Computed properties in Swift - Step-by-Step Execution

Concept Flow - Computed properties

Start

↓

Access computed property

↓

Run getter code

↓

Return computed value

↓

Use value

↓

Optional: Set computed property

↓

Run setter code

↓

Update internal state

↓

End

When you access a computed property, Swift runs its getter code to calculate and return a value. If you set it, the setter code runs to update internal data.

Execution Sample

Swift

struct Rectangle {
  var width: Double
  var height: Double
  var area: Double {
    return width * height
  }
}

Defines a Rectangle with width and height, and a computed property area that calculates width times height.

Execution Table

Step	Action	Code Executed	Result/Value
1	Create Rectangle instance	let rect = Rectangle(width: 3, height: 4)	rect.width = 3, rect.height = 4
2	Access computed property 'area'	rect.area	Runs getter: return 3 * 4
3	Getter returns value	return width * height	12
4	Use computed value	print(rect.area)	12
5	End	-	-

💡 No setter used; execution ends after getter returns computed value.

Variable Tracker

Variable	Start	After Step 1	After Step 2	Final
rect.width	-	3	3	3
rect.height	-	4	4	4
rect.area (computed)	-	-	-	12 (computed on access)

Key Moments - 2 Insights

Why doesn't the computed property 'area' store a value?

Because 'area' runs code every time you access it (see execution_table step 2 and 3), it calculates the value on the fly instead of storing it.

What happens if you try to set 'area' in this example?

This 'area' has no setter defined, so trying to set it will cause a compile error. The execution_table shows only getter usage.

Visual Quiz - 3 Questions

Look at the execution_table, what value does 'rect.area' return at step 3?

A 0

B 7

C 12

D Error

💡 Hint

Check the 'Result/Value' column at step 3 in execution_table.

At which step is the computed property 'area' accessed?

A Step 1

B Step 2

C Step 4

D Step 5

💡 Hint

Look for 'Access computed property' in the 'Action' column of execution_table.

If you add a setter to 'area', what would change in the execution_table?

A A new step for setting 'area' would appear

B The getter would stop running

C The rectangle's width and height would become constants

D No change at all

💡 Hint

Think about what happens when you set a computed property with a setter (see concept_flow).

Concept Snapshot

Computed properties in Swift look like variables but run code when accessed.
Use 'var name: Type { get { ... } set { ... } }' syntax.
Getter runs on access to calculate value.
Setter runs on assignment to update data.
They do not store values directly.
Useful for dynamic values based on other properties.

Full Transcript

This example shows a Swift struct Rectangle with width and height stored properties and a computed property area. When you create a Rectangle instance with width 3 and height 4, those values are stored. When you access rect.area, Swift runs the getter code which multiplies width and height and returns 12. This value is not stored but calculated each time. The execution table traces these steps clearly. If you try to set area, it would cause an error because no setter is defined. Computed properties let you create dynamic values that update automatically based on other data.