Logging in Swift Applications: Best Practices

Introduction: Why is logging needed in Swift?

Logging is a fundamental developer tool that allows you to track the application state, identify errors, and analyze user behavior. In the Swift ecosystem (iOS/macOS), properly configured logging can save hours of debugging and help avoid problems in production versions.

In this article, we will cover the best practices for logging in Swift: from Apple's built-in solutions to popular third-party libraries. You will learn how not to overload the application, how to protect sensitive data, and how to make logs useful for the entire team.

1. Built-in tools: OSLog and print

1.1 Why print() is bad

The simplest logging method is the print() command. However, it is not recommended for use in production versions:

• No severity levels (info, warning, error).
• Impossible to filter logs in the Xcode console.
• Text is output even in release builds (if not manually removed).
• No data structuring.

1.2 OSLog — the standard from Apple

Starting with iOS 10 and macOS 10.12, Apple recommends using os_log from the os.log framework. This is a built-in mechanism that is fast and secure.

import os.log

let logger = Logger(subsystem: "com.example.myapp", category: "network")

func fetchData() {    logger.info("Start of data loading")    // ... loading code    logger.warning("Server responded with a delay")    // ... error handling    logger.error("Failed to connect to server: \\(error.localizedDescription)")}

Advantages of OSLog:

• Automatic disabling in release builds (except for errors).
• Support for categories and subsystems for filtering.
• Integration with Console.app and Xcode.
• Security: private data is automatically hidden (marker private).

2. SwiftLog — a universal API

2.1 What is SwiftLog?

SwiftLog is an open-source library from Apple that provides a unified interface for logging. It is platform-independent and allows connecting different backends (OSLog, files, servers).

2.2 Integration and setup

Add SwiftLog via Swift Package Manager:

// Package.swiftdependencies: [    .package(url: "https://github.com/apple/swift-log.git", from: "1.5.0")]

Usage example:

import Logging

let logger = Logger(label: "com.example.app")

logger.info("Application launched")logger.warning("Low battery: \\(batteryLevel)%")logger.error("Critical error: \\(error)")

// Configuring the log levelLoggingSystem.bootstrap { label in    var handler = StreamLogHandler.standardOutput(label: label)    handler.logLevel = .debug    return handler}

Key features:

• Levels: trace, debug, info, notice, warning, error, critical.
• Metadata (any additional fields).
• Easily replaceable backend (e.g., for sending to Crashlytics).
• Zero cost when the level is disabled (faster than print).

3. Third-party libraries: CocoaLumberjack and Pulse

3.1 CocoaLumberjack — flexibility and power

One of the most popular libraries for Objective-C and Swift. It allows writing logs to files, console, remote server, and even a database.

import CocoaLumberjack

// SetupDDLog.add(DDOSLogger.sharedInstance) // Consolelet fileLogger = DDFileLogger()fileLogger.rollingFrequency = 60 * 60 * 24 // 24 hoursDDLog.add(fileLogger)

// UsageDDLogInfo("User authorized")DDLogWarn("Network timeout")DDLogError("