Property Observers

Part of the Object Oriented Programming section of Coddy's Swift journey. Lesson 13 of 57.

A property observer runs code whenever a stored property changes. didSet runs right after the new value is stored, and inside it oldValue holds the previous value:

class Player {
    var score = 0 {
        didSet {
            print("score \(oldValue) -> \(score)")
        }
    }
}

let p = Player()
p.score = 10
p.score += 5

Output:

score 0 -> 10
score 10 -> 15

willSet runs before the change, with the incoming value as newValue. Observers do not run when the property gets its initial value:

struct Thermostat {
    var target = 20 {
        willSet { print("changing to \(newValue)") }
        didSet { print("changed from \(oldValue)") }
    }
}

var t = Thermostat()
t.target = 22

Output:

changing to 22
changed from 20

didSet is a good place to keep a value valid. Assigning to the property inside its own didSet does not trigger the observer again:

struct Volume {
    var level = 5 {
        didSet {
            if level > 10 { level = 10 }
            if level < 0 { level = 0 }
        }
    }
}

var v = Volume()
v.level = 42
print(v.level)
v.level = -3
print(v.level)

Output:

10
0

Observers can also keep a history or update related state, which keeps that logic next to the property instead of scattered through the code:

class Account {
    var history: [Int] = []
    var balance = 0 {
        didSet { history.append(balance - oldValue) }
    }
}

let a = Account()
a.balance = 100
a.balance = 70
print(a.history)

Output:

[100, -30]

Challenge

Easy

Complete the class Tank. Its stored property level must stay between 0 and capacity: add a didSet that corrects values outside that range, and that appends a warning low to alerts whenever the corrected level ends below 20% of the capacity (and the previous level was not low).

The supplied code reads the capacity and then changes +n or -n. It prints the level after each change and finally alerts: with the number of warnings.

Your code goes in Tank.swift. main.swift holds the supplied input/output code and cannot be edited.

Try it yourself

// Supplied input/output code: keep it as it is
var input: [String] = []
while let line = readLine() { input.append(line) }
let tank = Tank(capacity: Int(input[0])!)
for cmd in input.dropFirst() {
    let n = Int(cmd.dropFirst())!
    tank.level += cmd.hasPrefix("+") ? n : -n
    print(tank.level)
}
print("alerts: \(tank.alerts.count)")