Episode #344

Using os_log to Log Messages

Series: Unified Logging and Activity Tracing

12 minutes
Published on July 3, 2018

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In this episode we show how you adopt the Unified Logging framework in code by using the OSLog type to define your log subsystems and categories, and how to use os_log to actually log events and messages.

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Defining Shared OSLog instances

You will likely want to log statements and reuse the same subsystem and category throughout your application. For this, you can define a struct with a static member:

import os.log

struct Log {
    static var general = OSLog(subsystem: "com.myapp.my_target", category: "general")
}

Logging Messages

Then you can use the shared log instance to log messages with os_log:

os_log("The app did something interesting", 
    log: Log.general,
    type: .info)

Including values in your log messages

We can include values in our log messages by using one of the built-in formatting type specifiers:

%dIntegers
%.2fFloats
%sStrings
%@Objects

Sometimes we may want to give the system some information on how we want these values to be interpreted and formatted on display. One example is using time_t to get a formatted date to be displayed:

os_log("The record was modified at %{time_t}d", 
    log: Log.general, 
    type: .info, 
    time_t(date.timeIntervalSince1970))

This episode uses Xcode 10.0-beta2, Swift 4.2.