How to create undo and redo buttons in GUI that can be pressed multiple times?

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John Doe 2020-5-14

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编辑： John Doe 2020-5-18

Hello everyone! I have a working GUI where I have a undo and redo buttons. I have saved the current and previous states in two variables and implemented them in the respective callbacks. But I can undo and redo for only once. I guess the data is being overwritten everytime. Is there a way around it? Or a easier solution?

Thanks in advance!

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Answer 1

Geoff Hayes 2020-5-14

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John - without seeing your code, we can't determine why it is allowing you to undo and redo only once. You mention that you have saved the current and previous states in two variables. Are these scalar variables? If I were to implement redo and undo functionality, then I might use an array of some kind that would push the states to the front of the array so that when I call undo, the front would be popped up and the variables (or whatever) updated with the new front of the array. The popped off data would be pushed to the front of the redo array so that if I chose to call redo, then it's front would be popped off. There is lots more that you would have to consider - when should the redo array be emptied, when should the undo array be emptied, etc.

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Geoff Hayes 2020-5-15

在 MATLAB Online 中打开

Hi John - I guess it all depends on how you've implemented the pushing and popping of the elements from the stacks. In your first scenario, let's suppose A, B, and C are done sequentially, then your stacks should look like

myUndoStack = {C, B, A};
myRedoStack = {};

If we undo back to A, then the stacks should (?) look like

myUndoStack = {A};
myRedoStack = {B, C};

and using redo to get back to C we shoud have

myUndoStack = {C, B, A};
myRedoStack = {};

By keeping the two stacks in sync, you should be able to keep undoing and redoing as long as you want. I think the rule would be if you pop off something from the undo stack then it should be pushed to the redo stack. And vice versa too.

You do need to consider how these stacks get cleared. If my state is

myUndoStack = {A};
myRedoStack = {B, C};

and I go to step D, then it might look like

myUndoStack = {D, A};
myRedoStack = {};

since we've taken a "new" path and so our redo stack should be cleared.

For your second scenario, let's suppose A, B, and C are done sequentially, then the stacks should look like

myUndoStack = {C, B, A};
myRedoStack = {};

Going back to B via undo should give us

myUndoStack = {B, A};
myRedoStack = {C};

and pressing redo should give us

myUndoStack = {C, B, A};
myRedoStack = {};

It could be that too much might be taken off of the stacks. If you undo back to a step, then that step shoud be at the front of the undo stack. Maybe that isn't happening?

Geoff Hayes 2020-5-16

在 MATLAB Online 中打开

John - I think that the problem is with the undo function callback

% --- Executes on button press in pushbutton4.
function pushbutton4_Callback(hObject, eventdata, handles)
% hObject    handle to pushbutton4 (see GCBO)
% eventdata  reserved - to be defined in a future version of MATLAB
% handles    structure with handles and user data (see GUIDATA)
global undostack pic redostack
if ~isempty(undostack)
    pic = undostack{1};
    undostack = undostack(2:end);
    redostack = [pic redostack];
end
imshow(pic);

In the above code, we are taking the pic from the undo stack, pushing it to the redo stack, and then displaying that image. But the image we want to push to the redo stack should be the current image. The code may look like this

% --- Executes on button press in pushbutton4.
function pushbutton4_Callback(hObject, eventdata, handles)
% hObject    handle to pushbutton4 (see GCBO)
% eventdata  reserved - to be defined in a future version of MATLAB
% handles    structure with handles and user data (see GUIDATA)
global undostack pic redostack
if ~isempty(undostack)
    redostack = [pic redostack];  % <--- save current image to redo stack
    pic = undostack{1};
    undostack = undostack(2:end);
end
imshow(pic);

Likewise, when redoing, the image pushed to the undo stack should be the image that is currently being shown and not the image that we are to show.

% --- Executes on button press in pushbutton5.
function pushbutton5_Callback(hObject, eventdata, handles)
% hObject    handle to pushbutton5 (see GCBO)
% eventdata  reserved - to be defined in a future version of MATLAB
% handles    structure with handles and user data (see GUIDATA)
global pic  redostack undostack
if ~isempty(redostack)
    undostack = [pic undostack]; % <--- save current image to undo stack
    pic = redostack{1};
    redostack = redostack(2:end);
end
imshow(pic);

I think that part of the confusion was how I described how this might work. I envisioned having the current image always at the top of the undo stack rather than separate.

Also, you don't need to use global variables. For GUIs created with GUIDE, you can use the handles structure to save data to and extract from. For example, in the load function callback

% --- Executes on button press in pushbutton1.
function pushbutton1_Callback(hObject, eventdata, handles)
% hObject    handle to pushbutton1 (see GCBO)
% eventdata  reserved - to be defined in a future version of MATLAB
% handles    structure with handles and user data (see GUIDATA)
handles.undostack={};
handles.redostack={};
%redostack= [pic redostack];
path = imgetfilz();
axes(handles.axes1);
handlespic = imread(path);
%undostack = [pic undostack];
imshow(handles.pic);
guidata(hObject, handles);  % <--- important! need to do this every time

You would then access these variables (in all other callbacks) via the handles structure. Note that whenver you update a field in this structure, you need to call

guidata(hObject, handles);

so that all other callbacks receive the updated structure.

Image Analyst 2020-5-17

在 MATLAB Online 中打开

Not sure which button is redo, but you can set the Enable property to off in GUIDE by double clicking on the button and changing the Enable property to off. Now when they run it, it will be disabled. Then in the callback for the undo button, you can enable the redo button.

handles.btnRedo.Enable = 'on';

Any time you want to disable it again, you can do this:

handles.btnRedo.Enable = 'off';

Of course, change the name to whatever you called the redo button.

Thanks for Accepting and Voting for Geoff's answer. It's how people earn reputation points and is a good way to thank answerers (of which Geoff is one of the best).

John Doe 2020-5-17