getappdata works to load a variable that was never set?

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Betzalel Fialkoff 2017-9-30

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https://ww2.mathworks.cn/matlabcentral/answers/359156-getappdata-works-to-load-a-variable-that-was-never-set

编辑： Jan 2017-10-1

Hi I've been using getappdata/setappdata to share variables across different callbacks in my GUI.

Basically what happens is that the user calls a one of the callbacks to load an image into the application and i used setappdata, so that the image can be called somewhere else.

the problem i discovered while debugging on the line of code : >> mask=getappdata(0,'mask');%recieve the mask

if i read this line even without having saved the variable "mask" (with setappdata) somehow the variable mask still gets loaded into the workspace.

does anyone know what causes this? how i can correct it? or if there is an alternative to sharing variables across callback

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Betzalel Fialkoff 2017-10-1

is there some way i can use getappdata without storing it in a root object? if not what is a good alternative?

Stephen23 2017-10-1

编辑：Stephen23 2017-10-1

"is there some way i can use getappdata without storing it in a root object?"

Read the setappdata / getappdata help, which list all of the different graphics object types that you can use.

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Answer 1

Jan 2017-9-30

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https://ww2.mathworks.cn/matlabcentral/answers/359156-getappdata-works-to-load-a-variable-that-was-never-set#answer_283812

编辑：Jan 2017-9-30

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It is a bad style to share data using the root object 0. This has the same problems as using global variables and you find thousands of explanations in this forum and the net about the severe problems caused by globals.

Better share variables of callbacks by storing them in the figure. Then you can e.g. run multiple instances of a GUI without interferences.

Your description is not clear. As far as I understand you run

data = getappdata(0, 'AVariableNotCreatedBefore')

and are surprised that [] is returned. This is exactly, what is defined in the documentation:

help getappdata
... If the application-defined data does
not exist, an empty matrix will be returned in VALUE.

Perhaps you want to check the existence at first:

if isappdata(0, 'AVariableNotCreatedBefore')
  data = getappdata(0, 'AVariableNotCreatedBefore');
else
  error('Request not existing variable');
end

Or perhaps you have the problem, that the globally set variable is not cleared, when the GUI is closed. This is expected also, because you stored it in the root object. Then the solution is mentioned already: Store the data in the figure and not globally.

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Betzalel Fialkoff 2017-10-1

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>>Your description is not clear. As far as I understand you run

>>data = getappdata(0, 'AVariableNotCreatedBefore') >>and are surprised that [] is returned

the problem is that it doesnt return []. it returns an image that was defined the last time i ran the GUI, but now my workspace is empty and when i run the line (in isolation using F9)

data = getappdata(0, 'AVariableNotCreatedBefore')

it returns the variable that was defined the last time i used the gui. though as far as i can tell it should return an [] because it should know that the variable exists.

I hope this clarifies. It sounds to me like this is what you were describing:

>>Or perhaps you have the problem, that the globally set variable is >>not cleared, when the GUI is closed. This is expected also, because >>you stored it in the root object. Then the solution is mentioned >>already: Store the data in the figure and not globally.

if so perhaps there is a way i can store it not in a root object? if not can you please direct me to an article that discusses storing the data in a figure.

Hope that helped to clarify the question. Thank You

Jan 2017-10-1

编辑：Jan 2017-10-1

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@Betzalel: Do you know the so called wrapper function guidata? It calls get/setappdata also internally using the figure handle. This is frequently used to store data locally in the figure:

function YourCallback(hObject, EventData, handles)
handles.Mask = (rand(3) < 0.5);  % For example
guidata(hObject, handles);  % Store modified handles struct in the figure
end
function AnotherCallback(hObject, EventData, handles)
Mask = handles.Mask;
...
end

guidata determines the parent figure belonging to the object hObject automatically. But you can do this with set/getappdata directly also:

function YourCallback(hObject, EventData, handles)
FigH = ancestor(hObject, 'figure');  % [EDITED, Typo fixed]
Mask = (rand(3) < 0.5);  % For example
setappdata(FigH, 'Mask', Mask);
end
function AnotherCallback(hObject, EventData, handles)
FigH = ancestor(hObject, 'figure');
Mask = getappdata(FigH, 'Mask');
...
end