Add result builders to MultiLogger and MultiTrackers 👷

Some of our "multiplexer" instances for logging (`Log.MultiLogger`),
analytics (`Analytics.MultiTracker`) and performance metrics
(`PerformanceMetrics.MultiTracker`) require type erasure on the child
elements to which they forward events, causing the setup to be
cumbersome and not intuitive.

By leveraging result builders we can make this a bit smoother to users
by abstracting/automating the type erasure "dance". Furthermore, it
unlocks control flow (e.g. `if/else`, `for ... in`, `if #available`)
which allows more advanced setups to be defined directly in the builder.

## Changes

- Add result builders to facilitate instantiating certain "multiplexer"
instances, most notably the ones that require type erasure:

  + `Log.MultiLogger` (requires erasing to `AnyMetadataLogDestination`)

  + `Analytics.MultiTracker` (requires erasing to `AnyAnalyticsTracker`)

  + `PerformanceMetrics.MultiTracker` (doesn't require erasing)

- Add relevant UTs.

Alicerce.xcodeproj/project.pbxproj

@@ -56,7 +56,7 @@
 		0A266F0F1ED33B65009CD0D7 /* CAGradientLayer.swift in Sources */ = {isa = PBXBuildFile; fileRef = 0A266F0E1ED33B65009CD0D7 /* CAGradientLayer.swift */; };
 		0A266F201ED374F5009CD0D7 /* AssertDumpsEqual.swift in Sources */ = {isa = PBXBuildFile; fileRef = 0A266F1F1ED374F5009CD0D7 /* AssertDumpsEqual.swift */; };
 		0A266F861ED59DC7009CD0D7 /* Alicerce.framework in Frameworks */ = {isa = PBXBuildFile; fileRef = 0A3C2D711EA7E3E800EFB7D4 /* Alicerce.framework */; };
-		0A266F8C1ED59FB6009CD0D7 /* MultiTrackerTestCase.swift in Sources */ = {isa = PBXBuildFile; fileRef = 1B57E9871EB15F510027AB30 /* MultiTrackerTestCase.swift */; };
+		0A266F8C1ED59FB6009CD0D7 /* Analytics+MultiTrackerTestCase.swift in Sources */ = {isa = PBXBuildFile; fileRef = 1B57E9871EB15F510027AB30 /* Analytics+MultiTrackerTestCase.swift */; };
 		0A266F901ED59FB6009CD0D7 /* Route+ComponentTests.swift in Sources */ = {isa = PBXBuildFile; fileRef = 0A3C2D0A1EA7E1EE00EFB7D4 /* Route+ComponentTests.swift */; };
 		0A266F911ED59FB6009CD0D7 /* Route+TrieNode_AddTests.swift in Sources */ = {isa = PBXBuildFile; fileRef = 0A3C2D0B1EA7E1EE00EFB7D4 /* Route+TrieNode_AddTests.swift */; };
 		0A266F921ED59FB6009CD0D7 /* Route+TrieNode_InitTests.swift in Sources */ = {isa = PBXBuildFile; fileRef = 0A3C2D0C1EA7E1EE00EFB7D4 /* Route+TrieNode_InitTests.swift */; };
@@ -635,7 +635,7 @@
 		1B4D4CB61F05016B00FA4260 /* URLRequest.swift */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.swift; path = URLRequest.swift; sourceTree = "<group>"; };
 		1B57E97C1EB150C80027AB30 /* Analytics+MultiTracker.swift */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.swift; path = "Analytics+MultiTracker.swift"; sourceTree = "<group>"; };
 		1B57E97E1EB1510D0027AB30 /* AnalyticsTracker.swift */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.swift; path = AnalyticsTracker.swift; sourceTree = "<group>"; };
-		1B57E9871EB15F510027AB30 /* MultiTrackerTestCase.swift */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.swift; path = MultiTrackerTestCase.swift; sourceTree = "<group>"; };
+		1B57E9871EB15F510027AB30 /* Analytics+MultiTrackerTestCase.swift */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.swift; path = "Analytics+MultiTrackerTestCase.swift"; sourceTree = "<group>"; };
 		1B57E9891EB1606F0027AB30 /* XCTest.framework */ = {isa = PBXFileReference; lastKnownFileType = wrapper.framework; name = XCTest.framework; path = Platforms/iPhoneOS.platform/Developer/Library/Frameworks/XCTest.framework; sourceTree = DEVELOPER_DIR; };
 		1B667A0920127C1600A8CD5A /* StackOrchestrator+Store.swift */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.swift; path = "StackOrchestrator+Store.swift"; sourceTree = "<group>"; };
 		1B667A0B20127C7000A8CD5A /* StackOrchestratorPerformanceMetricsTracker.swift */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.swift; path = StackOrchestratorPerformanceMetricsTracker.swift; sourceTree = "<group>"; };
@@ -1454,7 +1454,7 @@
 		1B57E9861EB15F3C0027AB30 /* Analytics */ = {
 			isa = PBXGroup;
 			children = (
-				1B57E9871EB15F510027AB30 /* MultiTrackerTestCase.swift */,
+				1B57E9871EB15F510027AB30 /* Analytics+MultiTrackerTestCase.swift */,
 				0A708F6C20E99D9A001784DA /* MockAnalyticsTracker.swift */,
 			);
 			path = Analytics;
@@ -1842,7 +1842,7 @@
 				4838FE5723A951E6007311F0 /* TopConstrainableProxyTestCase.swift in Sources */,
 				0A708F6E20E99D9F001784DA /* MockAnalyticsTracker.swift in Sources */,
 				3E8D61952546F90400C08EA2 /* ConstraintGroupToggleTestCase.swift in Sources */,
-				0A266F8C1ED59FB6009CD0D7 /* MultiTrackerTestCase.swift in Sources */,
+				0A266F8C1ED59FB6009CD0D7 /* Analytics+MultiTrackerTestCase.swift in Sources */,
 				0A85F0E720B3177E0095AFFB /* PublicKeyAlgorithmTestCase.swift in Sources */,
 				0A266F901ED59FB6009CD0D7 /* Route+ComponentTests.swift in Sources */,
 				0A266F911ED59FB6009CD0D7 /* Route+TrieNode_AddTests.swift in Sources */,

