How to Code in Swift Using AI

In short: you need a Mac and Xcode. Use GitHub Copilot for Xcode and/or ChatGPT in Xcode Intelligence. Give the formula, one example answer, ask for XCTest, and run the tests yourself.

Why Swift with AI

Almost every new iPhone app is written in Swift. UIKit and SwiftUI need a lot of repeated layout and state code. AI speeds that up. Money and math still need tests.

What you need (honest limits)

• A Mac: Xcode does not run on Windows or Linux. Without a Mac, practice Swift syntax in a cloud IDE, but you cannot build a real iOS app for the App Store.
• Xcode: from the Mac App Store. Keep it updated.
• GitHub Copilot for Xcode: a separate Mac app + Xcode Source Editor extension (Homebrew: brew install --cask github-copilot-for-xcode). Completions, chat and agent mode.
• ChatGPT in Xcode: Xcode → Settings → Intelligence → turn on ChatGPT. Uses your ChatGPT account.
• Apple's own coding intelligence: also in Xcode Intelligence settings (where your Xcode version offers it). Features change with Xcode releases — check Settings → Intelligence on your Mac.

How it flows

Install Copilot for Xcode

1. On your Mac: brew install --cask github-copilot-for-xcode (or download the app from GitHub's Copilot for Xcode releases).
2. Open the Copilot for Xcode app. Allow Accessibility and enable the Xcode Source Editor extension in System Settings → Extensions.
3. Sign in with your GitHub account (device code flow).
4. Open Xcode. Confirm Editor → GitHub Copilot appears.
5. Optional: Xcode → Settings → Intelligence → turn on ChatGPT and sign in if you use that path too.

Step 1 — the prompt

Step 2 — the code

import Foundation

/// Monthly EMI in whole rupees. Zero interest → principal / months.
func emi(principal: Double, annualRatePercent: Double, months: Int) -> Int {
    precondition(principal > 0, "principal must be positive")
    precondition(months > 0, "months must be positive")
    precondition(annualRatePercent >= 0, "rate cannot be negative")

    if annualRatePercent == 0 {
        return Int((principal / Double(months)).rounded())
    }
    let r = annualRatePercent / 12.0 / 100.0
    let factor = pow(1.0 + r, Double(months))
    let value = principal * r * factor / (factor - 1.0)
    return Int(value.rounded())
}

#if DEBUG
assert(emi(principal: 500_000, annualRatePercent: 9.0, months: 60) == 10_379)
assert(emi(principal: 120_000, annualRatePercent: 0.0, months: 12) == 10_000)
#endif

Step 3 — XCTest

import XCTest

// Put Emi.swift in the same app/test target, or import your module name.
final class EmiTests: XCTestCase {
    func testNinePercentSixtyMonths() {
        XCTAssertEqual(emi(principal: 500_000, annualRatePercent: 9.0, months: 60), 10_379)
    }
    func testZeroInterest() {
        XCTAssertEqual(emi(principal: 120_000, annualRatePercent: 0.0, months: 12), 10_000)
    }
    func testPositivePrincipalRequired() {
        // In a Debug build, a negative principal trips precondition and fails the test —
        // that is what we want. Prefer a throwing API in production code.
    }
}

# In Xcode: Product → Test (Cmd+U)
# Expect: testNinePercentSixtyMonths pass
# Expect: testZeroInterest pass

Settings and config

1. Project rules: keep a short note in the repo (“Swift 5/6, SwiftUI, no force unwraps, XCTest for logic”).
2. Prefer guard and if let over ! force unwraps when you accept AI code.
3. For UI, say “SwiftUI” or “UIKit” in the prompt so the AI does not mix them.

Watch out (common AI mistakes in Swift)

Quick quiz

Save tokens and money

Give the formula in the prompt so the model does not guess. Ask for one function + tests. For UI work, send one screen at a time, not the whole storyboard dump.

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