Remove println() for release version iOS Swift

I would like to globally ignore all println() calls in my Swift code if I am not in a Debug build. I can't find any robust step by step instructions for this and would appreciate guidance. is there a way to do this globally, or do I need to surround every println() with #IF DEBUG/#ENDIF statements?

The simplest way is to put your own global function in front of Swift's println:

func println(object: Any) {
    Swift.println(object)
}

When it's time to stop logging, just comment out the body of that function:

func println(object: Any) {
    // Swift.println(object)
}

Or you can make it automatic by using a conditional:

func println(object: Any) {
    #if DEBUG
        Swift.println(object)
    #endif
}

EDIT In Swift 2.0 println is changed to print. Unfortunately it now has a variadic first parameter; this is cool, but it means you can't easily override it because Swift has no "splat" operator so you can't pass a variadic in code (it can only be created literally). But you can make a reduced version that works if, as will usually be the case, you are printing just one value:

func print(items: Any..., separator: String = " ", terminator: String = "\n") {
    Swift.print(items[0], separator:separator, terminator: terminator)
}

In Swift 3, you need to suppress the external label of the first parameter:

func print(_ items: Any..., separator: String = " ", terminator: String = "\n") {
    Swift.print(items[0], separator:separator, terminator: terminator)
}

Updated for Swift 4.x:

With Swift 2.0/3.0 and Xcode 7/8 now out of beta, there have been some changes to how you disable the print function in release builds.

There are some important points mentioned by @matt and @Nate Birkholz above that are still valid.

1. The println() function has been replaced by print()

2. To use the #if DEBUG macro then you have to define the "Swift Compiler - Custom Flags -Other Flags" to contain the value -D DEBUG

3. I would recommend overriding the Swift.print() function in the global scope so that you can use the print() function as normal in your code, but it will remove output for non-debug builds. Here is a function signature that you can add at the global scope to do this in Swift 2.0/3.0:

    func print(items: Any..., separator: String = " ", terminator: String = "\n") {
    
        #if DEBUG
        
        var idx = items.startIndex
        let endIdx = items.endIndex
        
        repeat {
            Swift.print(items[idx], separator: separator, terminator: idx == (endIdx - 1) ? terminator : separator)
            idx += 1
        }
        while idx < endIdx
        
        #endif
    }
   

Note: We have set the default separator to be a space here, and the default terminator to be a newline. You can configure this differently in your project if you would like.

Hope this helps.

Update:

It is usually preferable to put this function at the global scope, so that it sits in front of Swift's print function. I find that the best way to organize this is to add a utility file to your project (like DebugOptions.Swift) where you can place this function at the global scope.

As of Swift 3 the ++ operator will be deprecated. I have updated the snippet above to reflect this change.

The problem with all these approaches, including mine, is that they do not remove the overhead of evaluating the print arguments. No matter which of them you use, this is going to be expensive:

print(myExpensiveFunction())

The only decent solution is to wrap the actual print call in conditional compilation (let's assume that DEBUG is defined only for debug builds):

#if DEBUG
print(myExpensiveFunction())
#endif

That, and only that, prevents myExpensiveFunction from being called in a release build.

However, you can push back evaluation one level by using autoclosure. Thus, you could rewrite my solution (this is Swift 3) like this:

func print(_ item: @autoclosure () -> Any, separator: String = " ", terminator: String = "\n") {
    #if DEBUG
    Swift.print(item(), separator: separator, terminator: terminator)
    #endif
}

This solves the problem just in the case where you are printing just one thing, which is usually true. That's because item() is not called in release mode. print(myExpensiveFunction()) thus ceases to be expensive, because the call is wrapped in a closure without being evaluated, and in release mode, it won't be evaluated at all.

Swift 5

Simply make a new file in your project and paste this code in:

func print(_ items: Any..., separator: String = " ", terminator: String = "\n") {
    #if DEBUG
    items.forEach {
        Swift.print($0, separator: separator, terminator: terminator)        
    }
    #endif
}

This function signature matches the default Swift one so it "overwrites" the function in your project. If needed you can still access the original by using Swift.print().

Once you've added the code above, keep using print() the as usual and it will only print in debug builds.

Note: Doing the forEach to print each item gets rid of annoying array brackets around the print statements that show up if you just pass items straight into Swift.print().

For anyone relatively new to Swift you may wonder what the heck $0 is. It just represents the first argument passed into the forEach block. The forEach statement could also be written like this:

items.forEach { item in
    Swift.print(item, separator: separator, terminator: terminator)        
}

Lastly if you're interested, the Swift declaration of print looks like this:

public func print(_ items: Any..., separator: String = " ", terminator: String = "\n")

My answer above mirrors the exact Swift implementation - although I never print more than one thing or change separator/terminators. But who knows, you may want to.

As noted, i am a student and need things defined a little more clearly to follow along. After lots of research, the sequence I needed to follow is:

Click on the project name at the top of the File Navigator at the left of the Xcode project window. This is line that has the name of the project, how many build targets there are, and the iOS SDK version.

