Fuel Savings: MPG and fill-up are sliders, not typed fields

The two Fuel Savings figures and the in-list fill-up control were text
fields. A typed figure could sit outside the slider's own bounds, so the
control and the number it claimed to show could disagree, and the decimal
pad needed a Done button to escape.

- Settings: "Average fuel economy" and "Typical fill-up" are now sliders
  with a read-out ("45 MPG", "£40" / "40 L") in place of the field. The
  warning copy reads "Set a realistic..." rather than "Enter...".
- FuelSavingsAdjustmentView (shared by the Stations Worth-the-Drive header
  and the Favourites tab) drops its text field and gains the same read-out;
  the typed-value machinery and the -focusFillAmount QA hook go with it.
- Slider bounds live in FuelSavingsCalculator (fillAmountSliderRange/Step,
  mpgSliderRange/Step) so the Settings control and the in-list control can
  never offer different ranges, and the fill-up slider range stays inside
  the calculator's validity ceiling (asserted by test).
- Two tests pin those invariants: every slider position is a valid amount,
  and both MPG bounds are valid MPG values. 155 tests, 0 failures.
- Comments that described the removed fields were updated; the keyboard
  Done-bar plumbing is kept (no numeric field remains in the app, so it can
  no longer fire) rather than deleted, so any field added later still has an
  escape route.

Verified on the simulator: Settings shows "Average fuel economy / 45 MPG"
with no field, and the Worth-the-Drive fill-up control shows the unit picker
with a "£40" read-out and a slider. The Settings "Typical fill-up" row sits
below the fold on every simulator available here (and synthetic input cannot
scroll it), so those pixels are unverified.

FuelBoard/FuelSavingsViews.swift

@@ -4,14 +4,23 @@ struct FuelSavingsAdjustmentView: View {
     @Binding var amount: FuelSavingsFillAmount
     let referencePricePencePerLitre: Double?
 
-    @State private var textValue: String = ""
-    /// Focus for the numeric field — also lets the DEBUG QA hook below raise the
-    /// keyboard on launch so the Done bar can be screenshotted (a script cannot
-    /// tap the simulator).
-    @FocusState private var isAmountFieldFocused: Bool
-
     private var range: ClosedRange<Double> {
-        amount.unit == .pounds ? 10...100 : 10...80
+        FuelSavingsCalculator.fillAmountSliderRange(for: amount.unit)
+    }
+
+    private var step: Double {
+        FuelSavingsCalculator.fillAmountSliderStep(for: amount.unit)
+    }
+
+    /// Read-out of the figure the slider is on. Deliberately NOT a text field:
+    /// a typed figure could sit outside the slider's own bounds, leaving the
+    /// control and the value it claimed to show disagreeing. Slider-only means
+    /// what you see is the figure the estimate actually uses.
+    private var valueText: String {
+        switch amount.unit {
+        case .pounds: return String(format: "£%.0f", amount.value)
+        case .litres: return String(format: "%.0f L", amount.value)
+        }
     }
 
     var body: some View {
@@ -25,33 +34,17 @@ struct FuelSavingsAdjustmentView: View {
             }
             .pickerStyle(.segmented)
             HStack {
-                Slider(value: sliderBinding, in: range, step: amount.unit == .pounds ? 5 : 1)
-                TextField(amount.unit == .pounds ? "£" : "Litres", text: valueBinding)
-                    .keyboardType(.decimalPad)
-                    .multilineTextAlignment(.trailing)
-                    .frame(width: 72)
-                    .textFieldStyle(.roundedBorder)
-                    .focused($isAmountFieldFocused)
-                    .accessibilityIdentifier("fillAmountField")
+                Slider(value: sliderBinding, in: range, step: step)
+                    .accessibilityLabel("Fill-up amount")
+                    .accessibilityValue(valueText)
+                Text(valueText)
+                    .monospacedDigit()
+                    .frame(width: 72, alignment: .trailing)
             }
             Text("Changes the estimate for this session only — your Settings default stays unchanged.")
                 .font(.caption2)
                 .foregroundStyle(.secondary)
         }
-        .onAppear {
-            syncText()
-#if DEBUG
-            // QA hook: raise the keyboard without a tap so the Done bar can be
-            // screenshotted on a simulator (synthetic taps do not reach it).
-            if ProcessInfo.processInfo.arguments.contains("-focusFillAmount") {
-                DispatchQueue.main.asyncAfter(deadline: .now() + 1.0) {
-                    isAmountFieldFocused = true
-                }
-            }
-#endif
-        }
-        .onChange(of: amount.value) { _, _ in syncText() }
-        .onChange(of: amount.unit) { _, _ in syncText() }
     }
 