Sources/Analytics/Trackers/Analytics+MultiTracker.swift

@@ -22,6 +22,15 @@ public extension Analytics {
             self.trackers = trackers
         }
 
+        /// Creates an analytics multi tracker instance.
+        /// - Parameter trackers: The result builder that outputs the analytics trackers to register.
+        public init(@TrackerBuilder trackers: () -> [AnyAnalyticsTracker<State, Action, ParameterKey>]) {
+
+            self.trackers = trackers()
+
+            assert(!self.trackers.isEmpty, "🙅‍♂️ Trackers shouldn't be empty, since it renders this tracker useless!")
+        }
+
         // MARK: - Tracking
 
         /// Tracks an analytics event, by propagating it to all the registered sub trackers.
@@ -33,3 +42,43 @@ public extension Analytics {
         }
     }
 }
+
+extension Analytics.MultiTracker {
+
+    @resultBuilder
+    public struct TrackerBuilder {
+
+        public typealias AnyAnalyticsTracker = Analytics.AnyAnalyticsTracker<State, Action, ParameterKey>
+
+        public static func buildExpression<Tracker: AnalyticsTracker>(_ tracker: Tracker) -> [AnyAnalyticsTracker]
+        where Tracker.State == State, Tracker.Action == Action, Tracker.ParameterKey == ParameterKey {
+
+            [tracker.eraseToAnyAnalyticsTracker()]
+        }
+
+        public static func buildExpression(_ tracker: AnyAnalyticsTracker) -> [AnyAnalyticsTracker] { [tracker] }
+
+        public static func buildExpression(_ trackers: [AnyAnalyticsTracker]) -> [AnyAnalyticsTracker] { trackers }
+
+        public static func buildBlock(_ trackers: [AnyAnalyticsTracker]...) -> [AnyAnalyticsTracker] {
+
+            trackers.flatMap { $0 }
+        }
+
+        public static func buildOptional(_ tracker: [AnyAnalyticsTracker]?) -> [AnyAnalyticsTracker] { tracker ?? [] }
+
+        public static func buildEither(first tracker: [AnyAnalyticsTracker]) -> [AnyAnalyticsTracker] { tracker }
+
+        public static func buildEither(second tracker: [AnyAnalyticsTracker]) -> [AnyAnalyticsTracker] { tracker }
+
+        public static func buildLimitedAvailability(_ tracker: [AnyAnalyticsTracker]) -> [AnyAnalyticsTracker] {
+
+            tracker
+        }
+
+        public static func buildArray(_ trackers: [[AnyAnalyticsTracker]]) -> [AnyAnalyticsTracker] {
+
+            trackers.flatMap { $0 }
+        }
+    }
+}

Sources/Logging/Loggers/Log+MultiLogger.swift

@@ -34,6 +34,41 @@ extension Log {
         /// The logger's log destination error callback closure.
         private let onError: LogDestinationErrorClosure?
 
