Callback functions in MATLAB - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a callback function in MATLAB?

A callback function in MATLAB is a function that is executed in response to an event, such as a button click or a timer finishing. It is called automatically when the event occurs.

beginner

How do you assign a callback function to a button in MATLAB GUI?

You assign a callback function to a button by setting the button's 'Callback' property to a function handle, for example: btn.Callback = @myCallbackFunction;

intermediate

What is the typical input signature of a MATLAB callback function?

A typical MATLAB callback function has two inputs: <code>src</code> (the source object that triggered the callback) and <code>event</code> (event data), for example: <code>function myCallback(src, event)</code>.

beginner

Why are callback functions useful in MATLAB programming?

Callback functions allow MATLAB programs to respond to user actions or events asynchronously, enabling interactive and dynamic behavior in GUIs and other event-driven applications.

intermediate

Can callback functions in MATLAB access variables from the main workspace directly?

No, callback functions do not have direct access to variables in the main workspace. To share data, you can use nested functions, guidata, or app properties.

What does a callback function in MATLAB do?

A Runs only when the program starts

B Runs only when manually called from the command window

C Runs continuously in a loop

D Runs automatically when a specific event happens

A callback function runs automatically when the event it is linked to occurs, such as a button press.

How do you specify a callback function for a UI button in MATLAB?

A Set the button's 'Callback' property to a function handle

B Write the function name as a string in the command window

C Use the 'run' command inside the button code

D Assign the function to a global variable

You assign a callback by setting the button's 'Callback' property to a function handle, like @myFunction.

What are the usual inputs to a MATLAB callback function?

A No inputs

B Only the event data

C Source object and event data

D Only the source object

Callback functions typically receive the source object and event data as inputs.

Which of these is NOT a way to share data with a callback function in MATLAB?

A Using guidata

B Directly accessing main workspace variables

C Using nested functions

D Using app properties

Callback functions cannot directly access main workspace variables; other methods like nested functions or guidata are used.

Why are callback functions important in MATLAB GUIs?

A They allow the program to respond to user actions

B They speed up the program execution

C They prevent errors automatically

D They store data permanently

Callback functions let the program react to user actions like clicks, making GUIs interactive.

Explain what a callback function is and how it works in MATLAB.

Think about how MATLAB responds when you click a button.

Describe how you can share data between the main program and a callback function in MATLAB.

Direct access to main workspace variables is not possible.

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