Capture lists in closures in Swift - Step-by-Step Execution

Concept Flow - Capture lists in closures

Define variable x = 10

↓

Create closure with capture list [x

↓

Closure captures current x value

↓

Change x to 20

↓

Call closure

↓

Closure uses captured x (10), not current x (20)

↓

End

This flow shows how a closure captures a variable's value at creation using a capture list, preserving it even if the variable changes later.

Execution Sample

Swift

var x = 10
let closure = { [x] in
  print(x)
}
x = 20
closure()

This code creates a closure that captures x's value at 10, then changes x to 20, but the closure prints the captured 10.

Execution Table

Step	Action	Variable x	Closure Capture	Output
1	Initialize x	10	N/A	N/A
2	Create closure with capture list [x]	10	Captures x=10	N/A
3	Change x to 20	20	Captured x=10 (unchanged)	N/A
4	Call closure	20	Captured x=10	10
5	End	20	Captured x=10	N/A

💡 Closure uses captured x=10, ignoring current x=20

Variable Tracker

Variable	Start	After Step 2	After Step 3	Final
x	10	10	20	20
closure capture x	N/A	10	10	10

Key Moments - 2 Insights

Why does the closure print 10 instead of 20 after x changes?

Because the closure captured x's value at creation (step 2), it keeps that value (10) even though x later changes to 20 (step 3). See execution_table rows 2 and 4.

What if we omit the capture list [x] in the closure?

Without a capture list, the closure captures x by reference, so it would print the current value of x (20) when called. This differs from the capture list behavior shown in step 2.

Visual Quiz - 3 Questions

Look at the execution table, what value does the closure capture at step 2?

A 10

B 20

C N/A

D Closure does not capture x

💡 Hint

Check the 'Closure Capture' column at step 2 in the execution_table.

At which step does variable x change to 20?

A Step 1

B Step 3

C Step 2

D Step 4

💡 Hint

Look at the 'Variable x' column in execution_table to find when x changes.

If we remove the capture list [x], what will the closure print when called?

A Error

B 10

C 20

D Nothing

💡 Hint

Recall that without capture list, closure captures variable by reference, so it uses current x value at call time.

Concept Snapshot

Capture lists in closures:
- Syntax: { [var] in ... }
- Captures variable's value at closure creation
- Later changes to variable do not affect captured value
- Useful to fix variable state inside closure
- Without capture list, closure captures by reference

Full Transcript

This example shows how Swift closures can capture variables using capture lists. We start with variable x = 10. When we create a closure with [x], it captures the current value 10. Then we change x to 20. When we call the closure, it prints 10, the captured value, not 20. This behavior helps keep a fixed value inside the closure even if the original variable changes later. Without the capture list, the closure would print the current value of x when called.