Reroute print and NSLogs to BOTH Xcode console and a file url [Swift + ObjC Solutions]

DebugLogger.swift

//
//  DebugLogger.swift
//
//  Created by Jonathan Cardasis. on 1/1/18.
//  Copyright ©2018 Jonathan Cardasis. All rights reserved.
//

#if DEBUG
import Foundation

class DebugLogger {
    static let shared: DebugLogger = {
        let logger = DebugLogger()
        logger.openConsolePipe()
        return logger
    }()
    
    static private var inputPipe = Pipe()
    static private var outputPipe = Pipe()
    
    static private var fileRedirectURL: URL?
    
    private init() { }
    
    func redirectOutputToFile(_ fileURL: URL) {
        DebugLogger.fileRedirectURL = fileURL
    }
    
    private func openConsolePipe() {
        // Route everything that comes in throught the outputPipe back to xcode console
        dup2(fileno(stdout), DebugLogger.outputPipe.fileHandleForWriting.fileDescriptor)
        
        // Route printing functions that go to sys pipes (stdout & stderr) into local inputPipe
        dup2(DebugLogger.inputPipe.fileHandleForWriting.fileDescriptor, fileno(stdout))
        dup2(DebugLogger.inputPipe.fileHandleForWriting.fileDescriptor, fileno(stderr))
        
        // Begin notification listener
        let inputReadHandler = DebugLogger.inputPipe.fileHandleForReading
        
        NotificationCenter.default.addObserver(self, selector: #selector(handlePipeNotification(notification:)), name: FileHandle.readCompletionNotification, object: inputReadHandler)
        
        inputReadHandler.readInBackgroundAndNotify()
    }
    
    @objc private func handlePipeNotification(notification: Notification) {
        // Listen for next input
        DebugLogger.inputPipe.fileHandleForReading.readInBackgroundAndNotify()
        
        if let data = notification.userInfo?[NSFileHandleNotificationDataItem] as? Data {
            // Write data back to output pipe for xcode to display
            DebugLogger.outputPipe.fileHandleForWriting.write(data)
            
            // Write to file if applicable
            if let fileURL = DebugLogger.fileRedirectURL {
                if !FileManager.default.fileExists(atPath: fileURL.path) {
                    FileManager.default.createFile(atPath: fileURL.path, contents: nil, attributes: nil)
                }
                
                do {
                    let fileHandle = try FileHandle(forWritingTo: fileURL)
                    fileHandle.seekToEndOfFile()
                    fileHandle.write(data)
                    fileHandle.closeFile()
                }  catch { /* pass */ }
            }
        }
        
    }
}
    
#endif

DebugLoggerObjC.m

//
//  DebugLogger.h
//
//  Created by Jonathan Cardasis. on 3/1/18.
//
#ifdef DEBUG

#import <Foundation/Foundation.h>

@interface DebugLogger : NSObject

+ (id)sharedInstance;
- (void)redirectOutputToFile:(NSURL*) fileURL;

@end

#endif

//----------------------------------------------//

//
//  DebugLogger.m
//
//  Created by Jonathan Cardasis. on 3/1/18.
//
#ifdef DEBUG

#import "DebugLogger.h"

@implementation DebugLogger

static NSPipe *inputPipe;
static NSPipe *outputPipe;
static NSURL *fileRedirectURL;


+ (id)sharedInstance {
    static dispatch_once_t s = 0;
    __strong static id _sharedObject = nil;
    
    dispatch_once(&s, ^{
        _sharedObject = [[self alloc] init];
        inputPipe = [[NSPipe alloc] init];
        outputPipe = [[NSPipe alloc] init];
        [_sharedObject openConsolePipe];
    });
    
    return _sharedObject;
}

- (void)redirectOutputToFile:(NSURL*) fileURL {
    fileRedirectURL = fileURL;
}


// MARK: - Private
- (void)openConsolePipe {
    // Route everything that comes in throught the outputPipe back to xcode console
    dup2(fileno(stdout), outputPipe.fileHandleForWriting.fileDescriptor);
    
    // Route printing functions that go to sys pipes (stdout & stderr) into local inputPipe
    dup2(inputPipe.fileHandleForWriting.fileDescriptor, fileno(stdout));
    dup2(inputPipe.fileHandleForWriting.fileDescriptor, fileno(stderr));
    
    // Begin notification listener
    NSFileHandle *inputReadHandler = inputPipe.fileHandleForReading;
    
    [[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(handlePipeNotification:) name:NSFileHandleReadCompletionNotification object:inputReadHandler];
    
    [inputReadHandler readInBackgroundAndNotify];
}

- (void)handlePipeNotification:(NSNotification*) notification {
    // Listen for next input
    [inputPipe.fileHandleForReading readInBackgroundAndNotify];
    
    NSDictionary *userInfo = notification.userInfo;
    if (userInfo == nil) {
        return;
    }
    
    NSData *data = (NSData*)userInfo[NSFileHandleNotificationDataItem];
    if (data == nil) {
        return;
    }
    
    [outputPipe.fileHandleForWriting writeData:data];
    
    NSURL *fileURL = fileRedirectURL;
    if (fileURL != nil) {
        if (![NSFileManager.defaultManager fileExistsAtPath:fileURL.path]) {
            [NSFileManager.defaultManager createFileAtPath:fileURL.path contents:nil attributes:nil];
        }
        
        NSError *err;
        NSFileHandle *fileHandle = [NSFileHandle fileHandleForWritingToURL:fileURL error:&err];
        
        if (err == nil) {
            [fileHandle seekToEndOfFile];
            [fileHandle writeData:data];
            [fileHandle closeFile];
        }
    }
}

@end
#endif

Example.swift

func main() {
  #if DEBUG
    let path = URL(fileURLWithPath: NSHomeDirectory()).appendingPathComponent("Documents/MyApp.log")
  
    DebugLogger.shared.redirectOutputToFile(path)
  #endif
  
  print("Swiftly logging")
  NSLog("Objectively saying something")
}

ExampleObjC.m

// ...
#ifdef DEBUG
    NSURL *debuggingLogURL = [[NSURL fileURLWithPath:NSHomeDirectory()] URLByAppendingPathComponent:@"Documents/MyApp.log"];
    [DebugLogger.sharedInstance redirectOutputToFile:debuggingLogURL];
#endif