Make a simple state machine to identify incorrect transitions

Core/Pixel.swift

@@ -161,6 +161,9 @@
     public static let retriedPixel = "retriedPixel"
 
     public static let time = "time"
+
+    public static let appState = "state"
+    public static let appEvent = "event"
 }
 
 public struct PixelValues {
@@ -268,7 +271,7 @@
                                                      allowedQueryReservedCharacters: allowedQueryReservedCharacters,
                                                      headers: headers)
         let request = APIRequest(configuration: configuration, urlSession: .session(useMainThreadCallbackQueue: true))
         request.fetch { _, error in
             Logger.general.debug("Pixel fired \(pixelName, privacy: .public) \(params, privacy: .public)")
             onComplete(error)
         }

Core/PixelEvent.swift

@@ -895,6 +895,9 @@ extension Pixel {
         case appDidShowUITime(time: BucketAggregation)
         case appDidBecomeActiveTime(time: BucketAggregation)
 
+        // MARK: Lifecycle
+        case appDidTransitionToUnexpectedState
+
     }
 
 }
@@ -1784,6 +1787,9 @@ extension Pixel.Event {
         case .appDidShowUITime(let time): return "m_debug_app-did-show-ui-time-\(time)"
         case .appDidBecomeActiveTime(let time): return "m_debug_app-did-become-active-time-\(time)"
 
+        // MARK: Lifecycle
+        case .appDidTransitionToUnexpectedState: return "m_debug_app-did-transition-to-unexpected-state"
+
         }
     }
 }

