This Logger framework simplifies the use of Apple's OSLog and works as an aggregator for other frameworks.

Getting Started

To start using Logger, just add the Logger.framework to the project and you're ready to start.

Best practices

In general, we should:

  • Think critically about what we log and whether that log message is really required to debug in production
  • Log as little information as is possible while still facilitating client debugging (i.e. stick to text instead of full objects)

Under absolutely no circumstances should we ever log:

  • Any personal data (firstnames, surnames, phone number, email address etc.)
  • Any technical data that may uniquely identify a user (IMEI, cifId, IBAN, ip address etc.)
  • Full objects (as this can contain sensitive data)
  • Arbitrary API responses (these are sure to contain personal data)

Note: Even in testing it is forbidden to log any of the above data - forgetting to remove a line of code could potentially result in a data breach and result in fines and sanctions for our clients.

Severity levels

Logger supports five severity levels: Debug, Verbose, Info, Warning and Error.


Never saved, only appears in the console. Very granular, contains the most detail. Will include params, model attributes etc. Considered audience is developers only.


Very similar to Info. Verbose should be used over Info when the event is not considered a major event, it can also be viewed as an event that is more useful for a developer/tester i.e endpoint with headers etc.


Normal logging that’s part of the general operation/flow of the app. Something happened but it’s trivial. Consider the audience to be non technical. For example; button clicked, new user created, new payment, payment status change etc.


Something thats concerning but not causing the app to stop working. By analysing warning logs, we should get a good picture of the health/performance of the app etc. Examples include, network retry attempts, timeouts etc.


Error occurred but it was recoverable. App won’t crash but fix should be high priority.

Info v Debug/Verbose

It’s difficult to distinguish between Info and Debug/Verbose. A general rule of thumb is to use a debug/verbose if a message contains some dynamic data i.e the developer/tester will more than likely be interested in this. On the other hand use info if it is plain text.

In most cases an action/event will begin with an info log, describing the event and be followed with a verbose log detailing how its being done.

Example: Financial cards app creating a physical card

[Cards] INFO: Creating physical card
[Networking] VERBOSE: Request: (POST) https://api.com/card -> Body: User(name: John Doe)
[Networking] DEBUG: Headers: Authorization: Bearer Itul6OhXwiJzO
[Networking] VERBOSE: Response: (200) -> Card(id: 123, name: John Doe)
[Cards] INFO: Physical card created.

Note: This is an illustrative example only. Response data and tokens etc. should not be logged.

OSLog mapping

Apple's OSLog severity levels are a bit different than the usual. Below is a table with the mapping in the Logger:

Logger OSLog
debug debug
verbose info
info default
warning error
error fault



Import the Logger into your AppDelegate file.

import Logger

Hint: to make Logger available throughout your files without importing it everywhere, use typealias and rename Logger to AppLogger. Start using AppLogger everywhere 👌🏼

typealias AppLogger = Logger


To quickly start logging a certain severity level, just use the default category like the examples below.


Logger.default.debug("Your debug message here.")


Logger.default.verbose("Your verbose message here.")


Logger.default.info("Your info message here.")


Logger.default.warning("Your warning message here.")


Logger.default.error("Your error message here.")


In Logger categories require a new logger instance. Out of the box, Logger supports the default category by using Logger.default.

To create a new Logger with a category for ui you can do:

let uiLogger = Logger(category: "ui")

Instead of creating a categorized logger every time it's needed and to simplify the access throughout the app, you can extend the Logger with a static variable. Example:

extension Logger {

  static let ui = Logger(category: "ui")

You'll now be able to access it by using Logger.ui

Logging Entities

A logging entity is defined by the protocol LoggingEntity and allows the usage of external logger frameworks by registering them into the Logger.

There are usually two types of loggers in the most common logging frameworks. One is a static method based logger, that uses class static methods to log. The other one is a class based logger, that uses object methods to log.

Registering a static method logger

To support this type of loggers, we should wrap their logic into a wrapper that implements LoggingEntity.

struct StaticExternalLoggerWrapperEntity: LoggingEntity {

  // this is optional: create an init that configures the logger.
  init(apiKey: String) {
    StaticExternalLogger.register(apiKey: apiKey)

  func debug(_ log: String) {
    StaticExternalLogger.debug(message: log)

  func verbose(_ log: String) {
    StaticExternalLogger.verbose(message: log)

  func info(_ log: String) {
    StaticExternalLogger.info(message: log)

  func warning(_ log: String) {
    StaticExternalLogger.warning(message: log)

  func error(_ log: String) {
    StaticExternalLogger.error(message: log)

Note that the wrapper should also handle categories appropriately depending on the logger's definition of category.

We can now register the logger as a LoggingEntity of our Logger:

let externalLogger = StaticExternalLoggerWrapperEntity(apiKey: "API_KEY")

Registering a class based logger

This can be supported using the same approach as above, but can also be supported by extending the original class and making it conform with LoggingEntity.

extension ClassExternalLogger: LoggingEntity {

  public func debug(_ log: String) {

  public func verbose(_ log: String) {

  public func info(_ log: String) {

  public func warning(_ log: String) {

  public func error(_ log: String) {

Note that the example above uses different naming for each severity (e.g.: debug is d). If the logger uses the exact same names and parameters, all it has to be done is:

extension ClassExternalLogger: LoggingEntity { }

We register the logger as a LoggingEntity the same way:

// Initialization of the logger. Add any necessary steps
let externalLogger = ClassExternalLogger()

Note that the loggers have to be register for all the categories you have. Example:

extension Logger {

  static let ui = Logger(category: "ui")

  static let networking = Logger(category: "networking")

// ...

func setupLoggers() {

  // ui category
  let uiStaticExternalLogger = StaticExternalLoggerWrapperEntity(apiKey: "API_KEY_FOR_UI_CATEGORY")

  let uiClassExternalLogger = ClassExternalLogger()

  Logger.ui.registerEntities([uiStaticExternalLogger, uiClassExternalLogger])

  // service category
  let serviceStaticExternalLogger = StaticExternalLoggerWrapperEntity(apiKey: "API_KEY_FOR_SERVICE_CATEGORY")

  let serviceClassExternalLogger = ClassExternalLogger()

  Logger.ui.registerEntities([serviceStaticExternalLogger, serviceClassExternalLogger])


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Last updated: Tue Dec 06 2022 11:50:21 GMT-0500 (GMT-05:00)