Merge branch 'add-promise'

10 files changed, 830 insertions, 20 deletions

diff --git a/ios/MullvadVPN.xcodeproj/project.pbxproj b/ios/MullvadVPN.xcodeproj/project.pbxproj
index df43bbe265..d159f98b47 100644
--- a/ios/MullvadVPN.xcodeproj/project.pbxproj
+++ b/ios/MullvadVPN.xcodeproj/project.pbxproj
@@ -111,7 +111,12 @@
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 		585CA70F25F8C44600B47C62 /* UIMetrics.swift in Sources */ = {isa = PBXBuildFile; fileRef = 585CA70E25F8C44600B47C62 /* UIMetrics.swift */; };
+		585DA8AF26B9492500B8C587 /* Promise.swift in Sources */ = {isa = PBXBuildFile; fileRef = 585DA8AE26B9492500B8C587 /* Promise.swift */; };
 		585FE2F124E1365400439C50 /* LogStreamer.swift in Sources */ = {isa = PBXBuildFile; fileRef = 585FE2F024E1365400439C50 /* LogStreamer.swift */; };
+		5860392726D91B8400554C79 /* PromiseTests.swift in Sources */ = {isa = PBXBuildFile; fileRef = 58A94AE526D23C3D001CB97C /* PromiseTests.swift */; };
+		5860392926DCE7AB00554C79 /* PromiseCompletion.swift in Sources */ = {isa = PBXBuildFile; fileRef = 5860392826DCE7AB00554C79 /* PromiseCompletion.swift */; };
+		5860392A26DCE7AB00554C79 /* PromiseCompletion.swift in Sources */ = {isa = PBXBuildFile; fileRef = 5860392826DCE7AB00554C79 /* PromiseCompletion.swift */; };
+		5860392B26DCEE6300554C79 /* PromiseCompletion.swift in Sources */ = {isa = PBXBuildFile; fileRef = 5860392826DCE7AB00554C79 /* PromiseCompletion.swift */; };
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@@ -179,6 +184,8 @@
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 		58B8743222B25A7600015324 /* WireguardAssociatedAddresses.swift in Sources */ = {isa = PBXBuildFile; fileRef = 58B8743122B25A7600015324 /* WireguardAssociatedAddresses.swift */; };
+		58B93A1826C54D7E00A55733 /* Locking.swift in Sources */ = {isa = PBXBuildFile; fileRef = 58BA692D23E99EFF009DC256 /* Locking.swift */; };
+		58B93A2526C683B300A55733 /* Promise.swift in Sources */ = {isa = PBXBuildFile; fileRef = 585DA8AE26B9492500B8C587 /* Promise.swift */; };
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@@ -192,6 +199,7 @@
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 		58BFA5CD22A7CE1F00A6173D /* ApplicationConfiguration.swift in Sources */ = {isa = PBXBuildFile; fileRef = 58BFA5CB22A7CE1F00A6173D /* ApplicationConfiguration.swift */; };
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@@ -216,6 +224,24 @@
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+		58E1336A26D2BE3700CC316B /* PromiseObserver.swift in Sources */ = {isa = PBXBuildFile; fileRef = 58E1336826D2BE3700CC316B /* PromiseObserver.swift */; };
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@@ -367,7 +393,9 @@
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@@ -468,12 +503,12 @@
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 		58F559042697002000F630D0 /* en */ = {isa = PBXFileReference; lastKnownFileType = text.plist.strings; name = en; path = en.lproj/AccountInput.strings; sourceTree = "<group>"; };
+		58F559062697002000F630D0 /* en */ = {isa = PBXFileReference; lastKnownFileType = text.plist.strings; name = en; path = en.lproj/Main.strings; sourceTree = "<group>"; };
+		58F559082697002100F630D0 /* en */ = {isa = PBXFileReference; lastKnownFileType = text.plist.strings; name = en; path = en.lproj/ConnectionPanel.strings; sourceTree = "<group>"; };
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@@ -626,6 +661,7 @@
 		58B0A2A1238EE67E00BC001D /* MullvadVPNTests */ = {
 			isa = PBXGroup;
 			children = (
+				58A94AE526D23C3D001CB97C /* PromiseTests.swift */,
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@@ -736,6 +772,7 @@
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 				5871FB9F254C26BF0051A0A4 /* NSRegularExpression+IPAddress.swift */,
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@@ -802,6 +839,21 @@
 			path = MullvadVPNScreenshots;
 			sourceTree = "<group>";
 		};
+		58E1338C26D2BFB500CC316B /* Promise */ = {
+			isa = PBXGroup;
+			children = (
+				585DA8AE26B9492500B8C587 /* Promise.swift */,
+				58E1336826D2BE3700CC316B /* PromiseObserver.swift */,
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+				58E1336C26D2BE7500CC316B /* AnyResult.swift */,
+				58E1337026D2BE9C00CC316B /* AnyOptional.swift */,
