Difference between CloseRequestFcn and DeleteFcn?

2023 1 2
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Hello, I use the 'CloseRequectFcn' Figure property in my GUI design to hande "program shutdown"-related tasks, like closing files and windows, stopping cameras, etc. Recently, I took over a project where the previous author used 'DeleteFcn' for this purpose. My question is, what is the correct way to use these two functions? Is there a case where one is better than the other? Is one "more correct" for handling program exit? The documentation doesn't seem to give a clear answer. Thanks!

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J. Alex Lee
J. Alex Lee 2023-1-3
maybe just use both and redirect to a common shutdown function; catch the case where someone wants to shut down the app by deleting the handle to it as well as close()-ing it, or using the GUI to close
M. A. Hopcroft
M. A. Hopcroft 2023-1-3
That is an interesting point, and your comment gives me more to think about. Now I'm re-reading the documentation, and focusing on the differences between the Figure properties and UI Figure properties. It seems like "delete" is something that happens after a "close" request. And the description of the "close()" command includes:
  • When implementing a CloseRequestFcn callback, do not use a call to close. Calling close in the body of the callback sets up a recursion that results in a MATLAB warning. Instead, implement the callback using the delete function. delete removes the figure without executing the CloseRequestFcn callback.
So this suggests that delete() is the final thing that happems when close() is called.
If a GUI is going to run in a interactive environment, there is a possibility that a user could call delete() on the gui figure, as you suggested, instead of close() (like clicking the window close button or a Quit menu item), so maybe it would make sense to have a DeleteFcn to catch this. But it seems like in a "normal" situation, or a deployed application, this is not likely to happen, so a CloseRequestFcn would be better.
The way the code is now, the DeleteFcn calls "close all", and this causes a recursion warning, where the DeleteFcn tries to close a window that has been deleted (as described above).
Rik
Rik 2023-1-3
You may consider moving the middle of your comment to the answer section.
J. Alex Lee
J. Alex Lee 2023-1-3
编辑：J. Alex Lee 2023-1-3
apparently you don't even need to call delete inside the deleteFcn for the deletion to happen...
f = figure("DeleteFcn",@(o,e)disp("called delete"))
delete(f)
will still close the figure.
So I would say make your closerequestfcn just call your deletefcn callback, and do everything in there.
M. A. Hopcroft
M. A. Hopcroft 2023-1-4
I think that is the right approach. In general, in a user interface, the user can close a window using their local window manager (i.e. clicking the close button), but figure delete is a programmatic function that won't happen without some call to delete() to make it happen. So you need a CloseRequestFcn to do any user interaction that you want when the window is closed by the user. A DeleteFcn handles any cleanup tasks that do not need user interaction. So there is a reason to user one or the other or both. And, in both cases, if no callback Fcn is specified, the figure is deleted.

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Rik
Rik 2023-1-3

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Summarizing what has been mentioned in comments so far:
CloseRequestFcn
The CloseRequestFcn responds to any calls to the close function, as well as clicking on the close button of the figure window. That means that any calls inside it to close will result in a recursion. Without branching logic this means it will end up triggering a warning arising from infinite recursion.
This function does not close the figure, so you need to call delete explicitly to do so. This has the benefit of implementing an 'are you sure' message.
DeleteFcn
The DeleteFcn will trigger just prior to any call to delete, including those from the default CloseRequestFcn. There is no need to explicitly call delete in this function.
This function is most usefull to save any persistent state just prior to closing a GUI. There is no way to avoid the closing of the figure inside this function (expect perhaps via an error, I didn't test this).

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M. A. Hopcroft
M. A. Hopcroft 2023-1-3
The CloseRequestFcn does not close the figure, but it is not expected to, and same for DeleteFcn. The call to close() or delete() is what closes the window or figure.

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