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App designer - file data store error, read function does not exist.

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I am trying to create a GUI with App Designer, which loads a file to UIAxes. With scrolling Mousewheel or a Slider I want to scroll through multiple files.
I used the FileDatastore approach where I create a dataarray, which includes the files. This works fine in the normal Matlab Editor, but in the App designer it shows an error, that the customreader function does not exist.
defined private function:
function data = customreader(~,filename)
dcm =;
din =;
din.readDicomObject(dcm, -1);
rows = dcm.getInt(;
cols = dcm.getInt(;
pixeldata = dcm.getInts(;
data = reshape(pixeldata, rows,cols);
function LoadTBButtonPushed(app, event)
%load file
folder = uigetdir();
if isequal(folder,0)
app.FileEditField.Value = 'no file selected';
location = folder;
fds = fileDatastore(location, 'ReadFcn', @customread, 'FileExtensions','.ima');
dataarray = readall(fds);
i = 1;
% Remove title, axis labels, and tick labels
title(app.UIAxes, []);
xlabel(app.UIAxes, []);
ylabel(app.UIAxes, []);
app.UIAxes.XAxis.TickLabels = {};
app.UIAxes.YAxis.TickLabels = {};
%show image
I = imshow(dataarray{1,1},'Parent', app.UIAxes, ...
'XData', [1 app.UIAxes.Position(3)], ...
'YData', [1 app.UIAxes.Position(4)], ...
'DisplayRange', []);
% Set limits of axes
app.UIAxes.XLim = [0 I.XData(2)];
app.UIAxes.YLim = [0 I.YData(2)];
Error using fileDatastore (line 102). Function customread does not exist.
is shown in Line: fds = fileDatastore(location, 'ReadFcn', @customread, 'FileExtensions','.ima');
I tried to get the letters app in there, but if I write @app.customreader, the next line sends an error.
Error using (line 28)
Error using ReadFcn @(varargin)app.customread(varargin{:}) function handle for file
No appropriate method, property, or field 'customread' for class 'prototyp_menu'.
Error in @(varargin)app.customread(varargin{:})', 'Documents\MATLAB\GUI_KM\Prototyp\prototyp_menu.mlapp', 240)" style="font-weight:bold">prototyp_menu>@(varargin)app.customread(varargin{:}) (line 240)
fds = fileDatastore(location, 'ReadFcn', @app.customread, 'FileExtensions','.ima');
Thanks for Your help!
Michael Werthmann
Michael Werthmann on 13 Feb 2019
@app.customreader worked!,
Thank you for your help and sorry for the dumb question...

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