+				58E1337426D2BEC400CC316B /* Promise+Optional.swift */,
+				58E1337826D2BEDD00CC316B /* Promise+ReceiveOn.swift */,
+				58E1338026D2BF5C00CC316B /* Promise+Result.swift */,
+			);
+			path = Promise;
+			sourceTree = "<group>";
+		};
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@@ -1062,12 +1114,14 @@
 			buildActionMask = 2147483647;
 			files = (
 				5896AE81246ACE81005B36CB /* KeychainMatchLimit.swift in Sources */,
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 				5857F23B24C8448600CF6F47 /* OperationProtocol.swift in Sources */,
 				58B0A2AA238EE6A900BC001D /* RelaySelector.swift in Sources */,
+				58E1337726D2BEC400CC316B /* Promise+Optional.swift in Sources */,
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@@ -1078,11 +1132,16 @@
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 				5857F23624C8445300CF6F47 /* OutputOperation.swift in Sources */,
+				58E1338326D2BF5C00CC316B /* Promise+Result.swift in Sources */,
 				58B0A2AC238EE6D500BC001D /* IPAddress+Codable.swift in Sources */,
 				58B0A2AD238EE6EC00BC001D /* MullvadEndpoint.swift in Sources */,
+				5860392B26DCEE6300554C79 /* PromiseCompletion.swift in Sources */,
 				58FAEDF4245088B300CB0F5B /* KeychainError.swift in Sources */,
+				5860392726D91B8400554C79 /* PromiseTests.swift in Sources */,
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 				5896AE88246D7FAF005B36CB /* CustomDateComponentsFormatting.swift in Sources */,
 				5857F23D24C8449A00CF6F47 /* TransformOperationObserver.swift in Sources */,
+				58C3478B26C1094F0060838B /* Promise.swift in Sources */,
 				5857F23524C8444E00CF6F47 /* InputOperation.swift in Sources */,
 				5857F23824C8446700CF6F47 /* AsyncBlockOperation.swift in Sources */,
 				582AE3122440CA0D00E6733A /* AccountTokenInputTests.swift in Sources */,
@@ -1092,8 +1151,10 @@
 				58CAF4EA26025927007C5886 /* PacketTunnelIpc.swift in Sources */,
 				5857F23E24C844A000CF6F47 /* OperationBlockObserver.swift in Sources */,
 				5857F23024C843ED00CF6F47 /* ChainedError.swift in Sources */,
+				58E1337B26D2BEDD00CC316B /* Promise+ReceiveOn.swift in Sources */,
 				58A8BE81239FBE62006B74AC /* IPEndpoint.swift in Sources */,
 				5896AE7F246ACE76005B36CB /* Keychain.swift in Sources */,
+				58E1336B26D2BE3700CC316B /* PromiseObserver.swift in Sources */,
 				5857F23C24C8449500CF6F47 /* OperationObserver.swift in Sources */,
 				58871D1825D5359B002297FA /* MullvadRest.swift in Sources */,
 				58871D2325D535D2002297FA /* IPAddressRange+Codable.swift in Sources */,
@@ -1113,7 +1174,9 @@
 				5840250122B1124600E4CFEC /* IPAddress+Codable.swift in Sources */,
 				5857F24724C882D700CF6F47 /* SelectLocationNavigationController.swift in Sources */,
 				580EE21224B322FC00F9D8A1 /* ResultOperation.swift in Sources */,
+				58E1337126D2BE9C00CC316B /* AnyOptional.swift in Sources */,
 				58BA693123EADA6A009DC256 /* SimulatorTunnelProvider.swift in Sources */,
+				58E1336D26D2BE7500CC316B /* AnyResult.swift in Sources */,
 				587B753B2666467500DEF7E9 /* NotificationBannerView.swift in Sources */,
 				58B993B12608A34500BA7811 /* LoginContentView.swift in Sources */,
 				58E6771F24ADFE7800AA26E7 /* SettingsNavigationController.swift in Sources */,
@@ -1127,6 +1190,7 @@
 				582BB1B1229569620055B6EF /* CustomNavigationBar.swift in Sources */,
 				584789E026529D72000E45FB /* SSLPinningURLSessionDelegate.swift in Sources */,
 				58ACF6492655365700ACE4B7 /* PreferencesViewController.swift in Sources */,
+				5860392926DCE7AB00554C79 /* PromiseCompletion.swift in Sources */,
 				588D2FE3248AC27F00E313F7 /* AsyncOperation.swift in Sources */,
 				5877153023981F7B001F8237 /* WireguardKeysViewController.swift in Sources */,
 				587B7536266528A200DEF7E9 /* NotificationManager.swift in Sources */,
@@ -1137,6 +1201,7 @@
 				58FAEDEF245069C700CB0F5B /* KeychainAttributes.swift in Sources */,
 				58CB0EE024B86751001EF0D8 /* MullvadRest.swift in Sources */,
 				580EE20924B3224200F9D8A1 /* RetryOperation.swift in Sources */,
+				58E1337526D2BEC400CC316B /* Promise+Optional.swift in Sources */,
 				58293FB125124117005D0BB5 /* CustomTextField.swift in Sources */,
 				582AE3102440A6CA00E6733A /* AccountTokenInput.swift in Sources */,
