produce a graph from that object.

properties
   LineColor = [0 0 0];
   LineType = '-';
end

The LineGraph Constructor

The constructor accepts a struct with x and y coordinate data, and property name/property value pairs:

function gobj = LineGraph(data,varargin)
   if nargin > 0
      gobj.Data = data;
      if nargin > 2
         for k=1:2:length(varargin)
            gobj.(varargin{k}) = varargin{k+1};
         end
      end
   end
end

Implement the draw Method

The LineGraph draw method uses property values to create a line object. The LineGraph class stores the line handle as protected class data. To support the use of no input arguments for the class constructor, draw checks the Data property to determine if it is empty before proceeding:

function gobj = draw(gobj)
   if isempty(gobj.Data)
      error('The LineGraph object contains no data')
   end
   h = line(gobj.Data.x,gobj.Data.y,...
      'Color',gobj.LineColor,...
      'LineStyle',gobj.LineType);
   gobj.Primitive = h;
   gobj.AxesHandle = get(h,'Parent');
end

Implement the zoom Method

The LineGraph zoom method follows the comments in the GraphInterface class which suggest using the camzoom function. camzoom provides a convenient interface to zooming and operates correctly with the push buttons created by the addButtons method.

Define the Property Set Methods

Property set methods provide a convenient way to execute code automatically when the value of a property changes for the first time in a constructor. (See Property Get and Set Methods.) The linegraph class uses set methods to update the line primitive data (which causes a redraw of the plot) whenever a property value changes. The use of property set methods provides a way to update the data plot quickly without requiring a call to the draw method. The draw method updates the plot by resetting all values to match the current property values.

Three properties use set methods: LineColor, LineType, and Data. LineColor and LineType are properties added by the LineGraph class and are specific to the line primitive used by this class. Other subclasses can define different properties unique to their specialization (for example, FaceColor).

The GraphInterface class implements the Data property set method. However, the GraphInterface class requires each subclass to define a method called updateGraph, which handles the update of plot data for the specific drawing primitive used.

The LineGraph Class

Here is the LineGraph class definition.

classdef LineGraph < graphics.GraphInterface
   properties
      LineColor = [0 0 0]
      LineType = '-'
   end
   
   methods
      function gobj = LineGraph(data,varargin)
         if nargin > 0
            gobj.Data = data;
            if nargin > 1
               for k=1:2:length(varargin)
                  gobj.(varargin{k}) = varargin{k+1};
               end
            end
         end
      end
      
      function gobj = draw(gobj)
         if isempty(gobj.Data)
            error('The LineGraph object contains no data')
         end
         h = line(gobj.Data.x,gobj.Data.y,...
            'Color',gobj.LineColor,...
            'LineStyle',gobj.LineType);
         gobj.Primitive = h;
         gobj.AxesHandle = h.Parent;
      end
      
      function zoom(gobj,factor)
         camzoom(gobj.AxesHandle,factor)
      end
      
      function updateGraph(gobj)
         set(gobj.Primitive,...
            'XData',gobj.Data.x,...
            'YData',gobj.Data.y)
      end
      
      function set.LineColor(gobj,color)
         gobj.LineColor = color;
         set(gobj.Primitive,'Color',color)
      end
      
      function set.LineType(gobj,ls)
         gobj.LineType = ls;
         set(gobj.Primitive,'LineStyle',ls)
      end
   end
end

Use the LineGraph Class

The LineGraph class defines the simple API specified by the graph base class and implements its specialized type of graph:

d.x = 1:10;
d.y = rand(10,1);
lg = graphics.LineGraph(d,'LineColor','b','LineType',':');
lg.draw;
lg.addButtons;

Clicking the Zoom In button shows the zoom method providing the callback for the button.

Window showing randomly drawn line segments

Changing properties updates the graph:

d.y = rand(10,1); 
lg.Data = d;
lg.LineColor = [0.9,0.1,0.6];

Now click Zoom Out and see the new results:

Window showing randomly drawn line segments