Callback functions in MATLAB

Introduction

Callback functions let your program respond when something happens, like a button click. They help your code wait and react to events.

When you want to run some code after a user clicks a button in a GUI.

When you need to update a plot after changing a slider value.

When you want to perform an action after a timer finishes.

When you want to handle user input or interaction in a program.

Syntax

MATLAB

function callbackFunction(src, event)
    % Your code here
end

% Assign callback to a UI control
button.ButtonPushedFcn = @callbackFunction;

The callback function usually takes two inputs: the source object and event data.

Use the @ symbol to pass the function handle as a callback.

Examples

This example shows a simple callback that prints a message when a button is clicked.

MATLAB

function buttonClicked(src, event)
    disp('Button was clicked!')
end

button.ButtonPushedFcn = @buttonClicked;

This callback reads the slider's current value and displays it when the slider moves.

MATLAB

function sliderMoved(src, event)
    val = src.Value;
    disp(['Slider value is ', num2str(val)])
end

slider.ValueChangedFcn = @sliderMoved;

Sample Program

This program creates a window with a button. When you click the button, it prints a message in the command window.

MATLAB

function simpleCallbackExample
    fig = uifigure('Name', 'Callback Example');
    btn = uibutton(fig, 'Text', 'Click me', 'Position', [100 100 100 30]);
    btn.ButtonPushedFcn = @buttonClicked;

    function buttonClicked(src, event)
        disp('You clicked the button!')
    end
end

simpleCallbackExample;

Important Notes

Callback functions run only when the event happens, so your program can wait without stopping.

Always define the callback function with the correct input arguments to avoid errors.

Summary

Callback functions let your program react to user actions or events.

Use the @ symbol to assign a function as a callback.

Callbacks usually receive the source object and event data as inputs.

Practice

1. What is the main purpose of a callback function in MATLAB GUI programming?

easy

A. To speed up the program execution by parallel processing

B. To store data permanently in a file

C. To execute code in response to user actions or events

D. To create new variables automatically

Solution

1. Step 1: Understand the role of callback functions

   Callback functions are designed to respond when a user interacts with a GUI element, like clicking a button.

2. Step 2: Identify the correct purpose

   Among the options, only executing code in response to user actions matches the callback function's role.

3. Final Answer:

   To execute code in response to user actions or events -> Option C

4. Quick Check:

   Callback = respond to events [OK]

Hint: Callbacks run when user interacts with GUI elements [OK]

Common Mistakes:

• Thinking callbacks store data permanently
• Confusing callbacks with parallel processing
• Assuming callbacks create variables automatically

2. Which of the following is the correct way to assign a callback function named myCallback to a push button btn in MATLAB?

easy

A. btn.Callback = myCallback;

B. btn.Callback = &myCallback;

C. btn.Callback = 'myCallback';

D. btn.Callback = @myCallback;

Solution

1. Step 1: Recall MATLAB callback syntax

   In MATLAB, the @ symbol is used to create a function handle for callbacks.

2. Step 2: Identify the correct assignment

   Assigning @myCallback correctly creates a handle to the function for the callback.

3. Final Answer:

   btn.Callback = @myCallback; -> Option D

4. Quick Check:

   Use @ for function handles [OK]

Hint: Use @ before function name for callbacks [OK]

Common Mistakes:

• Omitting @ symbol for function handle
• Using quotes instead of @ for function handle
• Using & which is not valid in MATLAB

3. Consider the following MATLAB code snippet for a button callback:

function myCallback(src, event)
    disp('Button pressed');
end

btn.Callback = @myCallback;

% When the button is clicked, what will be displayed?

medium

A. Button pressed

B. src event

C. Error: Undefined function

D. No output

Solution

1. Step 1: Understand the callback function behavior

   The function myCallback displays the text 'Button pressed' when called.

2. Step 2: Recognize the callback assignment

   The button's callback is set to @myCallback, so clicking the button triggers the function and prints the message.

3. Final Answer:

   Button pressed -> Option A

4. Quick Check:

   Callback prints message on click [OK]

Hint: Callback prints message when triggered [OK]

Common Mistakes:

• Expecting variable names to print
• Thinking callback causes error without GUI
• Assuming no output from disp function

4. What is wrong with this MATLAB callback assignment?

btn.Callback = myCallback;

Assuming myCallback is a function defined elsewhere.

medium

A. Missing @ symbol to create function handle

B. Callback property does not exist for buttons

C. Function name should be a string, not a variable

D. Callback functions cannot be assigned directly

Solution

1. Step 1: Check callback assignment syntax

   In MATLAB, callbacks require a function handle using the @ symbol.

2. Step 2: Identify the missing element

   The code assigns the function name without @, so MATLAB treats it as a variable, causing an error.

3. Final Answer:

   Missing @ symbol to create function handle -> Option A

4. Quick Check:

   Callbacks need @ before function name [OK]

Hint: Always use @ before function name for callbacks [OK]

Common Mistakes:

• Omitting @ symbol
• Confusing property names
• Thinking function name as string is correct

5. You want to create a MATLAB GUI button that changes its own label to 'Clicked!' when pressed. Which callback function correctly achieves this?

hard

A. function changeLabel()
btn.String = 'Clicked!';
end

B. function changeLabel(src, ~)
src.String = 'Clicked!';
end

C. function changeLabel(~, event)
event.String = 'Clicked!';
end

D. function changeLabel(src)
src.Label = 'Clicked!';
end

Solution

1. Step 1: Understand callback inputs

   The first input src is the source object (the button), which can be used to change its properties.

2. Step 2: Identify correct property and syntax

   The button's label text is stored in the String property, so setting src.String = 'Clicked!'; updates the label.

3. Final Answer:

   function changeLabel(src, ~) src.String = 'Clicked!'; end -> Option B

4. Quick Check:

   Use src.String to change button label [OK]

Hint: Use src.String to update button text inside callback [OK]

Common Mistakes:

• Using event instead of src to change properties
• Using wrong property name like Label
• Not including required input arguments