 				58FAEDF7245088E100CB0F5B /* Keychain.swift in Sources */,
@@ -1144,6 +1209,7 @@
 				58EF581125D69DB400AEBA94 /* StatusImageView.swift in Sources */,
 				58907D9524D17B4E00CFC3F5 /* DisconnectSplitButton.swift in Sources */,
 				5888AD87227B17950051EB06 /* SelectLocationViewController.swift in Sources */,
+				58E1336926D2BE3700CC316B /* PromiseObserver.swift in Sources */,
 				58293FB3251241B4005D0BB5 /* CustomTextView.swift in Sources */,
 				580EE20424B321EC00F9D8A1 /* OperationObserver.swift in Sources */,
 				58F19E35228C15BA00C7710B /* SpinnerActivityIndicatorView.swift in Sources */,
@@ -1183,6 +1249,7 @@
 				5888AD83227B11080051EB06 /* SelectLocationCell.swift in Sources */,
 				5891BF1C25E3E3EB006D6FB0 /* Bundle+ProductVersion.swift in Sources */,
 				5807E2C02432038B00F5FF30 /* String+Split.swift in Sources */,
+				58E1337926D2BEDD00CC316B /* Promise+ReceiveOn.swift in Sources */,
 				58CE5E66224146200008646E /* LoginViewController.swift in Sources */,
 				580EE21B24B3236900F9D8A1 /* InputOperation.swift in Sources */,
 				58EF580B25D69D7A00AEBA94 /* ProblemReportSubmissionOverlayView.swift in Sources */,
@@ -1206,6 +1273,7 @@
 				58293FAE2510CA58005D0BB5 /* ProblemReportViewController.swift in Sources */,
 				589AB4F7227B64450039131E /* BasicTableViewCell.swift in Sources */,
 				58B9EB152489139B00095626 /* DisplayChainedError.swift in Sources */,
+				585DA8AF26B9492500B8C587 /* Promise.swift in Sources */,
 				587B753F2668E5A700DEF7E9 /* NotificationContainerView.swift in Sources */,
 				5868BD33261DCD2600E6027F /* CustomSplitViewController.swift in Sources */,
 				58CCA01E2242787B004F3011 /* AccountTextField.swift in Sources */,
@@ -1230,6 +1298,7 @@
 				58C3A4B222456F1B00340BDB /* AccountInputGroupView.swift in Sources */,
 				58C4CB0124EBE5A700A22D49 /* LogEntryParser.swift in Sources */,
 				58F840B22464491D0044E708 /* ChainedError.swift in Sources */,
+				58E1338126D2BF5C00CC316B /* Promise+Result.swift in Sources */,
 				58FAEDFF24533A7000CB0F5B /* KeychainReturn.swift in Sources */,
 				58ACF64B26553C3F00ACE4B7 /* SettingsSwitchCell.swift in Sources */,
 				580EE20C24B3225F00F9D8A1 /* DelayOperation.swift in Sources */,
@@ -1246,12 +1315,16 @@
 				580EE21F24B3237F00F9D8A1 /* OutputOperation.swift in Sources */,
 				5850366825A47AC700A43E93 /* IPAddressRange+Codable.swift in Sources */,
 				580EE20224B321DB00F9D8A1 /* OperationProtocol.swift in Sources */,
+				58B93A1826C54D7E00A55733 /* Locking.swift in Sources */,
 				58FAEE0224533ABB00CB0F5B /* KeychainMatchLimit.swift in Sources */,
+				5860392A26DCE7AB00554C79 /* PromiseCompletion.swift in Sources */,
 				58FAEE0324533ABE00CB0F5B /* KeychainReturn.swift in Sources */,
 				58BFA5CD22A7CE1F00A6173D /* ApplicationConfiguration.swift in Sources */,
 				5850368D25A49E2200A43E93 /* PrivateKeyWithMetadata.swift in Sources */,
 				580EE20724B3222400F9D8A1 /* ExclusivityController.swift in Sources */,
 				58F840B02464382C0044E708 /* KeychainItemRevision.swift in Sources */,
+				58E1337A26D2BEDD00CC316B /* Promise+ReceiveOn.swift in Sources */,
+				58B93A2526C683B300A55733 /* Promise.swift in Sources */,
 				587AD7C723421D8600E93A53 /* TunnelSettings.swift in Sources */,
 				58F3C0962492617E003E76BE /* AsyncOperation.swift in Sources */,
 				580EE22924B3289300F9D8A1 /* AssociatedValue.swift in Sources */,
@@ -1266,10 +1339,14 @@
 				58BA692F23E99F5B009DC256 /* Locking.swift in Sources */,
 				580EE21624B3231200F9D8A1 /* OperationBlockObserver.swift in Sources */,
 				58CC40F024A602780019D96E /* ObserverList.swift in Sources */,
+				58E1337226D2BE9C00CC316B /* AnyOptional.swift in Sources */,
+				58E1338226D2BF5C00CC316B /* Promise+Result.swift in Sources */,
 				581503A724D6F4AE00C9C50E /* Logging.swift in Sources */,
 				58FAEE0424533AC000CB0F5B /* KeychainClass.swift in Sources */,
 				58AEEF6C2344A49D00C9BBD5 /* TunnelSettingsManager.swift in Sources */,
+				58E1336E26D2BE7500CC316B /* AnyResult.swift in Sources */,
 				581503A424D6F1EC00C9C50E /* ChainedError+Logger.swift in Sources */,
+				58E1336A26D2BE3700CC316B /* PromiseObserver.swift in Sources */,
 				5815039824D6ECAE00C9C50E /* CustomFormatLogHandler.swift in Sources */,
 				5840250522B11AB700E4CFEC /* MullvadEndpoint.swift in Sources */,
