Reuse Code in Apps Using Helper Functions

Helper functions are MATLAB^® functions that you define to use in your app. After you define a helper function, you can call it at different places in your app code. For example, you can call a helper function to update a plot after an app user changes a number on a slider or selects an item in a drop-down list. Creating a helper function allows you to single-source the common commands and avoid maintaining redundant code.

You can define helper functions in your app in App Designer or in a MATLAB program file. Choose an approach based on how the function code interacts with your app.

Approach	When to Use
Create helper function in App Designer.	The code updates the user interface or needs access to components in the app. You want to run the function only when the app is running.
Create helper function in MATLAB program file (`.m`).	You already have some code written in a MATLAB script or function, and you want to execute that code from your app. You want to share the code between multiple independent apps or projects in MATLAB. The code does not directly update any part of the user interface. You want to run or test the code independently of the app.

Create Helper Function in App Designer

There are two types of helper functions you can create in App Designer: private functions, which you can call only inside your app, and public functions, which you can call either inside or outside your app. Private functions are commonly used in single-window apps, while public functions are commonly used in multiwindow apps.

Create a helper function in App Designer from Code View. In the Editor tab of the toolstrip, click Function, then select Private Function or Public Function.

Function drop-down list with options "Private Function" and "Public Function"

When you make your selection, App Designer creates a template function within the app code and places your cursor in the body of that function. Then you can update the function name and its arguments, and add your code to the function body. The app argument is required, and you can use the app input to access components and properties in the app. You can include any additional arguments after the app argument.

For example, this function, named updatePlot, creates a surface plot of the peaks function. It accepts an additional argument n for specifying the number of samples to display in the plot.

function updatePlot(app,n)
    surf(app.UIAxes,peaks(n));
    colormap(app.UIAxes,winter);
end

Call Private Function

Define a private function when you want to call the function only from within the app where you define it. You can call the function from anywhere in your app.

For example, if your app contains a slider and a drop-down component that control parameters on the same plot, define the updatePlot function as a private function. Update the plotted data when either of the component values change by calling the updatePlot function from the component callback functions.

function SliderValueChanged(app,event)
    value = app.Slider.Value;
    updatePlot(app,value)        
end

function DropDownValueChanged(app,event)
    value = app.DropDown.Value;
    num = str2double(value);
    updatePlot(app,num)        
end

Call Public Function

Define a public function when you want to call the function from outside the app while the app is running. Public functions are useful when you have a multiwindow app composed of multiple app files. You can call a public function defined in one of the apps from another app.

For example, if you have a main app with a button that creates a secondary app with a plot, define the updatePlot function as a public function. Update the plot from the main app by calling the updatePlot function from the ButtonPushedFcn callback.

function CreatePlotButtonPushed(app,event)
    secondaryApp = MyPlotApp;
    updatePlot(secondaryApp,10)       
end

For more information about multiwindow apps, see Create Multiwindow Apps in App Designer.

Manage Helper Functions

You can manage helper functions that you define in App Designer by using the Code Browser. For example, you can use the Code Browser to perform these tasks:

View all helper functions defined in App Designer — In the Code Browser in Code View, select the Functions tab.
Search for a helper function — At the top of the Functions tab, use the Search box.
Rename a helper function — Double-click the function name and type a new name. App Designer automatically updates all references to the function in your app code when you change the name.
Navigate to a helper function — Click the function name to scroll the function definition into view. Right-click the function name and select Go to to place your cursor in the function.
Delete a helper function — Right-click the function name and select Delete.

Create Helper Function in MATLAB Program File

To reuse code between multiple independent apps or projects, define a helper function outside of App Designer, in a MATLAB program file. If you have existing code in a script, you can convert the script to a function. For more information, see Scripts vs. Functions.

To create a new function that your app can call, follow these steps:

Create a new function file in MATLAB. For more information, see Create Functions in Files.
Define function inputs for data the function needs from the app, such as component values.
Define function outputs for data the app needs from the function, such as results of calculations.
Add the function file to a place that the app file can access, such as in the same folder as the app file or on the MATLAB search path.

You can then call the function from your app code.

For example, to display the number of days between a selected date and the current date, you can create a MATLAB function that performs the date calculations. Create a function file named calculateDate.m in the same folder as your app file. Define the function to take in a date and return the number of days between the current day and that date.

function numdays = calculateDate(date)
dur = date - datetime("today");
numdays = days(dur);
end

Call this function from your app code in App Designer. For example, if your app contains a date picker, call the calculateDate function from the ValueChangedFcn callback for the date picker to update your app.

function DatePickerValueChanged(app,event)
    value = app.DatePicker.Value;
    numdays = calculateDate(value);
    app.CountdownLabel.Text = "Your date is " + numdays + " days away";
end

Example: Helper Function That Updates Plot

Open App

This app shows how to use a helper function to both initialize and update a plot. The app calls the updatePlot private helper function at the end of the startupFcn callback to initialize a plot when the app starts up. The UITableDisplayDataChanged callback also calls the updatePlot function to update the same plot when the user sorts columns or changes a value in the table.