Add device-driven temperature unit display

Author: cfdshark

README.md

@@ -130,6 +130,49 @@ The project now separates:
 
 This keeps the background logic stable and easy to reason about, while allowing the forecast rows and current-condition UI to become much more specific without changing the rest of the app's visual architecture.
 
+## Temperature Unit Behavior
+
+The app currently fetches temperatures from OpenWeather in **metric** units and keeps those values in Celsius inside the app's domain models.
+
+Temperature conversion happens only at presentation time:
+
+- if the phone's system temperature preference resolves to Celsius, the app displays `°C`
+- if the phone's system temperature preference resolves to Fahrenheit, the app converts the stored Celsius value and displays `°F`
+
+This applies consistently to:
+
+- the current temperature in the forecast header
+- each daily temperature shown in the forecast list
+
+### Current Limitation
+
+This is intentionally limited.
+
+Right now the app does **not** offer an in-app temperature preference. It only follows the phone's current system preference for temperature units. That is acceptable for a lightweight prototype, but it is not ideal for a production weather app because users may want control that differs from their device-wide regional setting.
+
+Examples:
+
+- a user may want Fahrenheit in the weather app even if their phone is configured for a metric region
+- a user may want Celsius in the app while traveling in a locale that defaults to Fahrenheit
+
+### Production Direction
+
+For a production-ready version, the app should add a dedicated settings surface where the user can explicitly choose their preferred temperature unit.
+
+That future settings flow should support:
+
+- `Use System Setting`
+- `Celsius`
+- `Fahrenheit`
+
+In that model:
+
+- the app would still be able to respect the phone setting by default
+- the user would be able to override that default when needed
+- the chosen preference would be persisted and used consistently across the entire app
+
+The current implementation does not include that settings page or persistence layer yet. It is a presentation-only improvement that makes the displayed unit match the phone setting without introducing additional app configuration UI.
+
 ## Configuration
 
 The current testing configuration is defined in [AppConfiguration.swift](/Users/blessingmabunda/Documents/WeatherApp/WeatherApp/App/AppConfiguration.swift).
@@ -159,7 +202,7 @@ xcodebuild test -project WeatherApp.xcodeproj -scheme WeatherApp -destination 'p
   - OpenWeather condition-to-icon mapping for bundled weather assets
   - theme/background mapping
   - forecast view-model state transitions
-  - presentation formatting helpers
+  - presentation formatting helpers, including Celsius/Fahrenheit display conversion
 - Protocol-based test doubles isolate location, network, and weather flows.
 - UI verification is handled through SwiftUI previews plus view-model coverage rather than snapshot tooling.
 

WeatherApp/Features/Forecast/Views/ForecastHeaderView.swift

@@ -20,7 +20,11 @@ struct ForecastHeaderView: View {
                 HStack(alignment: .center, spacing: 16) {
                     headerIcon
 
-                    Text("\(snapshot.currentTemperatureCelsius)°")
+                    Text(
+                        ForecastPresentationFormatter.temperatureString(
+                            celsius: snapshot.currentTemperatureCelsius
+                        )
+                    )
                         .font(.system(size: 52, weight: .bold, design: .rounded))
                         .foregroundStyle(.white)
                 }

WeatherApp/Shared/ForecastPresentationFormatter.swift

@@ -13,7 +13,11 @@ enum ForecastPresentationFormatter {
         return formatter.string(from: date)
     }
 
-    static func temperatureString(celsius: Int) -> String {
-        "\(celsius)°"
+    static func temperatureString(
+        celsius: Int,
+        unitPreference: TemperatureUnitPreference = TemperatureUnitPreference()
+    ) -> String {
+        let displayValue = unitPreference.displayValue(forCelsius: celsius)
+        return "\(displayValue)\(unitPreference.unitSuffix)"
     }
 }

WeatherApp/Shared/TemperatureUnitPreference.swift

@@ -0,0 +1,36 @@
+import Foundation
+
+enum TemperatureUnitPreference: Equatable {
+    case celsius
+    case fahrenheit
+
+    init(locale: Locale = .autoupdatingCurrent) {
+        let preferredUnit = UnitTemperature(forLocale: locale)
+
+        switch preferredUnit {
+        case UnitTemperature.fahrenheit:
+            self = .fahrenheit
+        default:
+            self = .celsius
+        }
+    }
+
+    func displayValue(forCelsius celsius: Int) -> Int {
+        switch self {
+        case .celsius:
+            return celsius
+        case .fahrenheit:
+            let measurement = Measurement(value: Double(celsius), unit: UnitTemperature.celsius)
+            return Int(measurement.converted(to: .fahrenheit).value.rounded())
+        }
+    }
+
+    var unitSuffix: String {
+        switch self {
+        case .celsius:
+            return "°C"
+        case .fahrenheit:
+            return "°F"
+        }
+    }
+}

WeatherAppTests/ForecastPresentationFormatterTests.swift

@@ -2,8 +2,34 @@ import XCTest
 @testable import WeatherApp
 
 final class ForecastPresentationFormatterTests: XCTestCase {
-    func testTemperatureStringUsesDegreeSuffix() {
-        XCTAssertEqual(ForecastPresentationFormatter.temperatureString(celsius: 24), "24°")
+    func testTemperatureStringUsesCelsiusWhenPreferred() {
+        XCTAssertEqual(
+            ForecastPresentationFormatter.temperatureString(
+                celsius: 24,
+                unitPreference: .celsius
+            ),
+            "24°C"
+        )
+    }
+
+    func testTemperatureStringConvertsToFahrenheitWhenPreferred() {
+        XCTAssertEqual(
+            ForecastPresentationFormatter.temperatureString(
+                celsius: 24,
+                unitPreference: .fahrenheit
+            ),
+            "75°F"
+        )
+    }
+
+    func testTemperatureStringConvertsNegativeValuesToFahrenheit() {
+        XCTAssertEqual(
+            ForecastPresentationFormatter.temperatureString(
+                celsius: -10,
+                unitPreference: .fahrenheit
+            ),
+            "14°F"
+        )
     }
 
     func testWeekdayStringReturnsWeekdayName() {

WeatherAppTests/TemperatureUnitPreferenceTests.swift

@@ -0,0 +1,26 @@
+import XCTest
+@testable import WeatherApp
+
+final class TemperatureUnitPreferenceTests: XCTestCase {
+    func testInitUsesFahrenheitLocalePreference() {
+        XCTAssertEqual(
+            TemperatureUnitPreference(locale: Locale(identifier: "en_US")),
+            .fahrenheit
+        )
+    }
+
+    func testInitUsesCelsiusLocalePreference() {
+        XCTAssertEqual(
+            TemperatureUnitPreference(locale: Locale(identifier: "en_ZA")),
+            .celsius
+        )
+    }
+
+    func testDisplayValueLeavesCelsiusUntouched() {
+        XCTAssertEqual(TemperatureUnitPreference.celsius.displayValue(forCelsius: 18), 18)
+    }
+
+    func testDisplayValueConvertsToRoundedFahrenheit() {
+        XCTAssertEqual(TemperatureUnitPreference.fahrenheit.displayValue(forCelsius: 18), 64)
+    }
+}