     private var unitBinding: Binding<FuelSavingsFillUnit> {
@@ -65,7 +58,6 @@ struct FuelSavingsAdjustmentView: View {
                 } else {
                     amount = FuelSavingsFillAmount(value: newUnit == .pounds ? 40 : 30, unit: newUnit)
                 }
-                syncText()
             }
         )
     }
@@ -76,23 +68,6 @@ struct FuelSavingsAdjustmentView: View {
             set: { amount.value = min(max($0, range.lowerBound), range.upperBound) }
         )
     }
-
-    private var valueBinding: Binding<String> {
-        Binding(
-            get: { textValue },
-            set: { raw in
-                textValue = raw
-                if let value = Double(raw), value > 0 {
-                    amount.value = min(value, FuelSavingsCalculator.maximumFillAmount(for: amount.unit))
-                }
-            }
-        )
-    }
-
-    private func syncText() {
-        let value = amount.value
-        textValue = value.rounded() == value ? String(format: "%.0f", value) : String(format: "%.1f", value)
-    }
 }
 
 struct FuelSavingsSummaryView: View {

FuelBoard/SettingsView.swift

@@ -71,9 +71,10 @@ struct SettingsView: View {
     @State private var mediumWidgetDiag: String?
     @State private var installedWidgets: String = ""
 
-    /// Which numeric field in the Fuel Savings section is being edited. Those
-    /// fields use `.decimalPad`, which has no Return key, so the keyboard's
-    /// Done button clears this to dismiss AND commit the typed value.
+    /// Which numeric field on this screen is being edited. `.decimalPad` has no
+    /// Return key, so the keyboard's Done button clears this to dismiss AND
+    /// commit the typed value. Both Fuel Savings controls are sliders now, so
+    /// nothing sets it — it stays for any numeric field this screen takes again.
     @FocusState private var focusedSavingsField: SavingsField?
 
     private enum SavingsField: Hashable {
@@ -153,14 +154,15 @@ struct SettingsView: View {
                     HStack {
                         Text("Average fuel economy")
                         Spacer()
-                        TextField("45", value: fuelSavingsMPGBinding, format: .number.precision(.fractionLength(0...1)))
-                            .keyboardType(.decimalPad)
-                            .multilineTextAlignment(.trailing)
-                            .frame(width: 80)
-                            .focused($focusedSavingsField, equals: .averageMPG)
-                        Text("MPG")
+                        Text(mpgText)
+                            .monospacedDigit()
                             .foregroundStyle(.secondary)
                     }
+                    Slider(value: fuelSavingsMPGBinding,
+                           in: FuelSavingsCalculator.mpgSliderRange,
+                           step: FuelSavingsCalculator.mpgSliderStep)
+                        .accessibilityLabel("Average fuel economy")
+                        .accessibilityValue(mpgText)
                     Picker("Fill-up unit", selection: fuelSavingsUnitBinding) {
                         ForEach(FuelSavingsFillUnit.allCases) { unit in
                             Text(unit.displayName).tag(unit)
@@ -170,14 +172,17 @@ struct SettingsView: View {
                     HStack {
                         Text("Typical fill-up")
                         Spacer()
-                        TextField("40", value: fuelSavingsAmountBinding, format: .number.precision(.fractionLength(0...1)))
-                            .keyboardType(.decimalPad)
-                            .multilineTextAlignment(.trailing)
-                            .frame(width: 90)
-                            .focused($focusedSavingsField, equals: .fillAmount)
+                        Text(fillUpText)
+                            .monospacedDigit()
+                            .foregroundStyle(.secondary)
                     }
+                    Slider(value: fuelSavingsAmountBinding,
+                           in: FuelSavingsCalculator.fillAmountSliderRange(for: fuelSavingsSettings.preferredAmount.unit),
+                           step: FuelSavingsCalculator.fillAmountSliderStep(for: fuelSavingsSettings.preferredAmount.unit))
+                        .accessibilityLabel("Typical fill-up")
+                        .accessibilityValue(fillUpText)
                     if !fuelSavingsSettings.isConfigured && fuelSavingsSettings.enabled {
-                        Text("Enter a realistic UK MPG and fill-up amount before Worth the Drive appears in sorting.")
+                        Text("Set a realistic UK MPG and fill-up amount before Worth the Drive appears in sorting.")
                             .font(.caption)
                             .foregroundStyle(.orange)
                     }
@@ -655,6 +660,21 @@ struct SettingsView: View {
         )
     }
 
+    /// "45 MPG" / "45.5 MPG". The slider is the only control, so this read-out
+    /// is the one place the figure is stated.
+    private var mpgText: String {
+        let mpg = fuelSavingsSettings.mpg
+        return mpg.rounded() == mpg ? String(format: "%.0f MPG", mpg) : String(format: "%.1f MPG", mpg)
+    }
+
+    /// "£40" / "40 L" — the fill-up slider's read-out.
+    private var fillUpText: String {
+        switch fuelSavingsSettings.preferredAmount.unit {
+        case .pounds: return String(format: "£%.0f", fuelSavingsSettings.preferredAmount.value)
+        case .litres: return String(format: "%.0f L", fuelSavingsSettings.preferredAmount.value)
+        }
+    }
+
     private func persistFuelSavingsSettings() {
         FuelStore.saveFuelSavingsSettings(fuelSavingsSettings)
     }

