Nike-Inc / Willow

Willow is a powerful, yet lightweight logging library written in Swift.


Build Status CocoaPods Compatible Carthage Compatible Platform

Willow is a powerful, yet lightweight logging library written in Swift.


  • Default Log Levels
  • Custom Log Levels
  • Simple Logging Functions using Closures
  • Configurable Synchronous or Asynchronous Execution
  • Thread-Safe Logging Output (No Log Mangling)
  • Custom Formatters through Dependency Injection per Log Level
  • Customizable Color Formatting for Console Output
  • Custom Writers through Dependency Injection per Log Level
  • Supports Multiple Simultaneous Writers
  • Shared Loggers Between Frameworks
  • Shared Locks or Queues Between Multiple Loggers
  • Comprehensive Unit Test Coverage
  • Complete Documentation


  • iOS 8.0+ / Mac OS X 10.10+ / tvOS 9.0+ / watchOS 2.0+
  • Xcode 7.3+


  • Need help? Open an issue.
  • Have a feature request? Open an issue.
  • Find a bug? Open an issue.
  • Want to contribute? Fork the repo and submit a pull request.



CocoaPods is a dependency manager for Cocoa projects. You can install it with the following command:

[sudo] gem install cocoapods

CocoaPods 1.0+ is required.

To integrate Willow into your project, specify it in your Podfile:

source ''
platform :ios, '9.0'

pod 'Willow', '~> 1.0'

Then, run the following command:

$ pod install


Carthage is a decentralized dependency manager that builds your dependencies and provides you with binary frameworks.

You can install Carthage with Homebrew using the following command:

brew update
brew install carthage

To integrate Willow into your Xcode project using Carthage, specify it in your Cartfile:

github "Nike-Inc/Willow" ~> 1.0

Run carthage update to build the framework and drag the built Willow.framework into your Xcode project.


Creating a Logger

import Willow
let defaultLogger = Logger()

The Logger initializer takes a single parameter which is a LoggerConfiguration. If no configuration is provided, the Logger will internally initialize a LoggerConfiguration with default parameters.

Logger Configuration

The LoggerConfiguration class is a container class to store all the configuration information to be applied to a particular Logger. Here are all the configurable parameters and their respective descriptions.

  • formatters: [LogLevel: [Formatter]] = [:] - The dictionary of formatters to apply to each associated log level.
  • writers: [LogLevel: [Writer]] = [.All: [ConsoleWriter()] - The dictionary of writers to write to for the associated log level. Writers can be used to log output to a specific destination such as the console or to a file.
  • executionMethod: ExecutionMethod = .Synchronous(lock: NSRecursiveLock()) - The execution method used when writing messages.

LoggerConfiguration and Logger objects can only be customized during initialization. If you need to change a Logger at runtime, it is advised to create an additional logger with a custom configuration to fit your needs. It is perfectly acceptable to have many different Logger instances running simutaneously.

There are two class methods that return custom LoggerConfiguration instances using combinations of the custom Formatter objects. These convenience methods make it VERY easy add timestamps as well as colored log message formatting to your Logger instance.

  • timestampConfiguration() - A logger configuration instance with a timestamp formatter applied to each log level.
  • coloredTimestampConfiguration() - A logger configuration instance with a timestamp and color formatter applied to each log level.

Thread Safety

The println function does not guarantee that the String parameter will be fully logged to the console. If two println calls are happening simultaneously from two different queues (threads), the messages can get mangled, or intertwined. Willow guarantees that messages are completely finished writing before starting on the next one.

It is important to note that by creating multiple Logger instances, you can potentially lose the guarantee of thread-safe logging. If you want to use multiple Logger instances, you should create a NSRecursiveLock or dispatch_queue_t that is shared between both configurations. For more info...see the Advanced Usage section.

Logging Messages with Closures

The logging syntax of Willow was optimized to make logging as lightweight and easy to remember as possible. Developers should be able to focus on the task at hand and not remembering how to write a log message.

Single Line Closures

let log = Logger()

log.debug { "Debug Message" }
// Debug Message { "Info Message" }
// Info Message
log.event { "Event Message" }
// Event Message
log.warn { "Warn Message" }
// Warn Message
log.error { "Error Message" }
// Error Message

The single line closure does not require a return declaration since it is implied in Swift. This makes it very easy to declare a closure. There are some VERY important performance considerations which is why Willow only accepts closures for all the Logger convenience methods. See the Closure Performance section for more information.

