Adding computed properties in Swift - Step-by-Step Execution

Concept Flow - Adding computed properties

Define struct/class

↓

Add stored properties

↓

Add computed property with getter

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Access computed property

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Return computed value based on stored properties

This flow shows how to add a computed property that calculates a value from stored properties when accessed.

Execution Sample

Swift

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

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

Execution Table

Step	Action	Stored Properties	Computed Property Access	Computed Property Result
1	Create Rectangle with width=5, height=10	width=5, height=10	N/A	N/A
2	Access area property	width=5, height=10	Calculate area = width * height	50
3	Change width to 7	width=7, height=10	N/A	N/A
4	Access area property again	width=7, height=10	Calculate area = width * height	70
5	End of execution	width=7, height=10	N/A	N/A

💡 Execution ends after accessing computed property twice and changing stored property once.

Variable Tracker

Variable	Start	After Step 1	After Step 3	Final
width	undefined	5	7	7
height	undefined	10	10	10
area (computed)	undefined	N/A	N/A	70

Key Moments - 2 Insights

Why does the computed property 'area' change when we change 'width'?

Because 'area' calculates its value each time it is accessed using the current 'width' and 'height' values, as shown in execution_table rows 2 and 4.

Is 'area' stored in memory like 'width' and 'height'?

'area' is not stored; it is computed on demand when accessed, which is why it updates automatically without storing a separate value (see execution_table steps 2 and 4).

Visual Quiz - 3 Questions

Look at the execution table, what is the value of 'area' after step 2?

A 15

B 7

C 50

D 10

💡 Hint

Check the 'Computed Property Result' column at step 2 in the execution_table.

At which step does the 'width' change?

A Step 2

B Step 3

C Step 4

D Step 5

💡 Hint

Look at the 'Stored Properties' column to see when 'width' changes from 5 to 7.

If we changed 'height' to 20 at step 3 instead of 'width', what would 'area' be at step 4?

A 100

B 50

C 70

D 200

💡 Hint

Recall area = width * height; if width=5 and height=20, area = 5*20 = 100.

Concept Snapshot

Adding computed properties in Swift:
- Use var with a code block { get { ... } } or shorthand { ... }
- Computed properties calculate values on access, no storage
- They can use other stored properties to compute results
- Changing stored properties affects computed property output
- Useful for dynamic values like area, fullName, etc.

Full Transcript

This example shows how to add a computed property in Swift. We define a Rectangle struct with stored properties width and height. Then we add a computed property area that returns width times height. When we create a Rectangle with width 5 and height 10, accessing area returns 50. If we change width to 7, accessing area again returns 70. The computed property recalculates its value each time based on current stored properties. This means area is not stored but computed on demand. This helps keep values up to date without extra storage.