Closures are reference types in Swift - Step-by-Step Execution

Concept Flow - Closures are reference types

Create closure instance

↓

Assign closure to variable A

↓

Assign variable A to variable B

↓

Modify closure via B

↓

Observe change via A

↓

Both A and B refer to same closure instance

This flow shows how closures are created and assigned to variables, and how multiple variables can reference the same closure instance, demonstrating closures are reference types.

Execution Sample

Swift

var closureA = { () -> Int in
    var count = 0
    return { count += 1; return count }
}()

var closureB = closureA
closureB()
closureA()

This code creates a closure that increments a count, assigns it to two variables, and calls it via both to show shared state.

Execution Table

Step	Action	Variable States	Output	Notes
1	Create closureA with internal count=0	closureA: closure with count=0	-	Closure instance created with count=0
2	Assign closureA to closureB	closureA: closure with count=0, closureB: same closure	-	closureB references same closure instance as closureA
3	Call closureB()	count increments to 1	1	Closure called via closureB, count becomes 1
4	Call closureA()	count increments to 2	2	Closure called via closureA, count becomes 2
5	End	closureA and closureB share count=2	-	Both variables reference same closure instance

💡 Execution ends after demonstrating shared state through closure references

Variable Tracker

Variable	Start	After Step 3	After Step 4	Final
closureA.count	0	1	2	2
closureB.count	0	1	2	2

Key Moments - 2 Insights

Why does calling closureA() after closureB() show the incremented count?

Because closureA and closureB reference the same closure instance, changes via closureB affect closureA as shown in steps 3 and 4 of the execution_table.

Is a new closure created when assigning closureA to closureB?

No, closureB points to the same closure instance as closureA, so no new closure is created (see step 2 in execution_table).

Visual Quiz - 3 Questions

Look at the execution_table at step 3, what is the value of count inside the closure after calling closureB()?

A 2

B 0

C 1

D Undefined

💡 Hint

Check the 'Variable States' column at step 3 in execution_table.

At which step does closureA and closureB share the same updated count value?

A Step 4

B Step 2

C Step 1

D Step 5

💡 Hint

Look at the 'Variable States' and 'Notes' columns at step 4 in execution_table.

If closureB was assigned a new closure instead of closureA, how would the variable_tracker change?

A closureA.count and closureB.count would increment together

B closureB.count would stay at 0 after calls

C closureA.count would reset to 0 after step 3

D closureB.count would be undefined

💡 Hint

Consider that separate closures have independent internal states, unlike shared references shown in variable_tracker.

Concept Snapshot

Closures in Swift are reference types.
Assigning a closure to another variable copies the reference, not the closure itself.
Calling the closure via any reference affects the same internal state.
This means closures share captured variables when referenced.
Use this to maintain shared state across calls.

Full Transcript

This visual execution trace shows how closures in Swift behave as reference types. We start by creating a closure that captures a variable count initialized to zero. This closure is assigned to closureA. Then closureA is assigned to closureB, so both variables point to the same closure instance. When closureB is called, it increments count to 1. Calling closureA next increments count to 2, showing that both variables share the same closure and internal state. The variable tracker confirms count changes are shared. Key moments clarify that no new closure is created on assignment and that both variables reference the same closure instance. The quiz tests understanding of closure state sharing and reference behavior. This demonstrates closures keep shared state because they are reference types in Swift.