Fix enabling feature flags on iOS 15

Tusker/Screens/Preferences/AdvancedPrefsView.swift

@@ -18,7 +18,6 @@ struct AdvancedPrefsView : View {
     @State private var mastodonCacheSize: Int64 = 0
     @State private var cloudKitStatus: CKAccountStatus?
     @State private var isShowingFeatureFlagAlert = false
-    @State private var featureFlagName = ""
     
     var body: some View {
         List {
@@ -32,23 +31,9 @@ struct AdvancedPrefsView : View {
         .listStyle(.insetGrouped)
         .appGroupedListBackground(container: PreferencesNavigationController.self)
         .onTapGesture(count: 3) {
-            featureFlagName = ""
             isShowingFeatureFlagAlert = true
         }
-        .alert("Enable Feature Flag", isPresented: $isShowingFeatureFlagAlert) {
-            TextField("Flag Name", text: $featureFlagName)
-                .textInputAutocapitalization(.never)
-                .autocorrectionDisabled()
-            
-            Button("Cancel", role: .cancel) {}
-            Button("Enable") {
-                if let flag = FeatureFlag(rawValue: featureFlagName) {
-                    preferences.enabledFeatureFlags.insert(flag)
-                }
-            }
-        } message: {
-            Text("Warning: Feature flags are intended for development and debugging use only. They are experimental and subject to change at any time.")
-        }
+        .modifier(FeatureFlagAlertModifier(showing: $isShowingFeatureFlagAlert))
         .navigationBarTitle(Text("Advanced"))
     }
     
@@ -251,6 +236,74 @@ extension StatusContentType {
     }
 }
 
+private struct FeatureFlagAlertModifier: ViewModifier {
+    @Binding var showing: Bool
+    @ObservedObject private var preferences = Preferences.shared
+    @State private var featureFlagName = ""
+    
+    func body(content: Content) -> some View {
+        if #available(iOS 16.0, *) {
+            content
+                .onChange(of: showing) {
+                    if $0 {
+                        featureFlagName = ""
+                    }
+                }
+                .alert("Enable Feature Flag", isPresented: $showing) {
+                    textField
+                    Button("Cancel", role: .cancel) {}
+                    Button("Enable", action: enableFlag)
+                } message: {
+                    warning
+                }
+        } else {
+            content
+                .sheet(isPresented: $showing) {
+                    NavigationView {
+                        List {
+                            Section {
+                                textField
+                            } footer: {
+                                warning
+                            }
+                        }
+                        .navigationTitle("Enable Feature Flag")
+                        .navigationBarTitleDisplayMode(.inline)
+                        .listStyle(.insetGrouped)
+                        .toolbar {
+                            ToolbarItem(placement: .cancellationAction) {
+                                Button("Cancel") {
+                                    showing = false
+                                }
+                            }
+                            ToolbarItem(placement: .confirmationAction) {
+                                Button("Enable", action: self.enableFlag)
+                            }
+                        }
+                    }
+                    .navigationViewStyle(.stack)
+                }
+        }
+    }
+    
+    private var textField: some View {
+        TextField("Flag Name", text: $featureFlagName)
+            .textInputAutocapitalization(.never)
+            .autocorrectionDisabled()
+    }
+    
+    private var warning: Text {
+        Text("Warning: Feature flags are intended for development and debugging use only. They are experimental and subject to change at any time.")
+    }
+    
+    private func enableFlag() {
+        if let flag = FeatureFlag(rawValue: featureFlagName) {
+            preferences.enabledFeatureFlags.insert(flag)
+            showing = false
+        }
+    }
+}
+
 #if DEBUG
 struct AdvancedPrefsView_Previews : PreviewProvider {
     static var previews: some View {