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Logging

Logging is very useful in all languages to allow debug applications and to help to solve bugs. In iOS development, we can use NSLog for logging apps written on Objective-C and print also in Swift. Basically, these solutions write a line of text in console. If we wanted to send logs of our iOS app, we would have to save them in a file first, for this purpose we can use a third party library as CocoaLumberjack.

NSLog

Use NSLog for debugging is very easy, it is just like a create a NSString. It has the following declaration:

void NSLog(NSString *format, ...);

The main characteristics of NSLog are:

  • NSLog is slow.
  • It is synchronous.
  • It outputs log to ASL and Xcode console.
  • It has not log levels what makes more difficult to tell the difference between errors, warnings, and other information.
  • It only works when the device is attached to the computer, then it is not a solution for remote access.

Examples of using:

NSLog(@"Hello world"); // Writes in console: Hello world

NSLog(@"Hello world %@", @"Tim"); // Writes in console: Hello world Tim

NSLog(@"Tim is %i years old", 43); // Writes in console: Tim is 43 years old

For more information about String format specifiers.

Logging an object

In many cases, it would be useful to print in console a complete object. If we try to print it directly, we will show the name of the class and its memory address in console.

Example:

// Header file
@interface Person: NSObject

@property (nonatomic, copy, readonly) NSString *name;
@property (nonatomic, copy, readonly) NSString *surname;

- (id)initWithName:(NSString *)aName andSurname:(NSString *)aSurname;

@end

// Implementation file

#import "Person.h"

@implementation Person

- (id)initWithName:(NSString *)aName andSurname:(NSString *)aSurname
{
    if (self = [super init]) {

        _name = aName;
        _surname = aSurname;

    }

    return self;
}

@end

// Using NSLog

Person *person = [[Person alloc] initWithName:@"Tim" andSurname:@"Cook"];

NSLog(@"%@", person); // Writes in console: <Person: 0x137641b00>

If we want to show a better log, we can override the property description with a proper string.

// Header file
@interface Person: NSObject

@property (nonatomic, copy, readonly) NSString *name;
@property (nonatomic, copy, readonly) NSString *surname;

- (id)initWithName:(NSString *)aName andSurname:(NSString *)aSurname;

@end

// Implementation file

#import "Person.h"

@implementation Person

- (id)initWithName:(NSString *)aName andSurname:(NSString *)aSurname
{
    if (self = [super init]) {

        _name = aName;
        _surname = aSurname;

    }

    return self;
}

- (NSString*)description
{
    return [NSString stringWithFormat:@"Name: %@, Surname: %@", self.name, self.surname];
}

@end

// Using NSLog

Person *person = [[Person alloc] initWithName:@"Tim" andSurname:@"Cook"];

NSLog(@"%@", person); // Writes in console: Name: Tim, Surname: Cook

Using DLog instead of NSLog

A good practice is not to show logs in your release version, in some cases can be useful but most of them should not be printed. Defining a macro DLog in the .pch file and using instead of NSLog, it will allow you to print logs only in debug mode but it will never print them on release mode.

Macro:

#ifndef DLog
#ifdef DEBUG
#define DLog(_format_, ...) NSLog(_format_, ## __VA_ARGS__)
#else
#define DLog(_format_, ...)
#endif

Swift’s print

We can use NSLog in Swift also, but it is a better practice to use print. Basically, print will add a newline at the end of its content as NSLog. Here, we can find a good comparative between print and NSLog (Note: the article compares NSLog vs println instead of print but apply also to print. println is deprecated since Swift 2.0).

Examples of using:

let name = "Tim"
let age = 43

print("Hello world"); // Writes in console: Hello world

print("Hello world \(name)"); // Writes in console: Hello world Tim

print("Tim is \(age) years old"); // Writes in console: Tim is 43 years old
Logging an object

In many cases, it would be useful to write in console a complete object. If we try to print it directly, we will show the name of the class in console.

Example:

class Person {

    var name: String
    var surname: String

    init(name: String, surname: String) {
        self.name = name
        self.surname = surname
    }

}

print(person) // Writes in console: Person

If the class implement the protocol CustomStringConvertible for other purposes, we will this string in console.

Example:

class Person {

    var name: String
    var surname: String

    init(name: String, surname: String) {
        self.name = name
        self.surname = surname
    }

}

extension Person: CustomStringConvertible {
    var description: String {
        return "Name: \(name) - Surname: \(surname)"
    }
}


print(person) // Writes in console: Name: Tim - Surname: Cook

But if we do not want to implement this protocol but you want to have a better description of the object in console, you can implement CustomDebugStringConvertible protocol.

Example:

class Person {

    var name: String
    var surname: String

    init(name: String, surname: String) {
        self.name = name
        self.surname = surname
    }

}

extension Person: CustomDebugStringConvertible {
    var debugDescription: String {
        return "Debugging --> Name: \(name) - Surname: \(surname)"
    }
}


print(person) // Writes in console: Debugging --> Name: Tim - Surname: Cook

Third party libraries

Although print and NSLog are very useful options and it is very easy to use them, they are quite simply. Moreover, these options only write in console or ASL but they do not save a log in a file then we are not able to send to us if we need it. For these reasons, there are some third party libraries which are more powerful and more customizable than the Apple ones.

CocoaLumberjack

  • Define different log levels.
  • Choose the output: Xcode console, Apple System Logs or/and file.
  • Define rolling frequency and maximum file size.
  • Faster than NSLog.

For more information about the installation, first steps and advanced configurations CocoaLumberjack GitHub

SwiftyBeaver

  • Define different log levels.
  • Choose the output: Xcode console, file or/and cloud.
  • Colored output depending on log level.
  • AES256 encryptation to send logs to cloud.
  • Easy configuration for each output.
  • Mac app to analyze logs sent from your app.

For more information about the installation, first steps and advanced configurations SwiftyBeaver GitHub

CleanroomLogger

  • Define different log levels, marking a special tag "log severity".
  • Make easier to find the exact line of code where is printing the log.
  • Has a function trace() which shows the exact execution path.
  • Is extensible, it allows to define custom implementations of different functions that could be used during the logging process.
  • Define rolling frequency.
  • Choose the output: Xcode console, Apple System Logs or/and file.

For more information about the installation, first steps and advanced configurations CleanroomLogger GitHub

Bibliography

Articles

GitHub