Property observers (willSet, didSet) in Swift - Step-by-Step Execution

Concept Flow - Property observers (willSet, didSet)

Property value about to change

↓

willSet called

↓

Property value changes

↓

didSet called

↓

Continue execution

When a property value changes, willSet runs before the change, and didSet runs after the change.

Execution Sample

Swift

var score: Int = 0 {
    willSet {
        print("About to set score to \(newValue)")
    }
    didSet {
        print("Score changed from \(oldValue) to \(score)")
    }
}
score = 10

This code shows how willSet and didSet run when the 'score' property changes from 0 to 10.

Execution Table

Step	Property 'score' Value	Event	willSet newValue	didSet oldValue	Output
1	0	Initial state	-	-	-
2	0 -> 10	Assign score = 10	10	-	About to set score to 10
3	10	After assignment	-	0	Score changed from 0 to 10

💡 Property 'score' changed from 0 to 10, observers finished.

Variable Tracker

Variable	Start	After willSet	After didSet	Final
score	0	0 (before change)	10 (after change)	10

Key Moments - 2 Insights

Why does willSet see the new value but didSet sees the old value?

willSet runs before the property changes, so it receives the newValue about to be set (see execution_table step 2). didSet runs after the change, so it sees the oldValue before the change (see execution_table step 3).

Can willSet or didSet change the property value inside their blocks?

No, changing the property inside willSet or didSet can cause unexpected behavior or infinite loops. They are meant only to observe changes, not modify them.

Visual Quiz - 3 Questions

Look at the execution table, what is the value of 'score' during willSet?

A 0 (old value)

B 10 (new value)

C Undefined

D It depends

💡 Hint

Check the 'Property 'score' Value' and 'willSet newValue' columns at step 2.

At which step does didSet run?

A Step 3

B Step 2

C Step 1

D After step 3

💡 Hint

Look at the 'Event' and 'Output' columns in the execution_table.

If we assign score = 20 next, what will willSet print?

A "Score changed from 10 to 20"

B "About to set score to 10"

C "About to set score to 20"

D Nothing

💡 Hint

willSet prints the new value about to be assigned (see execution_table step 2).

Concept Snapshot

Property observers let you watch changes to a property.
willSet runs just before the value changes, receiving newValue.
didSet runs just after the value changes, receiving oldValue.
Use them to react to changes without changing the value inside.
Syntax: var prop: Type { willSet { } didSet { } }

Full Transcript

Property observers in Swift let you run code before and after a property changes. The willSet observer runs right before the property value changes and receives the new value that will be set. The didSet observer runs right after the property changes and receives the old value before the change. In the example, when 'score' changes from 0 to 10, willSet prints the new value 10 before the change, and didSet prints the old value 0 after the change. This helps you track or respond to changes safely. You should not change the property inside these observers to avoid unexpected behavior.