By default, only the String returned by the closure will be logged. See the Formatters section for more information about customizing log message formats.

Multi-Line Closures

Logging a message is easy, but knowing when to add the logic necessary to build a log message and tune it for performance can be a bit tricky. We want to make sure logic is encapsulated and very performant. Willow log level closures allow you to cleanly wrap all the logic to build up the message.

log.debug {
    // First let's run a giant for loop to collect some info
    // Now let's scan through the results to get some aggregate values
    // Now I need to format the data
    return "Computed Data Value: \(dataValue)"
} {
    let countriesString = ",".join(countriesArray)
    return "Countries: \(countriesString)"

Unlike the Single Line Closures, the Multi-Line Closures require a return declaration.

Closure Performance

Willow works exclusively with logging closures to ensure the maximum performance in all situations. Closures defer the execution of all the logic inside the closure until absolutely necessary, including the string evaluation itself. In cases where the Logger instance is disabled, log execution time was reduced by 97% over the traditional log message methods taking a String parameter. Additionally, the overhead for creating a closure was measured at 1% over the traditional method making it negligible. In summary, closures allow Willow to be extremely performant in all situations.

Unfortunately, it is not possible to utilize @autoclosure in this scenario. Swift 1.1 allows @autoclosure declaration, but Swift 1.2+ does not, due to the way Willow supports Synchronous and Asynchronous logging. The Swift 1.2 @autoclosure declaration implies a @noescape which conflicts with the internal dispatch queue.

Disabling a Logger

The Logger class has an enabled property to allow you to completely disable logging. This can be helpful for turning off specific Logger objects at the app level, or more commonly to disable logging in a third-party library.

let log = Logger()
log.enabled = false

// No log messages will get sent to the registered Writers

log.enabled = true

// We're back in business...

Synchronous and Asynchronous Logging

Logging can greatly affect the runtime performance of your application or library. Willow makes it very easy to log messages synchronously or asynchronously. You can define this behavior when creating the LoggerConfiguration for your Logger instance.

let queue = dispatch_queue_create("serial.queue", DISPATCH_QUEUE_SERIAL)
let configuration = LoggerConfiguration(executionMethod: .Asynchronous(queue: queue))
let log = Logger(configuration: configuration)

Synchronous Logging

Synchronous logging is very helpful when you are developing your application or library. The log operation will be completed before executing the next line of code. This can be very useful when stepping through the debugger. The downside is that this can seriously affect performance if logging on the main thread.

Asynchronous Logging

Asynchronous logging should be used for deployment builds of your application or library. This will offload the logging operations to a separate dispatch queue that will not affect the performance of the main thread. This allows you to still capture logs in the manner that the Logger is configured, yet not affect the performance of the main thread operations.

These are large generalizations about the typical use cases for one approach versus the other. Before making a final decision about which approach to use when, you should really break down your use case in detail.


Log message customization is something that Willow specializes in. Some devs want to add a prefix to their library output, some want different timestamp formats, some even want colors! There's no way to predict all the types of custom formatting teams are going to want to use. This is where Formatter objects come in.

public protocol Formatter {
    func formatMessage(message: String, logLevel: LogLevel) -> String

The Formatter protocol has only a single API. It receives the message and logLevel and returns a newly formatted String. This is about as flexible as you can get. The Logger allows you to pass in your own Formatter objects and apply them to a LogLevel. Let's walk through a simple example for adding a prefix to only the Debug and Info log levels.

class PrefixFormatter: Formatter {
    func formatMessage(message: String, logLevel: Logger.LogLevel) -> String {
        return "[Willow] \(message)"

let prefixFormatter = PrefixFormatter()

let formatters: [Logger.LogLevel: [Formatter]] = [
    .Debug: [prefixFormatter],
    .Info: [prefixFormatter]

let configuration = LoggerConfiguration(formatters: formatters)
let log = Logger(configuration: configuration)

Formatter objects are very powerful and can manipulate the message in any way.

