Using os_log to log function arguments, or other dynamic data

Question

I'm trying to log function arguments into os_log like this:

func foo(x: String, y: [String:String]) {
    //...
    os_log("foo: \(x) \(y.description)", log: OSLog.default, type: .debug)
}

But getting error:

Cannot convert value of type 'String' to expected argument type 'StaticString'

So how can I log function arguments, or any other dynamic data?

Answer 1

See Logging:

Formatting Log Messages

To format a log message, use a standard NSString or printf format string, ...

and String Format Specifiers for the standard format string specifiers, such as %@ and %d.

In your case:

os_log("foo: %@ %@", log: .default, type: .debug, x, y.description)

The format string is restricted to static strings to prevent (unintentional) expansion of format string specifiers. Here is an example demonstrating the problem, using NSLog() because that does not restrict the format to constant strings:

let s = "50%"
NSLog("\(s)percent")
// Output: 500x0ercent

The %p expects a pointer on the variable argument list, which is not provided. This is undefined behavior, it can lead to crashes or unexpected output.

Answer 2

This is my approach:

import Foundation
import os.log

struct Log {
    enum LogLevel: String {
        case error = "⛔️"
        case warning = "⚠️"
        case debug = "💬"
    }

    static func debug(_ info: String, level: LogLevel = .debug, file: String = #file, function: String = #function, line: Int = #line) {
        os_log("%@ %@:%d %@: %@", type: .default, level.rawValue, (file as NSString).lastPathComponent, line, function, info)
    }

    static func warning(_ info: String, level: LogLevel = .warning, file: String = #file, function: String = #function, line: Int = #line) {
        os_log("%@ %@:%d %@: %@", type: .default, level.rawValue, (file as NSString).lastPathComponent, line, function, info)
    }

    static func error(_ error: NSError, level: LogLevel = .error, file: String = #file, function: String = #function, line: Int = #line) {
        os_log("%@ %@:%d %@: %@", type: .default, level.rawValue, (file as NSString).lastPathComponent, line, function, "\(error)")
    }
}

Usage:

Log.debug("MyLog")

Output example:

💬 AppDelegate.swift:26 application(_:didFinishLaunchingWithOptions:): MyLog