 				583BC70824FE4DC500C9DE04 /* Optional+DispatchQueue.swift in Sources */,
@@ -1286,6 +1363,7 @@
 				580EE22524B3243100F9D8A1 /* AsyncBlockOperation.swift in Sources */,
 				580EE20D24B3225F00F9D8A1 /* DelayOperation.swift in Sources */,
 				588534BF246193D90018B744 /* AutomaticKeyRotationManager.swift in Sources */,
+				58E1337626D2BEC400CC316B /* Promise+Optional.swift in Sources */,
 				58781CCE22AE8918009B9D8E /* RelayConstraints.swift in Sources */,
 				581503A024D6F01E00C9C50E /* LogRotation.swift in Sources */,
 				58781CD522AFBA39009B9D8E /* RelaySelector.swift in Sources */,
@@ -1443,22 +1521,6 @@
 			name = AppStorePaymentManager.strings;
 			sourceTree = "<group>";
 		};
-		58F559052697002000F630D0 /* Main.strings */ = {
-			isa = PBXVariantGroup;
-			children = (
-				58F559062697002000F630D0 /* en */,
-			);
-			name = Main.strings;
-			sourceTree = "<group>";
-		};
-		58F559072697002100F630D0 /* ConnectionPanel.strings */ = {
-			isa = PBXVariantGroup;
-			children = (
-				58F559082697002100F630D0 /* en */,
-			);
-			name = ConnectionPanel.strings;
-            sourceTree = "<group>";
-        };
 		58F558FC2696F09100F630D0 /* KeyboardNavigation.strings */ = {
 			isa = PBXVariantGroup;
 			children = (
@@ -1491,6 +1553,22 @@
 			name = AccountInput.strings;
 			sourceTree = "<group>";
 		};
+		58F559052697002000F630D0 /* Main.strings */ = {
+			isa = PBXVariantGroup;
+			children = (
+				58F559062697002000F630D0 /* en */,
+			);
+			name = Main.strings;
+			sourceTree = "<group>";
+		};
+		58F559072697002100F630D0 /* ConnectionPanel.strings */ = {
+			isa = PBXVariantGroup;
+			children = (
+				58F559082697002100F630D0 /* en */,
+			);
+			name = ConnectionPanel.strings;
+			sourceTree = "<group>";
+		};
 		58F559092697002100F630D0 /* HeaderBar.strings */ = {
 			isa = PBXVariantGroup;
 			children = (
diff --git a/ios/MullvadVPN/Promise/AnyOptional.swift b/ios/MullvadVPN/Promise/AnyOptional.swift
new file mode 100644
index 0000000000..95382ac864
--- /dev/null
+++ b/ios/MullvadVPN/Promise/AnyOptional.swift
@@ -0,0 +1,22 @@
+//
+//  AnyOptional.swift
+//  AnyOptional
+//
+//  Created by pronebird on 22/08/2021.
+//  Copyright © 2021 Mullvad VPN AB. All rights reserved.
+//
+
+import Foundation
+
+/// Protocol uniting all Optional types.
+protocol AnyOptional {
+    associatedtype Wrapped
+
+    func asConcreteType() -> Optional<Wrapped>
+}
+
+extension Optional: AnyOptional {
+    func asConcreteType() -> Optional<Wrapped> {
+        return self
+    }
+}
diff --git a/ios/MullvadVPN/Promise/AnyResult.swift b/ios/MullvadVPN/Promise/AnyResult.swift
new file mode 100644
index 0000000000..24e8e1fd11
--- /dev/null
+++ b/ios/MullvadVPN/Promise/AnyResult.swift
@@ -0,0 +1,23 @@
+//
+//  AnyResult.swift
+//  AnyResult
+//
+//  Created by pronebird on 22/08/2021.
+//  Copyright © 2021 Mullvad VPN AB. All rights reserved.
+//
+
+import Foundation
+
+/// Protocol uniting all Result types.
+protocol AnyResult {
+    associatedtype Success
+    associatedtype Failure: Error
+
+    func asConcreteType() -> Result<Success, Failure>
+}
+
+extension Result: AnyResult {
+    func asConcreteType() -> Result<Success, Failure> {
+        return self
+    }
+}
diff --git a/ios/MullvadVPN/Promise/Promise+Optional.swift b/ios/MullvadVPN/Promise/Promise+Optional.swift
new file mode 100644
index 0000000000..0fc38a1ae6
--- /dev/null
+++ b/ios/MullvadVPN/Promise/Promise+Optional.swift
@@ -0,0 +1,31 @@
+//
+//  Promise+Optional.swift
+//  Promise+Optional
+//
+//  Created by pronebird on 22/08/2021.
+//  Copyright © 2021 Mullvad VPN AB. All rights reserved.
+//
+
+import Foundation
+
+extension Optional {
+    func asPromise() -> Promise<Self> {
+        return .resolved(self)
+    }
+}
+
+extension Promise where Value: AnyOptional {
+    /// Map the value when present. Returns `defaultValue` otherwise.
+    func map<NewValue>(defaultValue: NewValue, transform: @escaping (Value.Wrapped) -> NewValue) -> Promise<NewValue> {
+        return then { value -> NewValue in
+            return value.asConcreteType().map(transform) ?? defaultValue
+        }
+    }
+
+    /// Map the value when present, producing new promise to compute the new value. Returns `defaultValue` otherwise.
