When I click an image, it doesn't enter its callback

I'm having difficulty writing an app with App Designer that allows me to click on an image and capture the coordinates of the click and then immediately draw a box around that clicked point.

I create an image and display it with this code fragment (I've left out the user interface and a lot of setup stuff):

% Draw a box around a known point 
dispImg = drawBox(dispImg, [TTx TTy], [app.roiBoxLen app.roiBoxLen], [255, 255, 0], 2);
% Insert Timestamp and Distance Text Boxes
ts = strrep(timestamp, '_', ' ');

dispImg = insertText(dispImg, [350,dims(1)-20], ['Range: ',num2str(TargetRange, '%0.2f'), ' nmi'],...
    'TextColor','g','FontSize',20, 'BoxColor', 'black', 'BoxOpacity',1,'AnchorPoint','LeftBottom');
dispImg = insertText(dispImg, [50,20], ts,'TextColor','g','FontSize',20, 'BoxColor', 'black','BoxOpacity',1);
dispImg = insertText(dispImg, [200,dims(1)-20], irType,'TextColor','g','FontSize',20, 'BoxColor', 'black','BoxOpacity',1,'AnchorPoint','LeftBottom');

drawnow;
% display the image on app.UIAxes
image(dispImg, 'Parent', app.UIAxes, 'ButtonDownFcn', @app.ImageClicked);

% then do something with the X,Y returned from app.ImageClicked

Then the callback (called "ImageClicked") looks like this

% Button down function: UIAxes
function ImageClicked(app, event)

% Check Matlab version, throw error if prior to r2020b
% Got this from a post on Matlab Central - not sure if needed
assert(~verLessThan('Matlab', '9.9'), 'ginput not supported prior to Matlab r2020b.')

coords = event.IntersectionPoint;

% For now, just display to the command line until I get this callback debugged
disp(coords);
end

The problem is that when I click the image, that callback is never hit.

It throws this error:

Error using ABSViewer/ImageClicked
Too many input arguments.

Error in ABSViewer>@(varargin)app.ImageClicked(varargin{:}) (line 93)
app.myImage.ButtonDownFcn = @app.ImageClicked; %create callback. IMPORTANT: You must use app.

If I include the code as an inline function (or whatever you call it in Matlab when you include a function's code within another function right after you define it as a callback), then it does work.

But I want to understand: why doesn't it hit its callback function like I think it should? This is my first attempt at doing a 'real' app with App Designer and it's very possible I could be misunderstanding how things work.

[Adam Danz]

Error in ABSViewer>@(varargin)app.ImageClicked(varargin{:}) (line 93)

Your ImageClicked function appears to only support 2 inputs and it looks like the varargin is supplying more than 2 inputs. See line 93, apparently.

[Greg Westbrook]

OK, forgive me but I made a mistake in writing my question - trying too many things late in the day yesterday I guess.

The error message that is thrown is actually the following.

Undefined function 'ImageClicked' for input arguments of type 'matlab.graphics.primitive.Image'.
Error while evaluating Image ButtonDownFcn.

Answer 1

[Adam Danz]

See Callbacks in App Designer.

Try this:

image(dispImg, 'Parent', app.UIAxes, 'ButtonDownFcn', @(src,event)ImageClicked(app,src,event));

and then define the callback function:

function ImageClicked(app, src, event)

Additionally, if the ButtonDownFcn is a proprety of an app component and if the ImageClicked function is a method in your app, this is a better approach (see documentation):

image(dispImg, 'ButtonDownFcn', @app.ImageClicked) % app is your app object

and then define the callback function:

function ImageClicked(app,src,event)

[Greg Westbrook]

Thanks! That was the solution to my problem.

[Stephen23]

Or without the anonymous function:

image(dispImg, 'Parent',app.UIAxes, 'ButtonDownFcn',{@ImageClicked,app});

https://www.mathworks.com/help/matlab/creating_plots/callback-definition.html#buhztrs-11

[Greg Westbrook]

I hate to display my ignorance but here goes:

When you say "without the anonymous function", why is the function anonymous?

[Stephen23]

"When you say "without the anonymous function", why is the function anonymous? "

This defines an anonymous function (which happens to call your function):

@(src,event)ImageClicked(app,src,event) % anonymous function

whereas this simply defines a function handle directly to your function:

@ImageClicked % function handle

Here is the MATLAB documentation on anonymous functions (sometimes known as "lambda" functions in other languages):

https://www.mathworks.com/help/matlab/matlab_prog/anonymous-functions.html

and all types of functions here:

https://www.mathworks.com/help/matlab/matlab_prog/types-of-functions.html

Anonymous functions are very useful, but in this case you can save yourself the extra workspace and one layer of the function stack. Note that in the code I gave, the cell array wrapped around it tells MATLAB to pass the extra arguments.

[Greg Westbrook]

Thanks! That makes sense.

[Adam Danz]

@Greg Westbrook, in App Designer, the app object is typically passed into the first argument. See some examples in the documentation for Callbacks in App Designer. This is optional for your ButtonDownFcn, in fact, you don't even need to pass it in at all if you're not using it, but if you do include it, you can decide whether to follow the standard in App Designer where the app is passed in arg 1 or the standard that additional arguments are passed into callbacks after arg 2.

I'll update my answer to include a third, and perhaps the best, option.

[Stephen23]

"the app object is typically passed into the first argument.."

Aah, I did not realize that once again, consistency was thrown to the wind. Thank you for the information!