Initial implementation

.gitignore

@@ -0,0 +1 @@
+.build

Package.resolved

@@ -0,0 +1,15 @@
+{
+  "originHash" : "041d5a9b2664fe8fba9d0ff5826e7f1955cca0b22d288fe2f2383df27258b52a",
+  "pins" : [
+    {
+      "identity" : "swift-log",
+      "kind" : "remoteSourceControl",
+      "location" : "https://github.com/apple/swift-log",
+      "state" : {
+        "revision" : "bbd81b6725ae874c69e9b8c8804d462356b55523",
+        "version" : "1.10.1"
+      }
+    }
+  ],
+  "version" : 3
+}

Package.swift

@@ -0,0 +1,31 @@
+// swift-tools-version: 6.0
+import PackageDescription
+
+let package = Package(
+	name: "swift-log-oslog",
+	platforms: [.macOS(.v11), .iOS(.v14), .tvOS(.v14), .watchOS(.v6)],
+	products: [
+		.library(
+			name: "LoggingOSLog",
+			targets: ["LoggingOSLog"]
+		)
+	],
+	dependencies: [
+		.package(url: "https://github.com/apple/swift-log", from: "1.10.1"),
+	],
+	targets: [
+		.target(
+			name: "LoggingOSLog",
+			dependencies: [.product(name: "Logging", package: "swift-log")],
+			path: "Sources/Logging OSLog"
+		),
+		.testTarget(
+			name: "LoggingOSLogTests",
+			dependencies: [
+				.product(name: "Logging", package: "swift-log"),
+				"LoggingOSLog"
+			],
+			path: "Tests/Logging OSLog Tests",
+		),
+	]
+)

Sources/Logging OSLog/Logging OSLog.swift

@@ -0,0 +1,75 @@
+import Foundation
+import os
+import Logging
+
+public struct LoggingOSLog: LogHandler {
+	public var logLevel: Logging.Logger.Level = .info
+	public var metadata: Logging.Logger.Metadata = [:]
+	public var metadataProvider: Logging.Logger.MetadataProvider?
+	private let oslogger: os.Logger
+
+	public init(label: String) {
+		let lastDotPosition = label.lastIndex(of: ".") ?? label.startIndex
+		let frontPart = label.prefix(upTo: lastDotPosition)
+		let backPart = label.suffix(from: lastDotPosition)
+		guard frontPart.count > 0 else {
+			let subsystem = Bundle.main.bundleIdentifier ?? "SwiftLogToOsLog"
+			self.init(subsystem: subsystem, category: label)
+			return
+		}
+		self.init(subsystem: String(frontPart), category: String(backPart))
+	}
+
+	public init(
+		subsystem: String = Bundle.main.bundleIdentifier ?? "SwiftLogToOsLog",
+		category: String,
+		logLevel: Logging.Logger.Level = .debug
+	) {
+		self.oslogger = os.Logger(subsystem: subsystem, category: category)
+		self.logLevel = logLevel
+	}
+
+	public init(oslog: os.Logger, logLevel: Logging.Logger.Level = .debug) {
+		self.oslogger = oslog
+		self.logLevel = logLevel
+	}
+
+	public func log(
+		level: Logging.Logger.Level,
+		message: Logging.Logger.Message,
+		metadata: Logging.Logger.Metadata?,
+		source: String,
+		file: String,
+		function: String,
+		line: UInt
+	) {
+		let metadataCSV = Self.joinMetadata(self.metadata, self.metadataProvider?.get(), metadata)
+		let messageParts = [message.description, metadataCSV]
+
+		let message = messageParts.compactMap { $0 }.joined(separator: " -> ")
+		self.oslogger.log(level: OSLogType.from(loggerLevel: level), "\(message)")
+	}
+
+	public subscript(metadataKey metadataKey: String) -> Logging.Logger.Metadata.Value? {
+		get {
+			return self.metadata[metadataKey]
+		}
+		set {
+			self.metadata[metadataKey] = newValue
+		}
+	}
+	
+	private static func joinMetadata(_ metadataList: Logging.Logger.Metadata?...) -> String? {
+		var metadataAggregator: Logging.Logger.Metadata = [:]
+		for metadata in metadataList {
+			guard let metadata = metadata else { continue }
+			metadataAggregator.merge(metadata) { return $1 }
+		}
+		return Self.joinMetadata(metadataAggregator)
+	}
+
+	private static func joinMetadata(_ metadata: Logging.Logger.Metadata, with separator: String = ", ") -> String? {
+		guard !metadata.isEmpty else { return nil }
+		return metadata.map { "\($0) = \($1)" }.joined(separator: separator)
+	}
+}

Sources/Logging OSLog/OSLog Extension.swift

@@ -0,0 +1,23 @@
+import Logging
+import os
+
+extension OSLogType {
+	static func from(loggerLevel: Logging.Logger.Level) -> Self {
+		switch loggerLevel {
+		case .trace:
+			return .debug
+		case .debug:
+			return .debug
+		case .info:
+			return .info
+		case .notice:
+			return .default
+		case .warning:
+			return .info
+		case .error:
+			return .error
+		case .critical:
+			return .fault
+		}
+	}
+}

Tests/Logging OSLog Tests/Logging Test.swift

@@ -0,0 +1,16 @@
+import Testing
+import Logging
+import LoggingOSLog
+
+@Test func isConstructable() {
+	let _ = LoggingOSLog(subsystem: "de.astzweig.app", category: "Image Processing")
+}
+
+@Test func canAddAsLoggingBackend() {
+	LoggingSystem.bootstrap(LoggingOSLog.init)
+}
+
+@Test func canLogMessage() {
+	let logger = Logging.Logger(label: "de.astzweig.loggingoslog.Test")
+	logger.info("Test message")
+}