+    func mapThen<NewValue>(defaultValue: NewValue, producePromise: @escaping (Value.Wrapped) -> Promise<NewValue>) -> Promise<NewValue> {
+        return then { value in
+            return value.asConcreteType().map(producePromise) ?? .resolved(defaultValue)
+        }
+    }
+}
diff --git a/ios/MullvadVPN/Promise/Promise+ReceiveOn.swift b/ios/MullvadVPN/Promise/Promise+ReceiveOn.swift
new file mode 100644
index 0000000000..a7ce8f039b
--- /dev/null
+++ b/ios/MullvadVPN/Promise/Promise+ReceiveOn.swift
@@ -0,0 +1,33 @@
+//
+//  Promise+ReceiveOn.swift
+//  Promise+ReceiveOn
+//
+//  Created by pronebird on 22/08/2021.
+//  Copyright © 2021 Mullvad VPN AB. All rights reserved.
+//
+
+import Foundation
+
+extension Promise {
+    /// Dispatch the upstream value on another queue.
+    func receive(on queue: DispatchQueue) -> Promise<Value> {
+        return Promise<Value> { resolver in
+            _ = self.observe { completion in
+                queue.async {
+                    resolver.resolve(completion: completion, queue: queue)
+                }
+            }
+        }
+    }
+
+    /// Dispatch the upstream value on another queue after delay.
+    func receive(on queue: DispatchQueue, after deadline: DispatchTime) -> Promise<Value> {
+        return Promise<Value> { resolver in
+            _ = self.observe { completion in
+                queue.asyncAfter(deadline: deadline) {
+                    resolver.resolve(completion: completion, queue: queue)
+                }
+            }
+        }
+    }
+}
diff --git a/ios/MullvadVPN/Promise/Promise+Result.swift b/ios/MullvadVPN/Promise/Promise+Result.swift
new file mode 100644
index 0000000000..94dd511227
--- /dev/null
+++ b/ios/MullvadVPN/Promise/Promise+Result.swift
@@ -0,0 +1,136 @@
+//
+//  Promise+Result.swift
+//  Promise+Result
+//
+//  Created by pronebird on 22/08/2021.
+//  Copyright © 2021 Mullvad VPN AB. All rights reserved.
+//
+
+import Foundation
+
+typealias _Promise = Promise
+
+extension Result {
+    typealias Promise = _Promise<Result<Success, Failure>>
+}
+
+extension Promise where Value: AnyResult {
+    typealias Success = Value.Success
+    typealias Failure = Value.Failure
+
+    static func failure(_ error: Failure) -> Result<Success, Failure>.Promise {
+        return Result<Success, Failure>.Promise(value: .failure(error))
+    }
+
+    static func success(_ value: Success) -> Result<Success, Failure>.Promise {
+        return Result<Success, Failure>.Promise(value: .success(value))
+    }
+
+    /// Replace value in Result. Passes failure result downstream.
+    func setOutput<NewSuccess>(_ newValue: NewSuccess) -> Result<NewSuccess, Failure>.Promise {
+        return map { _ in
+            return newValue
+        }
+    }
+    /// Returns a Promise containing resolved value or nil.
+    func success() -> Promise<Success?> {
+        return then { result -> Success? in
+            switch result.asConcreteType() {
+            case .success(let value):
+                return value
+            case .failure:
+                return nil
+            }
+        }
+    }
+
+    /// Map value. Passes failure result downstream.
+    func map<NewSuccess>(_ transform: @escaping (Success) -> NewSuccess) -> Result<NewSuccess, Failure>.Promise {
+        return then { result in
+            return result.asConcreteType().map(transform)
+        }
+    }
+
+    /// Perform actiion on success.
+    func onSuccess(_ onResolve: @escaping (Success) -> Void) -> Self {
+        return observe { completion in
+            if case .success(let value) = completion.unwrappedValue?.asConcreteType() {
+                onResolve(value)
+            }
+        }
+    }
+
+    /// Perform action on failure.
+    func onFailure(_ onResolve: @escaping (Failure) -> Void) -> Self {
+        return observe { completion in
+            if case .failure(let error) = completion.unwrappedValue?.asConcreteType() {
+                onResolve(error)
+            }
+        }
+    }
+
+    /// Map value producing Promise. Passes failure result downstream.
+    func mapThen<NewSuccess>(_ transform: @escaping (Success) -> Result<NewSuccess, Failure>.Promise) -> Result<NewSuccess, Failure>.Promise {
+        return then { result in
+            switch result.asConcreteType() {
+            case .success(let value):
+                return transform(value)
+            case .failure(let error):
+                return .failure(error)
+            }
+        }
+    }
+
+    /// Map failure. Passes successful result downstream.
+    func mapError<NewFailure>(_ transform: @escaping (Failure) -> NewFailure) -> Result<Success, NewFailure>.Promise {
+        return then { result in
+            return result.asConcreteType().mapError(transform)
+        }
+    }
+
+    /// Map value to Result. Passes failure result downstream.
+    func flatMap<NewSuccess>(_ transform: @escaping (Success) -> Result<NewSuccess, Failure>) -> Result<NewSuccess, Failure>.Promise {
+        return then { result in
+            return result.asConcreteType().flatMap(transform)
+        }
+    }
+
+    /// Map failure to Result. Passes successful result downstream.
