Callback functions in MATLAB - Mini Project: Build & Apply

Callback functions

📖 Scenario: Imagine you are creating a simple MATLAB program that reacts when a button is pressed. This is common in graphical user interfaces where you want something to happen after a user clicks a button.

🎯 Goal: You will build a MATLAB script that defines a callback function and links it to a button press event. When the button is pressed, the callback function will display a message.

📋 What You'll Learn

Create a function called buttonCallback that displays the text 'Button pressed!'

Create a variable called btn that represents a button using uicontrol

Set the Callback property of btn to the function handle @buttonCallback

Run the script and press the button to see the message

💡 Why This Matters

🌍 Real World

Callback functions are used in MATLAB GUIs to respond to user actions like button clicks, making programs interactive.

💼 Career

Understanding callbacks is important for engineers and scientists who build interactive tools and applications using MATLAB.

1

Create the callback function

Write a function called buttonCallback that takes one input argument src and displays the text 'Button pressed!' using disp.

MATLAB

% Define the callback function
% Your code here

Hint

Use the function keyword to define buttonCallback with one input argument src. Inside, use disp to show the message.

2

Create the button

Create a variable called btn using uicontrol with the style set to 'pushbutton' and the string set to 'Click me'.

MATLAB

function buttonCallback(src)
    disp('Button pressed!')
end

% Create the button
% Your code here

Hint

Use uicontrol with properties 'Style' set to 'pushbutton' and 'String' set to 'Click me'.

3

Link the callback function to the button

Set the Callback property of btn to the function handle @buttonCallback.

MATLAB

function buttonCallback(src)
    disp('Button pressed!')
end

btn = uicontrol('Style', 'pushbutton', 'String', 'Click me');

% Set the callback property
% Your code here

Hint

Use dot notation to set btn.Callback to @buttonCallback.

4

Test the button and callback

Run the script and press the button. Write a line to display the message 'Press the button to see the callback message.' using disp.

MATLAB

function buttonCallback(src)
    disp('Button pressed!')
end

btn = uicontrol('Style', 'pushbutton', 'String', 'Click me');
btn.Callback = @buttonCallback;

% Display instruction message
% Your code here

Hint

Use disp to show the instruction message before pressing the button.

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