Explore logging in Swift

2:44 - Example illustrating how to add logging to your app in three simple steps

// Add logging to your app in three simple steps
 
import os

let logger = Logger(subsystem: "com.example.Fruta", category: "giftcards")

func beginTask(url: URL, handler: (Data) -> Void) {
    launchTask(with: url) {
       handler($0)
    }

    logger.log("Started a task")
}

3:32 - An example code that logs a message with run-time data

// Add runtime data to the log messsage using string interpolation

import os

let logger = Logger(subsystem: "com.example.Fruta", category: "giftcards")

func beginTask(url: URL, handler: (Data) -> Void) {
    launchTask(with: url) {
       handler($0)
    }
    logger.log("Started a task \(taskId)")
}

4:28 - Example illustrating why nonnumeric types are redacted in the logs by default

logger.log("Paid with bank account \(accountNumber)")

5:01 - Code that shows how to mark public data so that it is displayed in the logs

logger.log("Ordered smoothie \(smoothieName, privacy: .public)")

6:03 - Code shown during first demo

import SwiftUI
import os

let logger = Logger(subsystem: "com.example.Fruta", category: "giftcards")

struct GiftCardView: View {
    // Denotes whether there is an active task for loading gift cards.
    @State private var taskRunning: Bool = false
    
    // A UUID that uniquely identifies a task.
    @State private var currentTaskID: UUID = UUID()
    
    // An unrecoverable error seen during execution.
    @State private var error: Error? = nil

    // A model that stores information about gift cards.
    @ObservedObject var model: GiftCardModel
    
    var body: some View {
        // Display a list of gifts which can be tapped on and scrolled through.
        GiftCardList(model: model, taskRunning: $taskRunning, currentTaskID: $currentTaskID, error: $error, downloadAction: beginTask, stopAction: endTask)
            .navigationTitle("Gift Cards")
    }
    
    // Start a task to download gift cards from a server.
    func beginTask(serverURL: URL, cardDownloadHandler: @escaping (Data) -> Void) {
        logger.log("Starting a new task for loading cards \(currentTaskID, privacy: .public)")        
        launchTask(with: serverURL) {
            cardDownloadHandler($0)
        }
    }
    
    // Stop the currently running task for downloading cards from a server.
    func endTask() {
        guard taskRunning else {
            logger.fault("Task \(currentTaskID, privacy: .public) is not runinng, cannot be stopped!")
            error = TaskError.noActiveTask
            return
        }
        taskRunning = false
        logger.log("Task \(currentTaskID, privacy: .public) interrupted")
    }
    
    // Start a URLSession dataTask with the given URL.
    func launchTask(with url: URL, handler: @escaping (Data) -> Void) {
        guard error == nil else {
            return
        }
        taskRunning = true
        let task = URLSession.shared.dataTask(with: url) {
            data, response, error in
            if let error = error {
                self.error = ConnectionError.other(error)
            }
            if let data = data {
                handler(data)
            }
        }
        task.resume()
    }

}

11:51 - Illustration of how debug-level logging will not evaluate the code that constructs log message

logger.debug("\(slowFunction(data))")

12:58 - Code that shows how to display run-time data with a fixed width and how to set precision of a floating-point value using formatting options

import SwiftUI
import os

let statisticsLogger = Logger(subsystem: "com.example.Fruta", category: "statistics")

// Log statistics about communication with a server.
func logStatistics(taskID: UUID, giftCardID: String, serverID: Int, seconds: Double) {
    statisticsLogger.log("\(taskID) \(giftCardID, align: .left(columns: GiftCard.maxIDLength)) \(serverID) \(seconds, format: .fixed(precision: 2))")
}

15:00 - Example of formatting log messages

logger.log("\(data, format: .hex, align: .right(columns: width))")

16:05 - Example illustrating the use of a hash to redact private data, which ensures that identical data get the same hash

logger.log("Paid with bank account: \(accountNumber, privacy: .private(mask: .hash))")