+        /// Creates a new multi logger instance, with the specified log destinations and modules.
+        ///
+        /// - Note:
+        /// Module filtering works as follows:
+        ///
+        /// A log message having a module parameter will only be logged _if the module is registered_ in the logger, and
+        /// the log message's level is *above* the module's registered minimum log level. On the other hand, if the
+        /// message is logged without module (i.e. using the `Logger`'s `log` API, i.e. *without* `module` parameter),
+        /// no module filtering will be made.
+        ///
+        /// - Parameters:
+        ///   - modules: The log modules and respective minimum log level to be registered. Used when the
+        ///   `ModuleLogger` APIs are used (i.e. with `module` parameter).
+        ///   - onError: The logger's log destination error callback closure.
+        ///   - destinations: The result builder which outputs log destinations to forward logging events to.
+        public init(
+            modules: [Module: Log.Level] = [:],
+            onError: LogDestinationErrorClosure? = nil,
+            @DestinationBuilder destinations: () -> [AnyMetadataLogDestination<MetadataKey>]
+        ) {
+
+            self.modules = modules
+
+            self.onError = onError ?? { destination, error in
+                Log.internalLogger.error("💥 LogDestination '\(destination)' failed operation with error: \(error)")
+            }
+
+            self.destinations = destinations()
+
+            assert(
+                !self.destinations.isEmpty,
+                "🙅‍♂️ Destinations shouldn't be empty, since it renders this logger useless!"
+            )
+        }
+
         /// Creates a new multi logger instance, with the specified log destinations and modules.
         ///
         /// - Note:
@@ -209,3 +244,47 @@ extension Log {
         }
     }
 }
+
+extension Log.MultiLogger {
+
+    @resultBuilder
+    public struct DestinationBuilder {
+
+        public typealias AnyLogDestination = AnyMetadataLogDestination<MetadataKey>
+
+        public static func buildExpression<Destination: MetadataLogDestination>(
+            _ destination: Destination
+        ) -> [AnyLogDestination] where Destination.MetadataKey == MetadataKey {
+
+            [destination.eraseToAnyMetadataLogDestination()]
+        }
+
+        public static func buildExpression(_ destinations: AnyLogDestination) -> [AnyLogDestination] { [destinations] }
+
+        public static func buildExpression(_ destinations: [AnyLogDestination]) -> [AnyLogDestination] { destinations }
+
+        public static func buildBlock(_ destinations: [AnyLogDestination]...) -> [AnyLogDestination] {
+
+            destinations.flatMap { $0 }
+        }
+
+        public static func buildOptional(_ destinations: [AnyLogDestination]?) -> [AnyLogDestination] {
+
+            destinations ?? []
+        }
+
+        public static func buildEither(first destination: [AnyLogDestination]) -> [AnyLogDestination] { destination }
+
+        public static func buildEither(second destination: [AnyLogDestination]) -> [AnyLogDestination] { destination }
+
+        public static func buildLimitedAvailability(_ destination: [AnyLogDestination]) -> [AnyLogDestination] {
+
+            destination
+        }
+
+        public static func buildArray(_ destinations: [[AnyLogDestination]]) -> [AnyLogDestination] {
+
+            destinations.flatMap { $0 }
+        }
+    }
+}

Sources/PerformanceMetrics/Trackers/PerformanceMetrics+MultiTracker.swift

@@ -19,6 +19,20 @@ public extension PerformanceMetrics {
         /// The tracker's token dictionary, containing the mapping between internal and sub trackers' tokens.
         private let tokens = Atomic<[Token<Tag> : [Token<Tag>]]>([:])
 
+        /// Creates a new performance metrics multi trcker instance, with the specified sub trackers.
+        ///
+        /// - Parameters:
+        ///   -trackers: The result builder to output thje sub trackers to forward performance measuring events to.
+        public init(@TrackerBuilder trackers: () -> [PerformanceMetricsTracker]) {
+
+            self.trackers = trackers()
+
+            assert(
+                self.trackers.isEmpty == false,
+                "🙅‍♂️ Trackers shouldn't be empty, since it renders this tracker useless!"
+            )
+        }
+
         /// Creates a new performance metrics multi trcker instance, with the specified sub trackers.
         ///
         /// - Parameters:
@@ -144,3 +158,52 @@ public extension PerformanceMetrics {
 