+    func flatMapError<NewFailure>(_ transform: @escaping (Failure) -> Result<Success, NewFailure>) -> Result<Success, NewFailure>.Promise {
+        return then { result in
+            result.asConcreteType().flatMapError(transform)
+        }
+    }
+}
+
+extension Promise where Value: AnyResult {
+    func tryAwait() throws -> PromiseCompletion<Value.Success> {
+        return try self.await().map { result in
+            return try result.asConcreteType().get()
+        }
+    }
+}
+
+extension Result {
+    func asPromise() -> Result<Success, Failure>.Promise {
+        return .resolved(self)
+    }
+
+    var error: Failure? {
+        switch self {
+        case .success:
+            return nil
+        case .failure(let error):
+            return error
+        }
+    }
+
+    var value: Success? {
+        switch self {
+        case .success(let value):
+            return value
+        case .failure:
+            return nil
+        }
+    }
+}
diff --git a/ios/MullvadVPN/Promise/Promise.swift b/ios/MullvadVPN/Promise/Promise.swift
new file mode 100644
index 0000000000..1e8d75be4f
--- /dev/null
+++ b/ios/MullvadVPN/Promise/Promise.swift
@@ -0,0 +1,262 @@
+//
+//  Promise.swift
+//  Promise
+//
+//  Created by pronebird on 03/08/2021.
+//  Copyright © 2021 Mullvad VPN AB. All rights reserved.
+//
+
+import Foundation
+
+private enum PromiseState<Value> {
+    case pending((PromiseResolver<Value>) -> Void, DispatchQueue?)
+    case executing
+    case resolved(Value, DispatchQueue?)
+    case cancelled
+}
+
+/// Class describing a block of asynchronous computation that can either resolve or be cancelled.
+final class Promise<Value> {
+    private var state: PromiseState<Value>
+    private var observers: [AnyPromiseObserver<Value>] = []
+    private let lock = NSRecursiveLock()
+
+    /// Returns Promise resolved with the given value.
+    class func resolved(_ value: Value) -> Self {
+        return Self.init(value: value)
+    }
+
+    /// Initialize Promise with the execution block.
+    init(body: @escaping (PromiseResolver<Value>) -> Void) {
+        state = .pending(body, nil)
+    }
+
+    /// Initialize resolved Promise with the given value.
+    init(value: Value) {
+        state = .resolved(value, nil)
+    }
+
+    deinit {
+        switch state {
+        case .resolved, .cancelled:
+            break
+        case .pending, .executing:
+            preconditionFailure("\(Self.self) is deallocated in \(state) state without being resolved or cancelled.")
+        }
+    }
+
+    /// Observe the result of Promise.
+    /// This method starts the promise execution if it hasn't started yet.
+    @discardableResult
+    func observe(_ receiveCompletion: @escaping (PromiseCompletion<Value>) -> Void) -> Self {
+        return lock.withCriticalBlock {
+            switch state {
+            case .resolved(let value, let queue):
+                let completion = PromiseCompletion<Value>.finished(value)
+                queue?.async { receiveCompletion(completion) } ?? receiveCompletion(completion)
+
+            case .cancelled:
+                receiveCompletion(.cancelled)
+
+            case .pending:
+                observers.append(AnyPromiseObserver<Value>(receiveCompletion))
+                execute()
+
+            case .executing:
+                observers.append(AnyPromiseObserver<Value>(receiveCompletion))
+            }
+            return self
+        }
+    }
+
+    /// Cancel Promise.
+    /// When Promise is cancelled, all downstream Promises pending execution are also cancelled.
+    func cancel() {
+        lock.withCriticalBlock {
+            switch state {
+            case .pending, .executing:
+                state = .cancelled
+                observers.forEach { observer in
+                    observer.receiveCompletion(.cancelled)
+                }
+                observers.removeAll()
+
+            case .cancelled, .resolved:
+                break
+            }
+        }
+    }
+
+    /// Trasform the value by producing a promise.
+    func then<NewValue>(_ onResolve: @escaping (Value) -> Promise<NewValue>) -> Promise<NewValue> {
+        return Promise<NewValue> { resolver in
+            _ = self.observe { completion in
+                switch completion {
+                case .finished(let value):
+                    _ = onResolve(value).observe { completion in
+                        resolver.resolve(completion: completion)
+                    }
+                case .cancelled:
+                    resolver.resolve(completion: .cancelled)
+                }
+            }
+        }
+    }
+
+    /// Transform the value.
+    func then<NewValue>(_ onResolve: @escaping (Value) -> NewValue) -> Promise<NewValue> {
+        return Promise<NewValue> { resolver in
+            _ = self.observe { completion in
+                resolver.resolve(completion: completion.map(onResolve))
+            }
+        }
+    }
+
+    /// Assign the cancellation token into the given variable.
+    /// Releasing the cancellation token cancels the given Promise and all downstream Promises.
+    func storeCancellationToken(in token: inout PromiseCancellationToken?) -> Self {
+        token = PromiseCancellationToken { [weak self] in
+            self?.cancel()
+        }
+        return self
+    }
+
+    /// Set the queue on which to execute the promise's body block.
+    func schedule(on queue: DispatchQueue) -> Self {
+        return lock.withCriticalBlock {
+            switch state {
+            case .pending(let block, _):
+                state = .pending(block, queue)
+            case .cancelled, .executing, .resolved:
+                break
+            }
+            return self
+        }
+    }
+
+    /// Block the given queue until the promise finished executing.
+    func block(on dispatchQueue: DispatchQueue) -> Promise<Value> {
+        return Promise { resolver in
+            dispatchQueue.async {
+                let completion = self.await()
+
+                resolver.resolve(completion: completion)
+            }
+        }
+    }
+
+    /// Block current queue until the promise finished executing.
