Adding dynamic properties to graphic handles

A client recently asked me to extend one of Matlab’s built-in graphic containers (uiflowcontainer in this specific case) with automatic scrollbars that would enable the container to act as a scroll-panel. The basic idea would be to dynamically monitor the container’s contents and when it is determined that they overflow the container’s boundaries, then attach horizontal/vertical scrollbars to enable scrolling the contents into view:

HG2 – addprop function

The addprop function is actually a public method of the dynamicprops class. Both the dynamicprops class as well as its addprop function are fully documented. What is NOT documented, as far as I could tell, is that all of Matlab’s builtin handle graphics objects indirectly inherit dynamicprops, via matlab.graphics.Graphics, which is a high-level superclass for all HG objects. The bottom line is that we can dynamically add run-time properties to any HG object, without affecting any other object. In other words, the new properties will only be added to the handles that we specifically request, and not to any others. This suits me just fine:

hProp = addprop(hPanel, 'hHorizontalScrollBar');
hPanel.hHorizontalScrollBar = hMyScrollbar;
hProp.SetAccess = 'private';  % make this property read-only

The new property hHorizontalScrollBar is now added to the hPanel handle, and can be accessed just like any other read-only property. For example:

>> get(hPanel, 'hHorizontalScrollBar')
ans =
    JavaWrapper
>> hPanel.hHorizontalScrollBar
ans =
    JavaWrapper
>> hPanel.hHorizontalScrollBar = 123
You cannot set the read-only property 'hHorizontalScrollBar' of UIFlowContainer.

Adding new methods is more tricky, since we do not have a corresponding addmethod function. The trick I used was to create a new property having the requested new method’s name, and set its read-only value to a handle of the requested function. For example:

hProp = addprop(hPanel, 'refresh');
hPanel.refresh = @myRefreshFunc;
hProp.SetAccess = 'private';  % make this property read-only

We can then invoke the new refresh “method” using the familiar dot-notation:

hPanel.refresh();

Note: if you ever need to modify the initial value in your code, you should revert the property’s SetAccess meta-property to 'public' before Matlab will enable you to modify the value:

try
    % This will raise an exception if the property already exists
    hProp = addprop(hPanel, propName);
catch
    % Property already exists - find it and set its access to public
    hProp = findprop(hPanel, propName);
    hProp.SetAccess = 'public';
end
hPanel.(propName) = newValue;

HG1 – schema.prop function

In HG1 (R2014a and earlier), we can use the undocumented schema.prop function to add a new property to any HG handle (which is a numeric value in HG1). Donn Shull wrote about schema.prop back in 2011, as part of his series of articles on UDD (Unified Data Dictionary, MCOS’s precursor). In fact, schema.prop is so useful that it has its own blog tag here and appears in no less than 15 separate articles (excluding today). With HG2’s debut 2 years ago, MathWorks tried very hard to rid the Matlab code corpus of all the legacy schema-based, replacing most major functionalities with MCOS-based HG2 code. But so far it has proven impossible to get rid of schema completely, and so schema code is still used extensively in Matlab to this day (R2015b). Search your Matlab path for “schema.prop” and see for yourself.
Anyway, the basic syntax is this:

hProp = schema.prop(hPanel, propName, 'mxArray');

The 'mxArray' specifies that the new property can accept any data type. We can limit the property to only accept certain types of data by specifying a less-generic data type, among those recognized by UDD (details).
Note that the meta-properties of the returned hProp are somewhat different from those of HG2’s hProp. Taking this into account, here is a unified function that adds/updates a new property (with optional initial value) to any HG1/HG2 object:

function addProp(hObject, propName, initialValue, isReadOnly)
    try
        hProp = addprop(hObject, propName);  % HG2
    catch
        try
            hProp = schema.prop(hObject, propName, 'mxArray');  % HG1
        catch
            hProp = findprop(hObject, propName);
        end
    end
    if nargin > 2
        try
            hProp.SetAccess = 'public';  % HG2
        catch
            hProp.AccessFlags.PublicSet = 'on';  % HG1
        end
        hObject.(propName) = initialValue;
    end
    if nargin > 3 && isReadOnly
        try
            % Set the property as read-only
            hProp.SetAccess = 'private';  % HG2
        catch
            hProp.AccessFlags.PublicSet = 'off';  % HG1
        end
    end
end