     }
 }
+
+extension PerformanceMetrics.MultiTracker {
+
+    @resultBuilder
+    public struct TrackerBuilder {
+
+        public static func buildExpression(_ tracker: PerformanceMetricsTracker) -> [PerformanceMetricsTracker] {
+
+            [tracker]
+        }
+
+        public static func buildExpression(_ trackers: [PerformanceMetricsTracker]) -> [PerformanceMetricsTracker] {
+
+            trackers
+        }
+
+        public static func buildBlock(_ trackers: [PerformanceMetricsTracker]...) -> [PerformanceMetricsTracker] {
+
+            trackers.flatMap { $0 }
+        }
+
+        public static func buildOptional(_ tracker: [PerformanceMetricsTracker]?) -> [PerformanceMetricsTracker] {
+
+            tracker ?? []
+        }
+
+        public static func buildEither(first tracker: [PerformanceMetricsTracker]) -> [PerformanceMetricsTracker] {
+
+            tracker
+        }
+
+        public static func buildEither(second tracker: [PerformanceMetricsTracker]) -> [PerformanceMetricsTracker] {
+
+            tracker
+        }
+
+        public static func buildLimitedAvailability(
+            _ tracker: [PerformanceMetricsTracker]
+        ) -> [PerformanceMetricsTracker] {
+
+            tracker
+        }
+
+        public static func buildArray(_ trackers: [[PerformanceMetricsTracker]]) -> [PerformanceMetricsTracker] {
+
+            trackers.flatMap { $0 }
+        }
+    }
+}

Tests/AlicerceTests/Analytics/MultiTrackerTestCase.swift → Tests/AlicerceTests/Analytics/Analytics+MultiTrackerTestCase.swift

@@ -1,7 +1,7 @@
 import XCTest
 @testable import Alicerce
 
-final class MultiTrackerTestCase: XCTestCase {
+final class Analytics_MultiTrackerTestCase: XCTestCase {
 
     enum MockState {
         case screen(name: String)
@@ -22,6 +22,72 @@ final class MultiTrackerTestCase: XCTestCase {
     typealias MultiTracker = Analytics.MultiTracker<MockState, MockAction, MockParameterKey>
     typealias MockSubTracker = MockAnalyticsTracker<MockState, MockAction, MockParameterKey>
 
+    // init
+
+    func testInit_WithResultBuilder_ShouldInstantiateCorrectTrackers() {
+
+        let subTracker1 = MockSubTracker()
+        let subTracker2 = MockSubTracker()
+        let subTracker3 = MockSubTracker()
+        let subTracker4 = MockSubTracker()
+        let subTrackerOpt = MockSubTracker()
+        let subTrackerTrue = MockSubTracker()
+        let subTrackerFalse = MockSubTracker()
+        let subTrackerArray = (1...3).map { _ in MockSubTracker() }
+        let subTrackerAvailable = MockSubTracker()
+
+        let optVar: Bool? = true
+        let optNil: Bool? = nil
+        let trueVar = true
+        let falseVar = false
+
+        let tracker = MultiTracker {
+            subTracker1
+            subTracker2
+
+            subTracker3.eraseToAnyAnalyticsTracker()
+
+            [subTracker4].map { $0.eraseToAnyAnalyticsTracker() }
+
+            if let _ = optVar { subTrackerOpt }
+            if let _ = optNil { subTrackerOpt }
+
+            if trueVar {
+                subTrackerTrue
+            } else {
+                subTrackerFalse
+            }
+
+            if falseVar {
+                subTrackerTrue
+            } else {
+                subTrackerFalse
+            }
+
+            for tracker in subTrackerArray { tracker }
+
+            if #available(iOS 1.337, *) { subTrackerAvailable }
+        }
+
+        XCTAssertDumpsEqual(
+            tracker.trackers,
+            (
+                [
+                    subTracker1,
+                    subTracker2,
+                    subTracker3,
+                    subTracker4,
+                    subTrackerOpt,
+                    subTrackerTrue,
+                    subTrackerFalse
+                ]
+                + subTrackerArray
+                + [subTrackerAvailable]
+            )
+            .map { $0.eraseToAnyAnalyticsTracker() }
+        )
+    }
+
     // track
 
     func testTrack_WithActionEvent_ShouldCallTrackOnAlltrackers() {

Tests/AlicerceTests/Analytics/MockAnalyticsTracker.swift

@@ -9,7 +9,9 @@ final class MockAnalyticsTracker<S, A, PK: AnalyticsParameterKey>: AnalyticsTrac
 
     var trackInvokedClosure: ((Event) -> Void)?
 
-    init() {}
+    let id: UUID
+
+    init(id: UUID = .init()) { self.id = id }
 
     func track(_ event: Event) { trackInvokedClosure?(event) }
 }

Tests/AlicerceTests/Logging/Destinations/MockMetadataLogDestination.swift

@@ -20,10 +20,13 @@ class MockMetadataLogDestination<Module: LogModule, MetadataKey: Hashable>: Meta
 
     var minLevel: Log.Level { mockMinLevel }
 
+    let id: UUID
+
     // MARK: - Lifecycle
 
-    public init(mockMinLevel: Log.Level = .verbose) {
+    public init(id: UUID = .init(), mockMinLevel: Log.Level = .verbose) {
 
+        self.id = id
         self.mockMinLevel = mockMinLevel
     }