Object-oriented programming: turn a script into objects
Keep each piece of data with the rules that use it, so features stop copying code.
Watch the narrated video
Every step
Problem
Steps 1–3Refactor
Steps 4–8Payoff
Steps 9–10Recap
Steps 11–12The code: before and after
Before
let names = ["Ana", "Ben", "Cara"]
let plans = ["basic", "pro", "pro"]
let months = [3, 12, 2]
for i in names.indices {
let price = plans[i] == "pro" ? 30 : 10
let total = price * months[i]
print("\(names[i]) owes \(total)")
}After
struct Subscription {
let name: String
let plan: String
let months: Int
func monthlyPrice() -> Int {
plan == "pro" ? 30 : 10
}
func total() -> Int {
monthlyPrice() * months
}
}
let subs = [
Subscription(name: "Ana", plan: "basic", months: 3),
Subscription(name: "Ben", plan: "pro", months: 12),
Subscription(name: "Cara", plan: "pro", months: 2),
]
for s in subs {
print("\(s.name) owes \(s.total())")
}
for s in subs where s.total() > 100 {
print("Reminder: \(s.name)")
}Transcript, step by step
Problem
Step 1: A gym bills its members Before
This gym script keeps its members in three arrays, one for names, one for plans and one for months. The loop walks them by index, picks a monthly price from the plan, and multiplies it by the months. Then it prints each bill, so Ana owes 30, Ben owes 360 and Cara owes 60.
Step 2: Add a reminder Code smell
The front desk now wants a reminder for every member who owes more than 100. The quick way is a second loop that copies the price and total lines, then prints a reminder for the big bills.
Step 3: Parallel arrays, a rule in a loop Code smell
In a script, the data sits in parallel arrays that belong together only by position, and the rule that turns them into a bill lives inside one loop, so nothing else can use it. You can see the price rule here and copied here, the total here and copied here, and one member spread across three arrays here. When the pro plan goes up to 35, two lines must change, and the one you miss keeps the old 30. So Ben's bill would say 420, while his reminder still works from 360. And every new detail about a member, like a start date, is a fourth array that must stay in the same order as the other three.
Refactor
Step 4: Refactor first Back it out
Let's take the second loop out again. We'll refactor first, so that adding the reminder becomes a small change, and then we'll add it.
Step 5: One member, one object Replace Array with Object
First, a struct called Subscription holds the three things we know about a member, its name, its plan and its months. Swift writes the initializer for a struct like this, so each field is named once and every call spells out its labels. The three arrays become one array of three Subscription objects. The loop now goes over that array with no index, and it reads s dot plan and s dot months. One member is now one object, so its name, plan and months can't drift apart.
Step 6: Name the bill rule Extract Function
The price and total lines move into a top-level function called total, which takes one subscription. The loop prints total of s, and every bill comes out the same as before.
Step 7: Move total to its data Move Method
Total only ever read the subscription's own plan and months, so it moves into Subscription as a method. It loses its parameter, because the fields it needs belong to the object it's called on. The loop now asks s dot total. Now the data and the rule that uses it live in one place, and that is what an object is.
Step 8: Give the price a name Extract Method
The price rule gets its own method, monthly price, which returns 30 for pro and 10 for any other plan. Total becomes monthly price times months, so the rule now has a name you can read and call. Both bodies are one expression, so Swift returns their value without the return keyword.
Payoff
Step 9: Add the reminder again Payoff
Now the reminder goes in as a second loop, and its where clause keeps only the subscriptions whose total is over 100. Subscription didn't change, and neither did the bill loop. Ben owes 360, so he's the one member who gets a reminder.
Step 10: One home for each rule Payoff
The price rule is in one place now, the total is in one place, and each member is one object. A pro price of 35 is one edit in monthly price, and both the bill and the reminder pick it up. A new member detail, like a start date, is one new field on one struct instead of a fourth array kept in order. Each member's name, plan and months now sit in one object with the rules that read them, and that is object-oriented programming in this code.
Recap
Step 11: Where we started, where we ended Before / after
On the left, three arrays hold each member by position, and the loop works out the price and the total inline. On the right, one Subscription object holds a member, and it owns the monthly price and total rules. The bill loop asks each subscription for its total, and it never sees the price rule.
Step 12: When to reach for it Takeaways
Reach for objects when a few values always travel together and several parts of the code compute from them. Skip them for a one-off script that runs once, because when nothing else uses its rules, a few lines top to bottom do the job. You'll spot the need by parallel arrays matched by index, and by the same calculation copied into a second loop. Swift's String works this way, because it keeps its text together with methods like has prefix and uppercased.