PlaygroundHandler

Demo.swift

import Logging

LoggingSystem.bootstrap { label in
    var handler = PlaygroundHandler(label: label)
    handler.logLevel = .trace
    return handler
}

let logger = Logger(label: "hoge")

logger.debug("hoge")
logger.error("hogeeeeeeeee")
logger.critical("hoge")

PlaygroundHandler.swift

import Foundation
import Logging

public struct PlaygroundHandler {
    private let label: String
    private var prettyMetadata: String?
    public var metadata = Logger.Metadata() {
        didSet {
            self.prettyMetadata = self.prettify(self.metadata)
        }
    }
    public var logLevel = Logger.Level.info

    public init(label: String) {
        self.label = label
    }

    private func prettify(_ metadata: Logger.Metadata) -> String? {
        !metadata.isEmpty
            ? metadata.lazy.sorted(by: { $0.key < $1.key }).map { "\($0)=\($1)" }.joined(separator: " ")
            : nil
    }

    private func timestamp() -> String {
        var buffer = [Int8](repeating: 0, count: 255)
        var timestamp = time(nil)
        let localTime = localtime(&timestamp)
        strftime(&buffer, buffer.count, "%Y-%m-%dT%H:%M:%S%z", localTime)
        return buffer.withUnsafeBufferPointer {
            $0.withMemoryRebound(to: CChar.self) {
                String(cString: $0.baseAddress!)
            }
        }
    }
}

extension PlaygroundHandler: LogHandler {
    public func log(level: Logger.Level, message: Logger.Message, metadata: Logger.Metadata?, source: String, file: String, function: String, line: UInt) {
        let prettyMetadata = metadata?.isEmpty ?? true
            ? self.prettyMetadata
            : self.prettify(self.metadata.merging(metadata!, uniquingKeysWith: { _, new in new }))
        print("\(self.timestamp()) \(level) \(self.label) :\(prettyMetadata.map { " \($0)" } ?? "") \(message)")
    }

    public subscript(metadataKey metadataKey: String) -> Logger.Metadata.Value? {
        get {
            return self.metadata[metadataKey]
        }
        set(newValue) {
            self.metadata[metadataKey] = newValue
        }
    }
}