Callback functions in MATLAB - Step-by-Step Execution

Concept Flow - Callback functions

Define callback function

↓

Assign callback to event

↓

Wait for event trigger

↓

Event occurs

↓

MATLAB calls callback

↓

Callback executes code

↓

Return control to main program

This flow shows how MATLAB waits for an event, then calls the assigned callback function to run specific code.

Execution Sample

MATLAB

btn = uibutton('Text','Click me');
btn.ButtonPushedFcn = @buttonCallback;

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

Creates a button and assigns a callback that displays a message when clicked.

Execution Table

Step	Action	Evaluation	Result
1	Define function buttonCallback	Function created	Ready to use
2	Create button btn	Button object created	Button visible with text 'Click me'
3	Assign btn.ButtonPushedFcn = @buttonCallback	Callback linked	Button now listens for clicks
4	User clicks button	Event triggered	MATLAB calls buttonCallback
5	buttonCallback runs	disp('Button pressed!')	Output: Button pressed!
6	Callback ends	Return to waiting state	Program waits for next event

💡 Execution stops waiting for user events; callback runs only on event trigger.

Variable Tracker

Variable	Start	After Step 2	After Step 3	After Step 5	Final
buttonCallback	undefined	defined	defined	defined	defined
btn	undefined	uibutton object	uibutton object with callback	uibutton object with callback	uibutton object with callback
event	undefined	undefined	undefined	event data passed to callback	undefined

Key Moments - 2 Insights

Why doesn't the callback run immediately after assignment?

The callback runs only when the event (button click) happens, as shown in execution_table step 4. Assigning the callback just links it to the event.

What are the inputs src and event in the callback function?

They provide information about the source object and event details when the callback runs, as seen in step 5 where buttonCallback executes.

Visual Quiz - 3 Questions

Look at the execution_table, what happens at step 4?

A The user clicks the button triggering the event

B The callback function is assigned

C The button is created

D The callback function finishes running

💡 Hint

Check the 'Action' and 'Evaluation' columns at step 4 in execution_table.

According to variable_tracker, what is the state of btn after step 3?

A Undefined

B uibutton object without callback

C uibutton object with callback assigned

D Callback function itself

💡 Hint

Look at the 'btn' row and 'After Step 3' column in variable_tracker.

If the user never clicks the button, what happens to the callback function?

A It runs once automatically

B It never runs

C It runs repeatedly in a loop

D It runs when the button is created

💡 Hint

Refer to the exit_note and step 4 in execution_table about when callbacks run.

Concept Snapshot

Callback functions in MATLAB:
- Define a function to run on an event
- Assign it to an object's event property (e.g., ButtonPushedFcn)
- MATLAB waits for the event
- When event occurs, MATLAB calls the callback
- Callback runs with source and event info
- Control returns after callback finishes

Full Transcript

In MATLAB, a callback function is a special function that runs when an event happens, like clicking a button. First, you define the callback function. Then, you create a UI object such as a button. You assign the callback function to the button's event property. MATLAB waits for the user to trigger the event. When the user clicks the button, MATLAB calls the callback function, which runs the code inside it. After the callback finishes, MATLAB waits for the next event. Variables like the button object and event data change during this process. The callback does not run immediately after assignment; it runs only when the event occurs.

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