DuckDuckGo.xcodeproj/project.pbxproj

@@ -980,6 +980,13 @@
 		CB9B873C278C8FEA001F4906 /* WidgetEducationView.swift in Sources */ = {isa = PBXBuildFile; fileRef = CB9B873B278C8FEA001F4906 /* WidgetEducationView.swift */; };
 		CB9B873E278C93C2001F4906 /* HomeMessage.xcassets in Resources */ = {isa = PBXBuildFile; fileRef = CB9B873D278C93C2001F4906 /* HomeMessage.xcassets */; };
 		CBAA195A27BFE15600A4BD49 /* NSManagedObjectContextExtension.swift in Sources */ = {isa = PBXBuildFile; fileRef = CBAA195927BFE15600A4BD49 /* NSManagedObjectContextExtension.swift */; };
+		CBAD0EF92CFE1D3B006267B8 /* Init.swift in Sources */ = {isa = PBXBuildFile; fileRef = CBAD0EF82CFE1D35006267B8 /* Init.swift */; };
+		CBAD0EFB2CFE1D41006267B8 /* Launched.swift in Sources */ = {isa = PBXBuildFile; fileRef = CBAD0EFA2CFE1D3F006267B8 /* Launched.swift */; };
+		CBAD0EFD2CFE1D4B006267B8 /* Active.swift in Sources */ = {isa = PBXBuildFile; fileRef = CBAD0EFC2CFE1D48006267B8 /* Active.swift */; };
+		CBAD0EFF2CFE1D50006267B8 /* Inactive.swift in Sources */ = {isa = PBXBuildFile; fileRef = CBAD0EFE2CFE1D4E006267B8 /* Inactive.swift */; };
+		CBAD0F012CFE1D57006267B8 /* Background.swift in Sources */ = {isa = PBXBuildFile; fileRef = CBAD0F002CFE1D54006267B8 /* Background.swift */; };
+		CBAD0F062CFE2711006267B8 /* AppStateMachine.swift in Sources */ = {isa = PBXBuildFile; fileRef = CBAD0F052CFE270D006267B8 /* AppStateMachine.swift */; };
+		CBAD0F082CFE27E2006267B8 /* AppStateTransitions.swift in Sources */ = {isa = PBXBuildFile; fileRef = CBAD0F072CFE27D5006267B8 /* AppStateTransitions.swift */; };
 		CBC83E3429B631780008E19C /* Configuration in Frameworks */ = {isa = PBXBuildFile; productRef = CBC83E3329B631780008E19C /* Configuration */; };
 		CBC88EE12C7F834300F0F8C5 /* SpecialErrorPageUserScriptTests.swift in Sources */ = {isa = PBXBuildFile; fileRef = CBC88EE02C7F834300F0F8C5 /* SpecialErrorPageUserScriptTests.swift */; };
 		CBC88EE52C8097B500F0F8C5 /* URLCredentialCreator.swift in Sources */ = {isa = PBXBuildFile; fileRef = CBC88EE42C8097B500F0F8C5 /* URLCredentialCreator.swift */; };
@@ -2811,6 +2818,13 @@
 		CBA1DE942AF6D579007C9457 /* pt */ = {isa = PBXFileReference; lastKnownFileType = text.plist.strings; name = pt; path = pt.lproj/InfoPlist.strings; sourceTree = "<group>"; };
 		CBAA195927BFE15600A4BD49 /* NSManagedObjectContextExtension.swift */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.swift; path = NSManagedObjectContextExtension.swift; sourceTree = "<group>"; };
 		CBAA195B27C3982A00A4BD49 /* PrivacyFeatures.swift */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.swift; path = PrivacyFeatures.swift; sourceTree = "<group>"; };
+		CBAD0EF82CFE1D35006267B8 /* Init.swift */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.swift; path = Init.swift; sourceTree = "<group>"; };
+		CBAD0EFA2CFE1D3F006267B8 /* Launched.swift */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.swift; path = Launched.swift; sourceTree = "<group>"; };
+		CBAD0EFC2CFE1D48006267B8 /* Active.swift */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.swift; path = Active.swift; sourceTree = "<group>"; };
+		CBAD0EFE2CFE1D4E006267B8 /* Inactive.swift */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.swift; path = Inactive.swift; sourceTree = "<group>"; };
+		CBAD0F002CFE1D54006267B8 /* Background.swift */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.swift; path = Background.swift; sourceTree = "<group>"; };
+		CBAD0F052CFE270D006267B8 /* AppStateMachine.swift */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.swift; path = AppStateMachine.swift; sourceTree = "<group>"; };
+		CBAD0F072CFE27D5006267B8 /* AppStateTransitions.swift */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.swift; path = AppStateTransitions.swift; sourceTree = "<group>"; };
 		CBB6B2542AF6D543006B777C /* lt */ = {isa = PBXFileReference; lastKnownFileType = text.plist.strings; name = lt; path = lt.lproj/InfoPlist.strings; sourceTree = "<group>"; };
 		CBC7AB542AF6D583008CB798 /* ro */ = {isa = PBXFileReference; lastKnownFileType = text.plist.strings; name = ro; path = ro.lproj/InfoPlist.strings; sourceTree = "<group>"; };
 		CBC88EE02C7F834300F0F8C5 /* SpecialErrorPageUserScriptTests.swift */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.swift; path = SpecialErrorPageUserScriptTests.swift; sourceTree = "<group>"; };
@@ -5434,6 +5448,28 @@
 			name = Resources;
 			sourceTree = "<group>";
 		};
+		CBAD0EF72CFE1D14006267B8 /* AppStates */ = {
+			isa = PBXGroup;
+			children = (
+				CBAD0EF82CFE1D35006267B8 /* Init.swift */,
+				CBAD0EFA2CFE1D3F006267B8 /* Launched.swift */,
+				CBAD0EFC2CFE1D48006267B8 /* Active.swift */,
+				CBAD0EFE2CFE1D4E006267B8 /* Inactive.swift */,