+    func await() -> PromiseCompletion<Value> {
+        let condition = NSCondition()
+        condition.lock()
+        defer { condition.unlock() }
+
+        var returnValue: PromiseCompletion<Value>!
+        _ = observe { completion in
+            returnValue = completion
+            condition.signal()
+        }
+
+        condition.wait()
+        return returnValue
+    }
+
+    /// Execute the promise's body if still pending execution.
+    private func execute() {
+        lock.withCriticalBlock {
+            guard case .pending(let block, let queue) = state else { return }
+
+            state = .executing
+
+            let resolver = PromiseResolver(promise: self)
+
+            queue?.async { block(resolver) } ?? block(resolver)
+        }
+    }
+
+    /// Resolve Promise with the given value.
+    ///
+    /// Provide the optional `queue` parameter which will be used to dispatch the resolved value to observers added
+    /// after the promise was already resolved. When providing a `queue`, the call to `resolve()` must happen on
+    /// the same queue.
+    fileprivate func resolve(value: Value, queue: DispatchQueue?) {
+        lock.withCriticalBlock {
+            switch state {
+            case .pending, .executing:
+                // Oblige caller to resolve the value on the same queue.
+                queue.map { dispatchPrecondition(condition: .onQueue($0)) }
+
+                state = .resolved(value, queue)
+
+                observers.forEach { observer in
+                    observer.receiveCompletion(.finished(value))
+                }
+                observers.removeAll()
+
+            case .cancelled, .resolved:
+                break
+            }
+
+        }
+    }
+
+}
+
+final class PromiseCancellationToken {
+    private let handler: () -> Void
+    fileprivate init(_ handler: @escaping () -> Void) {
+        self.handler = handler
+    }
+
+    deinit {
+        handler()
+    }
+}
+
+struct PromiseResolver<Value> {
+    private let promise: Promise<Value>
+
+    /// Private initializer.
+    fileprivate init(promise: Promise<Value>) {
+        self.promise = promise
+    }
+
+    /// Resolve the promise with `PromiseCompletion`.
+    func resolve(completion: PromiseCompletion<Value>) {
+        resolve(completion: completion, queue: nil)
+    }
+
+    /// Resolve the promise with `PromiseCompletion` and ptiona queue on which to dispatch the value too observers added
+    /// after the promise was already resolved.
+    func resolve(completion: PromiseCompletion<Value>, queue: DispatchQueue?) {
+        switch completion {
+        case .finished(let value):
+            resolve(value: value, queue: queue)
+        case .cancelled:
+            promise.cancel()
+        }
+    }
+
+    /// Resolve Promise with the given value.
+    func resolve(value: Value) {
+        resolve(value: value, queue: nil)
+    }
+
+    /// Resolve the promise with the given value and optional queue on which to dispatch the value to observers added
+    /// after the promise was already resolved.
+    fileprivate func resolve(value: Value, queue: DispatchQueue?) {
+        promise.resolve(value: value, queue: queue)
+    }
+
+    /// Set cancellation handler.
+    func setCancelHandler(_ cancellation: @escaping () -> Void) {
+        _ = promise.observe { completion in
+            switch completion {
+            case .finished:
+                break
+            case .cancelled:
+                cancellation()
+            }
+        }
+    }
+}
diff --git a/ios/MullvadVPN/Promise/PromiseCompletion.swift b/ios/MullvadVPN/Promise/PromiseCompletion.swift
new file mode 100644
index 0000000000..b424e29518
--- /dev/null
+++ b/ios/MullvadVPN/Promise/PromiseCompletion.swift
@@ -0,0 +1,51 @@
+//
+//  PromiseCompletion.swift
+//  PromiseCompletion
+//
+//  Created by pronebird on 30/08/2021.
+//  Copyright © 2021 Mullvad VPN AB. All rights reserved.
+//
+
+import Foundation
+
+/// Promise result type.
+enum PromiseCompletion<Value> {
+    /// Promise is finished with value.
+    case finished(Value)
+
+    /// Promise is cancelled.
+    case cancelled
+
+    /// Return the contained value, otherwise `nil`.
+    var unwrappedValue: Value? {
+        switch self {
+        case .finished(let value):
+            return value
+        case .cancelled:
+            return nil
+        }
+    }
+
+    /// Map the contained value, producing new `PromiseCompletion` type.
+    func map<NewValue>(_ transform: (Value) throws -> NewValue) rethrows -> PromiseCompletion<NewValue> {
+        switch self {
+        case .finished(let value):
+            return .finished(try transform(value))
+        case .cancelled:
+            return .cancelled
+        }
+    }
+}
+
+extension PromiseCompletion: Equatable where Value: Equatable {
+    static func == (lhs: PromiseCompletion<Value>, rhs: PromiseCompletion<Value>) -> Bool {
+        switch (lhs, rhs) {
+        case (.finished(let lhsValue), .finished(let rhsValue)):
+            return lhsValue == rhsValue
+        case (.cancelled, .cancelled):
+            return true
+        case (.finished, .cancelled), (.cancelled, .finished):
+            return false
+        }
+    }
+}
diff --git a/ios/MullvadVPN/Promise/PromiseObserver.swift b/ios/MullvadVPN/Promise/PromiseObserver.swift
new file mode 100644
index 0000000000..89e2059249
--- /dev/null
+++ b/ios/MullvadVPN/Promise/PromiseObserver.swift
@@ -0,0 +1,27 @@
+//
+//  PromiseObserver.swift
+//  PromiseObserver
+//
+//  Created by pronebird on 22/08/2021.
