I’m having troubles using this in a GUI that I’m coding by hand.
As a “help” pushbutton callback, I have the following code:

function [] = pb_call5(varargin)
  jDesktop = com.mathworks.mde.desk.MLDesktop.getInstance;
  jClassName = 'com.mathworks.mlwidgets.help.HelpPopup';
  jPosition = java.awt.Rectangle(200,200,650,600);
  jTextArea = jDesktop.getMainFrame.getFocusOwner;
  helpTopic = 'LiqVapGui';
  javaMethodEDT('showHelp',jClassName,jTextArea,[],jPosition,helpTopic);
end

However, I am receiving an error stating that
“??? Error using ==> javaMethodEDT
Java exception occurred:
java.lang.IllegalArgumentException: ‘parent’ cannot be null”

When stepping through the debugger, I find that jTextArea is empty. However, if I re-type that line while in debug mode, it works just fine. Do you know any way that I can fix this problem?

-Nathan

@Nathan – I assume you haven’t placed this in your GUIDE-generated _OpeningFcn(), as explained ^[16]. Another cause of this might be that when you run the code, jTextArea is still unavailable by the time the javaMethodEDT is called. In step-by-step debugging this is not an issue, since there is plenty of time for jTextArea to be set. It is also possible that there is no FocusOwner in the Desktop (unlike the case of step-by-step debugging) – this can be forced using the commandwindow function. As a final alternative, try using the getClient() approach to getting the Command-window TextArea, as I explained in my recent ^[17]:

jDesktop = com.mathworks.mde.desk.MLDesktop.getInstance;
try
  cmdWin = jDesktop.getClient('Command Window');
  jTextArea = cmdWin.getComponent(0).getViewport.getComponent(0);
catch
  commandwindow;
  jTextArea = jDesktop.getMainFrame.getFocusOwner;
end

I am not using GUIDE (as implied by “coding by hand”), but your getClient() method worked for me. Thanks for the quick response and your help. This website is very insightful.

-Nathan

Hi Yair,

When I try to use html in help popup, I was told there is no setHtmlText of browser. So I ask myself is there any update in matlab 2017b. But I tried getComponent with other indices which doesn’t work. Maybe you would have a clue. Thanks a lot.

browser = jPopup.getContentPane.getComponent(1).getComponent(0);
browser.setHtmlText(html);

@Peng – the code above was relevant as of the time I wrote the article, 10 years ago (2009). In recent Matlab releases the code needs to be modified a bit:

browser = jPopup.getContentPane.getComponent(1).getComponent(1).getComponent.getBrowser;
browser.loadHTML(html)  % to display a string containing HTML code
browser.loadURL(url)    % to display an online webpage (e.g. 'google.com')

I have a problem with putting this into the startup.m to be executed at the beginning of the MATLAB session.

jDesktop = com.mathworks.mde.desk.MLDesktop.getInstance;
try
    cmdWin = jDesktop.getClient('Command Window');
    jTextArea = cmdWin.getComponent(0).getViewport.getComponent(0);
catch
    commandwindow;
    jTextArea = jDesktop.getMainFrame.getFocusOwner;
end
jClassName = 'com.mathworks.mlwidgets.help.HelpPopup';
jPosition = java.awt.Rectangle(0, 0, 400, 300);
helpTopic = 'surf';
% Variant 1:
javaMethodEDT('showHelp', jClassName, jTextArea, [], jPosition, helpTopic);
% Variant 2:
%jniSig = 'showHelp(Ljavax.swing.JComponent;Lcom.mathworks.mwswing.binding.KeyStrokeList;Ljava.awt.Rectangle;Ljava.lang.String;)';
%awtinvoke(jClassName,jniSig,jTextArea,[],jPosition,helpTopic);

My startup script uses similar code that opens and closes a dummy popup with custom window size that would be applied for subsequent F1-popups.
In R2018a and R2020b Prerelease the above example works well as standalone code. Nevertheless, when used in startup.m it does not create a popup during the startup (and for variant 1 it also results in “java.lang.IllegalArgumentException: ‘parent’ cannot be null” exception). I remember that in some earlier releases a similar solution used to work (pause(1) or drawnow were helpful in case of errors), but now I cannot find any fix. Maybe you would know if it is still possible to open a popup with the desired size during MATLAB startup.

@Peter – the graphics system might not be fully initialized by the time that the startup script is being executed by the Matlab engine. I suggest placing the relevant code in a small function and in startup.m simply start a single-shot timer with a StartDelay of several seconds, that will run the new function in its TimerFcn callback.