+				CBAD0F002CFE1D54006267B8 /* Background.swift */,
+			);
+			path = AppStates;
+			sourceTree = "<group>";
+		};
+		CBAD0F042CFE1DA2006267B8 /* AppLifecycle */ = {
+			isa = PBXGroup;
+			children = (
+				CBAD0F052CFE270D006267B8 /* AppStateMachine.swift */,
+				CBAD0F072CFE27D5006267B8 /* AppStateTransitions.swift */,
+				CBAD0EF72CFE1D14006267B8 /* AppStates */,
+			);
+			path = AppLifecycle;
+			sourceTree = "<group>";
+		};
 		D62EC3B72C24695800FC9D04 /* DuckPlayer */ = {
 			isa = PBXGroup;
 			children = (
@@ -6317,6 +6353,7 @@
 		F1C5ECF31E37812900C599A4 /* Application */ = {
 			isa = PBXGroup;
 			children = (
+				CBAD0F042CFE1DA2006267B8 /* AppLifecycle */,
 				83BE9BC2215D69C1009844D9 /* AppConfigurationFetch.swift */,
 				CB24F70E29A3EB15006DCC58 /* AppConfigurationURLProvider.swift */,
 				84E341951E2F7EFB00BDBA6F /* AppDelegate.swift */,
@@ -7499,6 +7536,7 @@
 				BDE91CDE2C62B90F0005CB74 /* UnifiedFeedbackRootView.swift in Sources */,
 				D65625A12C232F5E006EF297 /* SettingsDuckPlayerView.swift in Sources */,
 				D6FEB8B52B74994000C3615F /* HeadlessWebViewCoordinator.swift in Sources */,
+				CBAD0EF92CFE1D3B006267B8 /* Init.swift in Sources */,
 				9F96F73F2C914C57009E45D5 /* OnboardingGradient.swift in Sources */,
 				6FE1273D2C204C2500EB5724 /* FavoritesView.swift in Sources */,
 				8528AE81212F15D600D0BD74 /* AppRatingPrompt.xcdatamodeld in Sources */,
@@ -7536,6 +7574,7 @@
 				8590CB69268A4E190089F6BF /* DebugEtagStorage.swift in Sources */,
 				C1CDA3162AFB9C7F006D1476 /* AutofillNeverPromptWebsitesManager.swift in Sources */,
 				D668D9272B6937D2008E2FF2 /* SubscriptionITPViewModel.swift in Sources */,
+				CBAD0F012CFE1D57006267B8 /* Background.swift in Sources */,
 				F1CA3C371F045878005FADB3 /* PrivacyStore.swift in Sources */,
 				31DE43C42C2C60E800F8C51F /* DuckPlayerModalPresenter.swift in Sources */,
 				37FCAAC029930E26000E420A /* FailedAssertionView.swift in Sources */,
@@ -7602,6 +7641,7 @@
 				BDFF031D2BA3D2BD00F324C9 /* DefaultNetworkProtectionVisibility.swift in Sources */,
 				F1BE54581E69DE1000FCF649 /* TutorialSettings.swift in Sources */,
 				1EE52ABB28FB1D6300B750C1 /* UIImageExtension.swift in Sources */,
+				CBAD0EFF2CFE1D50006267B8 /* Inactive.swift in Sources */,
 				858650D12469BCDE00C36F8A /* DaxDialogs.swift in Sources */,
 				9F5E5AB02C3E4C6000165F54 /* ContextualOnboardingPresenter.swift in Sources */,
 				310D091B2799F54900DC0060 /* DownloadManager.swift in Sources */,
@@ -7642,6 +7682,7 @@
 				859DB8132CE6263C001F7210 /* TextZoomStorage.swift in Sources */,
 				D65625952C22D382006EF297 /* TabViewController.swift in Sources */,
 				8C4838B5221C8F7F008A6739 /* GestureToolbarButton.swift in Sources */,
+				CBAD0EFD2CFE1D4B006267B8 /* Active.swift in Sources */,
 				310ECFDD282A8BB0005029B3 /* EnableAutofillSettingsTableViewCell.swift in Sources */,
 				859DB8172CE6263C001F7210 /* TextZoomLevel.swift in Sources */,
 				BDE91CD62C6294020005CB74 /* FeedbackCategoryProviding.swift in Sources */,
@@ -7737,6 +7778,7 @@
 				D6F93E3E2B50A8A0004C268D /* SubscriptionSettingsView.swift in Sources */,
 				1D200C9B2BA31A6A00108701 /* AboutView.swift in Sources */,
 				851B12CC22369931004781BC /* AtbAndVariantCleanup.swift in Sources */,
+				CBAD0F062CFE2711006267B8 /* AppStateMachine.swift in Sources */,
 				D668D92B2B696840008E2FF2 /* IdentityTheftRestorationPagesFeature.swift in Sources */,
 				85F2FFCF2211F8E5006BB258 /* TabSwitcherViewController+KeyCommands.swift in Sources */,
 				3157B43327F497E90042D3D7 /* SaveLoginView.swift in Sources */,
@@ -7929,6 +7971,7 @@
 				311BD1B12836C0CA00AEF6C1 /* AutofillLoginListAuthenticator.swift in Sources */,
 				B652DF13287C373A00C12A9C /* ScriptSourceProviding.swift in Sources */,
 				854A012B2A54412600FCC628 /* ActivityViewController.swift in Sources */,
+				CBAD0F082CFE27E2006267B8 /* AppStateTransitions.swift in Sources */,
 				F1CA3C391F045885005FADB3 /* PrivacyUserDefaults.swift in Sources */,
 				6F655BE22BAB289E00AC3597 /* DefaultTheme.swift in Sources */,
 				6FE1274B2C20943500EB5724 /* ShortcutItemView.swift in Sources */,
@@ -8015,6 +8058,7 @@
 				983D71B12A286E810072E26D /* SyncDebugViewController.swift in Sources */,
 				6FDA1FB32B59584400AC962A /* AddressDisplayHelper.swift in Sources */,
 				F103073B1E7C91330059FEC7 /* BookmarksDataSource.swift in Sources */,
+				CBAD0EFB2CFE1D41006267B8 /* Launched.swift in Sources */,
 				6FD3F80F2C3EF4F000DA5797 /* DeviceOrientationEnvironmentValue.swift in Sources */,
 				85864FBC24D31EF300E756FF /* SuggestionTrayViewController.swift in Sources */,
 				D64648AF2B5993890033090B /* SubscriptionEmailViewModel.swift in Sources */,

