Computed properties in Swift

Derived on read, observed on write.

struct Leg {
    var distanceNm: Double
    var hours: Double

    // Computed from the stored properties on every read.
    var speedKts: Double { distanceNm / hours }

    // didSet observes writes to a stored property.
    var protests = 0 {
        didSet { print("Protest count now \(protests)") }
    }
}

var beat = Leg(distanceNm: 4.2, hours: 0.75)
print("Averaging \(beat.speedKts) kts")
beat.protests = 1
beat.distanceNm = 5.0
print("Recomputed: \(beat.speedKts) kts")

How it works

1. var speedKts: Double { ... } recomputes from stored state.
2. didSet runs after every write to protests.
3. Changing distanceNm changes the next speedKts read.

Keywords and builtins used here

• Double
• Leg
• didSet
• print
• struct
• var

The run, in numbers

Lines

18

Characters to type

442

Tokens

90

Three-star pace

65 tpm

At the three-star pace of 65 tokens a minute, this run takes about 83 seconds.

Type this snippet

Step 3 of 3 in Structs & classes, step 18 of 27 in Language basics.

← Previous Next →