Color Formatters

There is a special Formatter in Willow called a ColorFormatter. It was designed to take a foreground and backround color in the form of a UIColor or NSColor. It then formats the message to match the coloring scheme of the XcodeColors plugin. This allows you to change the foreground and background colors of logging output in the Xcode console. This can make it much easier to dig through thousands of lines of logging output.

let purple = UIColor.purpleColor()
let blue = UIColor.blueColor()
let green = UIColor.greenColor()
let orange = UIColor.orangeColor()
let red = UIColor.redColor()
let white = UIColor.whiteColor()
let black = UIColor.blackColor()

let colorFormatters: [Logger.LogLevel: [Formatter]] = [
    LogLevel.Debug: [ColorFormatter(foregroundColor: purple, backgroundColor: nil)],
    LogLevel.Info: [ColorFormatter(foregroundColor: blue, backgroundColor: nil)],
    LogLevel.Event: [ColorFormatter(foregroundColor: green, backgroundColor: nil)],
    LogLevel.Warn: [ColorFormatter(foregroundColor: black, backgroundColor: orange)],
    LogLevel.Error: [ColorFormatter(foregroundColor: white, backgroundColor: red)]

let configuration = LoggerConfiguration(formatters: colorFormatters)
let log = Logger(configuration: configuration)

The XcodeColors plugin is available through Alcatraz and can be installed with the click of a button.

Multiple Formatters

Multiple Formatter objects can be stacked together onto a single log level to perform multiple actions. Let's walk through using the TimestampFormatter (prefixes the message with a timestamp) in combination with the ColorFormatter objects from the previous example.

let purple = UIColor.purpleColor()
let blue = UIColor.blueColor()
let green = UIColor.greenColor()
let orange = UIColor.orangeColor()
let red = UIColor.redColor()
let white = UIColor.whiteColor()
let black = UIColor.blackColor()

let timestampFormatter = TimestampFormatter()

let formatters: [Logger.LogLevel: [Formatter]] = [
    LogLevel.Debug: [timestampFormatter, ColorFormatter(foregroundColor: purple, backgroundColor: nil)],
    LogLevel.Info: [timestampFormatter, ColorFormatter(foregroundColor: blue, backgroundColor: nil)],
    LogLevel.Event: [timestampFormatter, ColorFormatter(foregroundColor: green, backgroundColor: nil)],
    LogLevel.Warn: [timestampFormatter, ColorFormatter(foregroundColor: black, backgroundColor: orange)],
    LogLevel.Error: [timestampFormatter, ColorFormatter(foregroundColor: white, backgroundColor: red)]

let configuration = LoggerConfiguration(formatters: formatters)
let log = Logger(configuration: configuration)

Willow doesn't have any hard limits on the total number of Formatter objects that can be applied to a single log level. Just keep in mind that performance is key.

The default ConsoleWriter will execute the formatters in the same order they were added into the Array. In the previous example, Willow would log a much different message if the ColorFormatter was inserted before the TimestampFormatter.


Writing log messages to various locations is an essential feature of any robust logging library. This is made possible in Willow through the Writer protocol.

public protocol Writer {
    func writeMessage(message: String, logLevel: LogLevel, formatters: [Formatter]?)

Again, this is an extremely lightweight design to allow for ultimate flexibility. As long as your Writer classes conform, you can do anything with those log messages that you want. You could write the message to the console, append it to a file, send it to a server, etc. Here's a quick look at the implementation of the default ConsoleWriter created by the Logger if you don't specify your own.

public class ConsoleWriter: Writer {
    public func writeMessage(message: String, logLevel: LogLevel, formatters: [Formatter]?) {
        var mutableMessage = message
        formatters?.map { mutableMessage = $0.formatMessage(mutableMessage, logLevel: logLevel) }

Multiple Writers

So what about logging to both a file and the console at the same time? No problem. You can pass multiple Writer objects into the Logger initializer. The Logger will execute each Writer in the order it was passed in. For example, let's create a FileWriter and combine that with our ConsoleWriter.

public class FileWriter: Writer {
    public func writeMessage(var message: String, logLevel: Logger.LogLevel, formatters: [Formatter]?) {
        var mutableMessage = message
        formatters?.map { mutableMessage = $0.formatMessage(mutableMessage, logLevel: logLevel) }
        // Write the formatted message to a file (I'll leave this to you!)

let writers: [LogLevel: Writer] = [.All: [FileWriter(), ConsoleWriter()]]

let configuration = LoggerConfiguration(writers: writers)
let log = Logger(configuration: configuration)

Writer objects can also be selective about which formatters they want to run for a particular log level. All the examples run all the formatters, but you can be selective if you want to be.