DuckDuckGo/AppDelegate.swift

@@ -117,6 +117,8 @@ import os.log
 
     private var didFinishLaunchingStartTime: CFAbsoluteTime?
 
+    private let appStateMachine = AppStateMachine()
+
     override init() {
         super.init()
 
@@ -131,6 +133,7 @@ import os.log
     // swiftlint:disable:next cyclomatic_complexity
     func application(_ application: UIApplication, didFinishLaunchingWithOptions launchOptions: [UIApplication.LaunchOptionsKey: Any]?) -> Bool {
 
+        appStateMachine.handle(.launching(application, launchOptions: launchOptions))
         didFinishLaunchingStartTime = CFAbsoluteTimeGetCurrent()
         defer {
             if let didFinishLaunchingStartTime {
@@ -587,6 +590,8 @@ import os.log
     func applicationDidBecomeActive(_ application: UIApplication) {
         guard !testing else { return }
 
+        appStateMachine.handle(.activating(application))
+
         defer {
             if let didFinishLaunchingStartTime {
                 let launchTime = CFAbsoluteTimeGetCurrent() - didFinishLaunchingStartTime
@@ -690,6 +695,7 @@ import os.log
     }
 
     func applicationWillResignActive(_ application: UIApplication) {
+        appStateMachine.handle(.suspending(application))
         Task { @MainActor in
             await refreshShortcuts()
             await vpnWorkaround.removeRedditSessionWorkaround()
@@ -782,6 +788,7 @@ import os.log
     }
 
     func applicationDidEnterBackground(_ application: UIApplication) {
+        appStateMachine.handle(.backgrounding(application))
         displayBlankSnapshotWindow()
         autoClear?.startClearingTimer()
         lastBackgroundDate = Date()
@@ -827,6 +834,7 @@ import os.log
 
     func application(_ app: UIApplication, open url: URL, options: [UIApplication.OpenURLOptionsKey: Any] = [:]) -> Bool {
         Logger.sync.debug("App launched with url \(url.absoluteString)")
+        appStateMachine.handle(.openURL(url))
 
         // If showing the onboarding intro ignore deeplinks
         guard mainViewController?.needsToShowOnboardingIntro() == false else {