FuelBoard/StationsView.swift

@@ -202,9 +202,10 @@ struct StationsView: View {
             // large title at rest, scrolls it away and fades the compact
             // inline title in. No custom toolbar needed — the native mechanism
             // only renders text, which is exactly what we want here.
-            // The Worth the Drive fill-up box uses `.decimalPad`, which has no
-            // Return key — give the keyboard a Done button and let a scroll
-            // dismiss it too.
+            // No numeric field is left on this screen — the Worth the Drive
+            // fill-up control is a slider — so no keyboard should appear here.
+            // The Done bar and interactive dismissal stay as the escape hatch
+            // for any field that returns.
             .keyboardDoneButton()
             .scrollDismissesKeyboard(.interactively)
             .navigationTitle("FuelBoard")

FuelBoardTests/Tests/FuelBoardSharedTests/FuelSavingsEstimateTests.swift

@@ -432,4 +432,35 @@ final class FuelSavingsEstimateTests: XCTestCase {
         )
         XCTAssertEqual(worse.capsuleHeadline(), "£0.31 worse off after travel")
     }
+
+    // MARK: - Slider bounds (Settings and the in-list control share these)
+
+    /// Both fill-up controls are slider-only now, so every position the slider
+    /// can reach must be a usable amount — otherwise the control could sit on a
+    /// figure the calculator rejects.
+    func testFillAmountSliderBoundsAreAlwaysValidAmounts() {
+        for unit in FuelSavingsFillUnit.allCases {
+            let range = FuelSavingsCalculator.fillAmountSliderRange(for: unit)
+            let step = FuelSavingsCalculator.fillAmountSliderStep(for: unit)
+            XCTAssertLessThan(range.lowerBound, range.upperBound, "\(unit) range must be usable")
+            XCTAssertGreaterThan(step, 0, "\(unit) step must advance")
+            XCTAssertGreaterThan(range.lowerBound, 0, "\(unit) must never offer a zero fill")
+            XCTAssertLessThanOrEqual(range.upperBound,
+                                     FuelSavingsCalculator.maximumFillAmount(for: unit),
+                                     "\(unit) slider must stay inside the valid ceiling")
+            for position in [range.lowerBound, range.upperBound, (range.lowerBound + range.upperBound) / 2] {
+                XCTAssertTrue(FuelSavingsFillAmount(value: position, unit: unit).isValid,
+                              "\(position) \(unit) must be a valid amount")
+            }
+        }
+    }
+
+    func testMPGSliderBoundsAreInsideTheValidMPGRange() {
+        let range = FuelSavingsCalculator.mpgSliderRange
+        XCTAssertLessThan(range.lowerBound, range.upperBound)
+        XCTAssertGreaterThan(FuelSavingsCalculator.mpgSliderStep, 0)
+        XCTAssertTrue(FuelSavingsCalculator.isValidMPG(range.lowerBound))
+        XCTAssertTrue(FuelSavingsCalculator.isValidMPG(range.upperBound))
+        XCTAssertTrue(FuelSavingsCalculator.isValidMPG(45), "a typical UK figure must be reachable")
+    }
 }

Shared/FuelSavings.swift

@@ -224,6 +224,30 @@ enum FuelSavingsCalculator {
         }
     }
 
+    /// Bounds for every fill-up SLIDER (Settings and the in-list adjustment
+    /// share them, so the two controls can never offer different ranges). The
+    /// validity ceiling above is deliberately wider, so a stored outlier is
+    /// displayed rather than destroyed by the control that shows it.
+    static func fillAmountSliderRange(for unit: FuelSavingsFillUnit) -> ClosedRange<Double> {
+        switch unit {
+        case .pounds: return 10...100
+        case .litres: return 10...80
+        }
+    }
+
+    static func fillAmountSliderStep(for unit: FuelSavingsFillUnit) -> Double {
+        switch unit {
+        case .pounds: return 5
+        case .litres: return 1
+        }
+    }
+
+    /// Bounds for the average-MPG slider. The validity bounds (5...200) are
+    /// wide enough for any stored figure but far too wide to fine-tune by
+    /// touch; every real-world UK car sits inside this range.
+    static let mpgSliderRange: ClosedRange<Double> = 10...100
+    static let mpgSliderStep: Double = 1
+
     static func isValidMPG(_ mpg: Double) -> Bool {
         mpg.isFinite && mpg >= minimumMPG && mpg <= maximumMPG
     }