+//  Copyright © 2021 Mullvad VPN AB. All rights reserved.
+//
+
+import Foundation
+
+protocol PromiseObserver {
+    associatedtype Value
+
+    func receiveCompletion(_ completion: PromiseCompletion<Value>)
+}
+
+final class AnyPromiseObserver<Value>: PromiseObserver {
+    private let onReceiveCompletion: (PromiseCompletion<Value>) -> Void
+
+    init(_ receiveCompletionHandler: @escaping (PromiseCompletion<Value>) -> Void) {
+        onReceiveCompletion = receiveCompletionHandler
+    }
+
+    func receiveCompletion(_ completion: PromiseCompletion<Value>) {
+        onReceiveCompletion(completion)
+    }
+}
diff --git a/ios/MullvadVPNTests/PromiseTests.swift b/ios/MullvadVPNTests/PromiseTests.swift
new file mode 100644
index 0000000000..616d492a3d
--- /dev/null
+++ b/ios/MullvadVPNTests/PromiseTests.swift
@@ -0,0 +1,147 @@
+//
+//  PromiseTests.swift
+//  PromiseTests
+//
+//  Created by pronebird on 22/08/2021.
+//  Copyright © 2021 Mullvad VPN AB. All rights reserved.
+//
+
+import XCTest
+
+class PromiseTests: XCTestCase {
+
+    override func setUpWithError() throws {
+        // Put setup code here. This method is called before the invocation of each test method in the class.
+    }
+
+    override func tearDownWithError() throws {
+        // Put teardown code here. This method is called after the invocation of each test method in the class.
+    }
+
+    func testObserveResolvedPromise() throws {
+        let expect = expectation(description: "Wait for promise")
+
+        Promise(value: 1)
+            .observe { completion in
+                XCTAssertEqual(completion, .finished(1))
+                expect.fulfill()
+            }
+
+        wait(for: [expect], timeout: 1)
+    }
+
+    func testObservePromise() throws {
+        let expect = expectation(description: "Wait for promise")
+        Promise<Int> { resolver in
+            resolver.resolve(value: 1)
+        }
+        .observe { completion in
+            XCTAssertEqual(completion, .finished(1))
+            expect.fulfill()
+        }
+
+        wait(for: [expect], timeout: 1)
+    }
+
+    func testReceiveOn() throws {
+        let expect = expectation(description: "Wait for promise")
+        let queue = DispatchQueue(label: "TestQueue")
+
+        Promise(value: 1)
+            .receive(on: queue)
+            .observe { completion in
+                dispatchPrecondition(condition: .onQueue(queue))
+                expect.fulfill()
+            }
+
+        wait(for: [expect], timeout: 1)
+    }
+
+    func testScheduleOn() throws {
+        let expect = expectation(description: "Wait for promise")
+        let queue = DispatchQueue(label: "TestQueue")
+
+        Promise<Int> { resolver in
+            dispatchPrecondition(condition: .onQueue(queue))
+            resolver.resolve(value: 1)
+        }
+        .schedule(on: queue)
+        .observe { completion in
+            expect.fulfill()
+        }
+
+        wait(for: [expect], timeout: 1)
+    }
+
+    func testBlockOn() throws {
+        let expect1 = expectation(description: "Wait for promise")
+        let expect2 = expectation(description: "Wait for queue to be unblocked")
+        let queue = DispatchQueue(label: "TestQueue")
+
+        Promise<Int> { resolver in
+            DispatchQueue.main.async {
+                resolver.resolve(value: 1)
+            }
+        }
+        .block(on: queue)
+        .observe { completion in
+            dispatchPrecondition(condition: .onQueue(queue))
+            expect1.fulfill()
+        }
+
+        queue.async {
+            expect2.fulfill()
+        }
+
+        wait(for: [expect1, expect2], timeout: 1, enforceOrder: true)
+    }
+
+    func testCancellation() throws {
+        let cancelExpectation = expectation(description: "Expect cancellation handler to trigger")
+        let completionExpectation = expectation(description: "Expect promise to complete")
+
+        let promise = Promise<Int> { resolver in
+            let work = DispatchWorkItem {
+                XCTFail()
+                resolver.resolve(value: 1)
+            }
+
+            resolver.setCancelHandler {
+                work.cancel()
+                cancelExpectation.fulfill()
+            }
+
+            DispatchQueue.main.async(execute: work)
+        }.observe { completion in
+            XCTAssertEqual(completion, .cancelled)
+            completionExpectation.fulfill()
+        }
+
+        promise.cancel()
+
+        wait(for: [cancelExpectation, completionExpectation], timeout: 1)
+    }
+
+    func testOptionalMapNoneWithDefaultValue() {
+        let value: Int? = nil
+
+        value.asPromise()
+            .map(defaultValue: 1) { _ in
+                return 2
+            }.observe { completion in
+                XCTAssertEqual(completion.unwrappedValue, 1)
+            }
+    }
+
+    func testOptionalMapSomeWithDefaultValue() {
+        let value: Int? = 0
+
+        value.asPromise()
+            .map(defaultValue: 1) { _ in
+                return 2
+            }.observe { completion in
+                XCTAssertEqual(completion.unwrappedValue, 2)
+            }
+    }
+
+}