DuckDuckGo/AppLifecycle/AppStateMachine.swift

@@ -0,0 +1,53 @@
+//
+//  AppStateMachine.swift
+//  DuckDuckGo
+//
+//  Copyright © 2024 DuckDuckGo. All rights reserved.
+//
+//  Licensed under the Apache License, Version 2.0 (the "License");
+//  you may not use this file except in compliance with the License.
+//  You may obtain a copy of the License at
+//
+//  http://www.apache.org/licenses/LICENSE-2.0
+//
+//  Unless required by applicable law or agreed to in writing, software
+//  distributed under the License is distributed on an "AS IS" BASIS,
+//  WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+//  See the License for the specific language governing permissions and
+//  limitations under the License.
+//
+
+import UIKit
+
+enum AppEvent {
+
+    case launching(UIApplication, launchOptions: [UIApplication.LaunchOptionsKey: Any]?)
+    case activating(UIApplication)
+    case backgrounding(UIApplication)
+    case suspending(UIApplication)
+
+    case openURL(URL)
+
+}
+
+protocol AppState {
+
+    func apply(event: AppEvent) -> any AppState
+
+}
+
+protocol AppEventHandler {
+
+    func handle(_ event: AppEvent)
+
+}
+
+final class AppStateMachine: AppEventHandler {
+
+    private(set) var currentState: any AppState = Init()
+
+    func handle(_ event: AppEvent) {
+        currentState = currentState.apply(event: event)
+    }
+
+}

DuckDuckGo/AppLifecycle/AppStateTransitions.swift

@@ -0,0 +1,118 @@
+//
+//  AppStateTransitions.swift
+//  DuckDuckGo
+//
+//  Copyright © 2024 DuckDuckGo. All rights reserved.
+//
+//  Licensed under the Apache License, Version 2.0 (the "License");
+//  you may not use this file except in compliance with the License.
+//  You may obtain a copy of the License at
+//
+//  http://www.apache.org/licenses/LICENSE-2.0
+//
+//  Unless required by applicable law or agreed to in writing, software
+//  distributed under the License is distributed on an "AS IS" BASIS,
+//  WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+//  See the License for the specific language governing permissions and
+//  limitations under the License.
+//
+
+import os.log
+import Core
+
+extension Init {
+
+    func apply(event: AppEvent) -> any AppState {
+        switch event {
+        case .launching(let application, let launchOptions):
+            return Launched(application: application, launchOptions: launchOptions)
+        default:
+            return handleUnexpectedEvent(event)
+        }
+    }
+
+}
+
+extension Launched {
+
+    func apply(event: AppEvent) -> any AppState {
+        switch event {
+        case .activating(let application):
+            return Active(application: application)
+        case .openURL:
+            return self
+        case .launching, .suspending, .backgrounding:
+            return handleUnexpectedEvent(event)
+        }
+    }
+
+}
+
+extension Active {
+
+    func apply(event: AppEvent) -> any AppState {
+        switch event {
+        case .suspending(let application):
+            return Inactive(application: application)
+        case .launching, .activating, .backgrounding, .openURL:
+            return handleUnexpectedEvent(event)
+        }
+    }
+
+}
+
+extension Inactive {
+
+    func apply(event: AppEvent) -> any AppState {
+        switch event {
+        case .backgrounding(let application):
+            return Background(application: application)
+        case .activating(let application):
+            return Active(application: application)
+        case .launching, .suspending, .openURL:
+            return handleUnexpectedEvent(event)
+        }
+    }
+
+}
+
+extension Background {
+
+    func apply(event: AppEvent) -> any AppState {
+        switch event {
+        case .activating(let application):
+            return Active(application: application)
+        case .openURL:
+            return self
+        case .launching, .suspending, .backgrounding:
+            return handleUnexpectedEvent(event)
+        }
+    }
+
+}
+
+extension AppEvent {
+
+    var rawValue: String {
+        switch self {
+        case .launching: return "launching"
+        case .activating: return "activating"
+        case .backgrounding: return "backgrounding"
+        case .suspending: return "suspending"
+        case .openURL: return "openURL"
+        }
+    }
+
+}
+
+extension AppState {
+
+    func handleUnexpectedEvent(_ event: AppEvent) -> Self {
+        Logger.lifecycle.error("Invalid transition (\(event.rawValue)) for state (\(type(of: self)))")
+        DailyPixel.fireDailyAndCount(pixel: .appDidTransitionToUnexpectedState,
+                                     withAdditionalParameters: [PixelParameters.appState: String(describing: type(of: self)),
+                                                                PixelParameters.appEvent: event.rawValue])
+        return self
+    }
+
+}