Per LogLevel Writers

It is also possible to specify different combinations of Writer objects for each LogLevel. Let's say we want to log .Warn and .Error messages to the console, and we want to log all messages to a file writer.

let writers: [LogLevel: Writer] = [
    .All: [FileWriter()],
    [.Warn, .Error]: [ConsoleWriter()]

let configuration = LoggerConfiguration(writers: writers)
let log = Logger(configuration: configuration)

Advanced Usage

Creating Custom Log Levels

Depending upon the situation, the need to support additional log levels may arise. Willow can easily support additional log levels through the art of bitmasking. Since the internal RawValue of a LogLevel is a UInt, Willow can support up to 32 log levels simultaneously for a single Logger. Since there are 7 default log levels, Willow can support up to 27 custom log levels for a single logger. That should be more than enough to handle even the most complex of logging solutions.

Creating custom log levels is very simple. Here's a quick example of how to do so. First, you must create a LogLevel extension and add your custom values.

extension LogLevel {
    private static let Verbose = LogLevel(rawValue: 0b00000000_00000000_00000001_00000000)

Now that we have a custom log level called Verbose, we need to extend the Logger class to be able to easily call it.

extension Logger {
    public func verbose(message: () -> String) {
        logMessage(message, withLogLevel: .Verbose)

Finally, using the new log level is a simple as...

let log = Logger()
log.verbose { "My first verbose log message!" }

The All log level contains a bitmask where all bits are set to 1. This means that the All log level will contain all custom log levels automatically.

Shared Loggers between Frameworks

Defining a single Logger and sharing that instance several frameworks could be very advantageous. Especially with the addition of Frameworks in iOS 8. Now that we're going to be creating more frameworks inside our own apps to be shared between apps, extensions and third party libraries, wouldn't it be nice if we could share Logger instances?

Let's walk through a quick example of a Math framework sharing a Logger with it's parent Calculator app.

//=========== Inside Math.swift ===========
public var log = Logger(configuration: LoggerConfiguration(writers: [.Warn, .Error]: [ConsoleWriter()])) // We're going to replace this

//=========== Calculator.swift ===========
import Math

let writers = [.All: [FileWriter(), ConsoleWriter()]]
var log = Logger(configuration: LoggerConfiguration(writers: writers))

// Replace the Math.log with the Calculator.log to share the same Logger instance
Math.log = log

It's very simple to swap out a pre-existing Logger with a new one.

Multiple Loggers, One Queue

The previous example showed how to share Logger instances between multiple frameworks. Something more likely though is that you would want to have each third party library or internal framework to have their own Logger with their own configuration. The one thing that you really want to share is the NSRecursiveLock or dispatch_queue_t that they run on. This will ensure all your logging is thread-safe. Here's the previous example demonstrating how to create multiple Logger instances with different configurations and share the queue.

//=========== Inside Math.swift ===========
public var log = Logger(configuration: LoggerConfiguration(writers: [.Warn, .Error]: [ConsoleWriter()])) // We're going to replace this

//=========== Calculator.swift ===========
import Math

// Create a single queue to share
let sharedQueue = dispatch_queue_create("com.math.logger", DISPATCH_QUEUE_SERIAL)

// Create the Calculator.log with multiple writers and a .Debug log level
let writers = [.All: [FileWriter(), ConsoleWriter()]]
let configuration = LoggerConfiguration(
    writers: writers, 
    executionMethod: .Asynchronous(queue: sharedQueue)

var log = Logger(configuration: configuration)

// Replace the Math.log with a new instance with all the same configuration values except a shared queue
let mathConfiguration = LoggerConfiguration(
    formatters: Math.log.configuration.formatters,
    writers: Math.log.configuration.writers,
    executionMethod: .Asynchronous(queue: sharedQueue)

Math.log = Logger(configuration: mathConfiguration)

Willow is a very lightweight library, but its flexibility allows it to become very powerful if you so wish.


Why 5 default log levels? And why are they so named?

Simple...simplicity and elegance. Contextually it gets difficult to understand which log level you need if you have too many. However, that doesn't mean that this is always the perfect solution for everyone or every use case. This is why there are 5 default log levels, with support for easily adding additional ones.

As for the naming, here's my own mental breakdown of each log level for an iOS app (obviously it depends on your use case).

  • Debug - Highly detailed information of a context
  • Info - Summary information of a context
  • Event - User driven interactions such as button taps, view transitions, selecting a cell
  • Warn - An error occurred but it is recoverable
  • Error - A non-recoverable error occurred

When should I use Willow?

If you are starting a new iOS project in Swift and want to take advantage of many new conventions and features of the language, Willow would be a great choice. If you are still working in Objective-C, a pure Objective-C library such as the amazing CocoaLumberjack would probably be more appropriate.

Where did the name Willow come from?

Willow is named after the one, the only, Willow tree.


Willow is released under the MIT license. See LICENSE for details.


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