Choose the Build Settings tab and scroll down to the "Swift Compiler - Custom Flags" section near the bottom. Click the Down Arrow next to Other Flags to expand the section.

Click on the Debug line to select it. Place your mouse cursor over the right side of the line and double-click. A list view will appear. Click the + button at the lower left of the list view to add a value. A text field will become active.

In the text field, enter the text -D DEBUG and press Return to commit the line.

Add a new Swift file to your project. You are going to want to make a custom class for the file, so enter text along the lines of the following:

class Log {

  var intFor : Int

  init() {
    intFor = 42
   }

  func DLog(message: String, function: String = __FUNCTION__) {
    #if DEBUG
      println("\(function): \(message)")
    #endif
  }
}

I was having trouble getting the class to be accepted by Xcode today, so the init may be a bit more heavyweight than necessary.

Now you will need to reference your custom class in any class in which you intend to use the new custom function in place of println() Add this as a property in every applicable class:

   let logFor = Log()

Now you can replace any instances of println() with logFor.DLog(). The output also includes the name of the function in which the line was called.

Note that inside class functions I couldn't call the function unless I made a copy of the function as a class function in that class, and println() is also a bit more flexible with the input, so I couldn't use this in every instance in my code.

Here is a function that I use, which works perfectly in Swift 3:

func gLog<T>( _ object: @autoclosure() -> T, _ file: String = #file, _ function: String = #function, _ line: Int = #line)
    {
    #if DEBUG
        let value = object()
        let stringRepresentation: String

        if let value = value as? CustomDebugStringConvertible
            {
            stringRepresentation = value.debugDescription
            }
        else if let value = value as? CustomStringConvertible
            {
            stringRepresentation = value.description
            }
        else
            {
            fatalError("gLog only works for values that conform to CustomDebugStringConvertible or CustomStringConvertible")
            }

        let fileURL = NSURL(string: file)?.lastPathComponent ?? "Unknown file"
        let queue = Thread.isMainThread ? "UI" : "BG"
    let gFormatter = DateFormatter()
    gFormatter.dateFormat = "HH:mm:ss:SSS"
        let timestamp = gFormatter.string(from: Date())

        print("✅ \(timestamp) {\(queue)} \(fileURL) > \(function)[\(line)]: " + stringRepresentation + "\n")
    #endif
    }

Here is an example of the output it generates:

Explanation:

• the green checkmark is used to enable you to quickly see your print (gLog) messages in the console, where they can sometimes get lost in a sea of other messages

• the time/date stamp

• the thread it is being run on -- in my case it is either the MainThread (which I call UI), or not the MainThread (which I call BG, for background thread)

• the name of the file that the gLog message resides in

• the function within the file that the gLog message resides in

• the line number of the gLog message

• the actual gLog message you would like to print out

Hope this is useful to someone else!

Tested with Swift 2.1 & Xcode 7.1.1

There's an easy way to exclude all print statements from release versions, once you know that empty functions are removed by the Swift compiler.

Side note : In the era of Objective-C, there was a pre-parser which could be used to remove NSLog statements before the compiler kicked in, like described in my answer here. But since Swift no longer has a pre-parser this approach is no longer valid.

Here's what I use today as an advanced and easily configurable log function, without ever having to worry about removing it in release builds. Also by setting different compiler flags, you can tweak the information that is logged as needed.

You can tweak the function as needed, any suggestion to improve it is welcome!

// Gobal log() function
//
// note that empty functions are removed by the Swift compiler -> use #if $endif to enclose all the code inside the log()
// these log() statements therefore do not need to be removed in the release build !
//
// to enable logging
//
// Project -> Build Settings -> Swift Compiler - Custom flags -> Other Swift flags -> Debug
// add one of these 3 possible combinations :
//
//      -D kLOG_ENABLE
//      -D kLOG_ENABLE -D kLOG_DETAILS
//      -D kLOG_ENABLE -D kLOG_DETAILS -D kLOG_THREADS
//
// you can just call log() anywhere in the code, or add a message like log("hello")
//
func log(message: String = "", filePath: String = #file, line: Int = #line, function: String = #function) {
            #if kLOG_ENABLE
        
            #if kLOG_DETAILS
           
            var threadName = ""
            #if kLOG_THREADS
                threadName = NSThread.currentThread().isMainThread ? "MAIN THREAD" : (NSThread.currentThread().name ?? "UNKNOWN THREAD")
                threadName = "[" + threadName + "] "
            #endif

            let fileName = NSURL(fileURLWithPath: filePath).URLByDeletingPathExtension?.lastPathComponent ?? "???"
                
            var msg = ""
            if message != "" {
                msg = " - \(message)"
            }
            
            NSLog("-- " + threadName + fileName + "(\(line))" + " -> " + function + msg)
        #else
            NSLog(message)
        #endif
    #endif
}

Here's where you set the compiler flags :

An example output with all flags on looks like this :

   2016-01-13 23:48:38.026 FoodTracker[48735:4147607] -- [MAIN THREAD] ViewController(19) -> viewDidLoad() - hello

The code with the log() looks like this :

    override func viewDidLoad() { log("hello")
    super.viewDidLoad()
    
   // Handle the text field's user input through delegate callbacks
   nameTextField.delegate = self
}

Even simpler, after making sure -D DEBUG is set for the OTHER_SWIFT_FLAGS Debug build settings:

#if !DEBUG
    func print(_ items: Any..., separator: String = " ", terminator: String = "\n") { }
#endif

XCode 8 introduced a few new build settings.
In particular one referred to Active Compilation Conditions does in a similar way what Other Flags settings did.

