Closures as function parameters in Swift - Step-by-Step Execution

Concept Flow - Closures as function parameters

Define function with closure parameter

↓

Call function, pass closure

↓

Function executes closure

↓

Closure runs, returns result

↓

Function uses closure result

↓

End

This flow shows how a function takes a closure as a parameter, calls it, and uses its result.

Execution Sample

Swift

func greetUser(action: () -> String) {
    let message = action()
    print(message)
}

greetUser { "Hello, Swift!" }

This code defines a function that takes a closure returning a string, calls it, and prints the message.

Execution Table

Step	Action	Evaluation	Result
1	Define function greetUser with closure parameter	No execution yet	Function ready to be called
2	Call greetUser with closure { "Hello, Swift!" }	Closure passed as argument	Function starts execution
3	Inside greetUser, call closure action()	Closure runs	"Hello, Swift!"
4	Assign closure result to message	message = "Hello, Swift!"	message holds "Hello, Swift!"
5	Print message	print("Hello, Swift!")	Output: Hello, Swift!
6	Function greetUser ends	No more code	Execution complete

💡 Function ends after printing the closure's returned string.

Variable Tracker

Variable	Start	After Step 3	After Step 4	Final
message	undefined	"Hello, Swift!"	"Hello, Swift!"	"Hello, Swift!"

Key Moments - 3 Insights

Why do we call action() inside the function?

Because action is a closure passed in, calling action() runs the closure and gets its returned value, as shown in step 3 of the execution_table.

What type of value does the closure return?

The closure returns a String, which is assigned to the variable message in step 4 of the execution_table.

Can the closure be written outside the function call?

Yes, but here it is passed inline for simplicity, as shown in step 2 of the execution_table.

Visual Quiz - 3 Questions

Look at the execution_table, what is the value of message after step 4?

A "Hello, Swift!"

B undefined

C Closure itself

D nil

💡 Hint

Check the variable_tracker row for message after step 4.

At which step does the closure actually run?

A Step 4

B Step 2

C Step 3

D Step 5

💡 Hint

Look at the execution_table action and evaluation columns for when action() is called.

If the closure returned "Hi!" instead, what would be printed?

A "Hello, Swift!"

B "Hi!"

C Nothing

D Error

💡 Hint

The printed output depends on the closure's return value shown in step 5.

Concept Snapshot

func functionName(parameter: () -> ReturnType) {
  let result = parameter()
  // use result
}

Call with closure:
functionName { returnValue }

The closure runs inside the function when called as parameter().

Full Transcript

This example shows how a Swift function can take a closure as a parameter. The function greetUser takes a closure named action that returns a String. When greetUser is called, it runs the closure by calling action(), stores the returned string in message, and prints it. The closure is passed inline as { "Hello, Swift!" }. The execution steps show defining the function, calling it with the closure, running the closure, assigning its result, printing, and ending. The variable message starts undefined and holds the closure's returned string after step 3. Key points include understanding that calling action() runs the closure and returns its value, which the function uses. The quiz checks understanding of when the closure runs, what message holds, and how changing the closure affects output.