> "Active Compilation Conditions" is a new build setting for passing conditional compilation flags to the Swift compiler.

As per XCode 8 (tested in 8.3.2) you will get this by default:

So without any config you can write the following:

#if DEBUG
    print("⚠️ Something weird happened")
#endif

I strongly recommend you that if you use this approach extensively create a class/struct/function that wraps this logging logic. You may want to extend this further down the road.

Varun Naharia has the better solution so far. I would combine his answer with Rivera's ...

1. create a -D DEBUG flag on the compiler directives, build settings.

2. then add this code:

    #if !DEBUG
     public func print(_ items: Any..., separator: String = " ", terminator: String = "\n") {
    }
    #endif
   

This code will convert every print into nothing for release.

Swift 4 Xcode 10.0

maybe you could use this

func dPrint(_ message: @autoclosure () -> Any) {
    #if DEBUG
    print(message())
    #endif
}

The reason of using @autoclosure is that if you pass a function as the message parameter, the function will be called only in debug mode, it will cause a performance hit.

unlike the Swift.print(_ items: Any..., separator: String = default, terminator: String = default) function, my solution has only one parameter, because in most cases, we don't pass multiple parameters as the print function only shows information in console, we can just convert the parameters to String: "\(param1)"+"\(param2)", right? hope u like my solution

for my solution i make it simple

import UIKit

class DLog: NSObject {

   init(title:String, log:Any) {
       #if DEBUG
           print(title, log)
       #endif
    
   }

}

then to show it just call

_ = DLog(title:"any title", log:Any)

You can also use a breakpoint, set it to continue after evaluation, and write the print message in the breakpoint!

You could define debug_println whose contents would be roughly:

#if DEBUG
  println()
#endif

My Solution is use this code in AppDelegate before class

// Disable console log in live app
#if !arch(x86_64) && !arch(i386)
    public func debugPrint(items: Any..., separator: String = " ", terminator: String = "\n") {

    }
    public func print(_ items: Any..., separator: String = " ", terminator: String = "\n") {

    }
#endif

class AppDelegate: UIResponder, UIApplicationDelegate {
// App Delegate Code 

}

Even simpler: take advantage of the fact that asserts are removed from release builds and only from there call the print. This removes all log calls (yes, even the calls to Log.da) as they are empty when building for release.

But I also heard that prints are removed for release builds, but not been able to find it in writing. So for now, I am using something like this Log below. I have a more meaty version on GitHub with emojis (for readability) and log topics (for consistency):

https://github.com/Gatada/JBits/blob/master/Project/Utility/Log.swift

public enum Log {
    
    /// A date formatter used to create the timestamp in the log.
    ///
    /// This formatter is only created if it is actually used, reducing the
    /// overhead to zero.
    static var formatter: DateFormatter?
    
    // MARK: - API

    /// Call to print message in debug area.
    ///
    /// Asserts are removed in release builds, which make
    /// the function body empty, which caused all calls to
    /// be removed as well.
    ///
    /// Result is zero overhead for release builds.
    public static func da(_ message: String) {
        assert(debugAreaPrint(message))
    }
    
    // MARK: - Helpers
    
    /// The function that actually does the printing. It returns `true` to
    /// prevent the assert from kicking in on debug builds.
    private static func debugAreaPrint(_ message: String) -> Bool {
        print("\(timestamp) - \(message)")
        return true
    }

    /// Creates a timestamp used as part of the temporary logging in the debug area.
    static private var timestamp: String {
        
        if formatter == nil {
            formatter = DateFormatter()
            formatter!.dateFormat = "HH:mm:ss.SSS"
        }
        
        let date = Date()
        return formatter!.string(from: date)
    }
}

In code:

Log.da("This is only handled in a debug build.")

Seen in the Xcode debug area only when running a debug build: > 13:36:15.047 - This is only handled in a debug build.

My Project was developed in Objective C, but from the past year I have started merging new code in Swift, So In Swift below solution worked for me, I have added that code in My Swift constant file :

func print(_ items: Any..., separator: String = " ", terminator: String = "\n") {
    #if DEBUG
    items.forEach {
        Swift.print($0, separator: separator, terminator: terminator)
    }
    #endif
}

This works for me (add this as a global function in the project)

func print(_ items: Any...) {
    #if DEBUG
        Swift.print(items[0])
    #endif
}