How can I use NSError in my iPhone App

Question

I am working on catching errors in my app  and I am looking into using NSError   I am slightly confused about how to use it  and how to populate it   Could someone provide an example on how I populate then use NSError

User · Answer

I would like to add some more suggestions based on my most recent implementation. I've looked at some code from Apple and I think my code behaves in much the same way.

The posts above already explain how to create NSError objects and return them, so I won't bother with that part. I'll just try to suggest a good way to integrate errors (codes, messages) in your own app.

I recommend creating 1 header that will be an overview of all the errors of your domain (i.e. app, library, etc..). My current header looks like this:

FSError.h

FOUNDATION_EXPORT NSString *const FSMyAppErrorDomain;

enum {
    FSUserNotLoggedInError = 1000,
    FSUserLogoutFailedError,
    FSProfileParsingFailedError,
    FSProfileBadLoginError,
    FSFNIDParsingFailedError,
};

FSError.m

#import "FSError.h" 

NSString *const FSMyAppErrorDomain = @"com.felis.myapp";

Now when using the above values for errors, Apple will create some basic standard error message for your app. An error could be created like the following:

+ (FSProfileInfo *)profileInfoWithData:(NSData *)data error:(NSError **)error
{
    FSProfileInfo *profileInfo = [[FSProfileInfo alloc] init];
    if (profileInfo)
    {
        /* ... lots of parsing code here ... */

        if (profileInfo.username == nil)
        {
            *error = [NSError errorWithDomain:FSMyAppErrorDomain code:FSProfileParsingFailedError userInfo:nil];            
            return nil;
        }
    }
    return profileInfo;
}

The standard Apple-generated error message (error.localizedDescription) for the above code will look like the following:

Error Domain=com.felis.myapp Code=1002 "The operation couldn’t be completed. (com.felis.myapp error 1002.)"

The above is already quite helpful for a developer, since the message displays the domain where the error occured and the corresponding error code. End users will have no clue what error code 1002 means though, so now we need to implement some nice messages for each code.

For the error messages we have to keep localisation in mind (even if we don't implement localized messages right away). I've used the following approach in my current project:

1) create a strings file that will contain the errors. Strings files are easily localizable. The file could look like the following:

FSError.strings

"1000" = "User not logged in.";
"1001" = "Logout failed.";
"1002" = "Parser failed.";
"1003" = "Incorrect username or password.";
"1004" = "Failed to parse FNID."

2) Add macros to convert integer codes to localized error messages. I've used 2 macros in my Constants+Macros.h file. I always include this file in the prefix header (MyApp-Prefix.pch) for convenience.

Constants+Macros.h

// error handling ...

#define FS_ERROR_KEY(code)                    [NSString stringWithFormat:@"%d", code]
#define FS_ERROR_LOCALIZED_DESCRIPTION(code)  NSLocalizedStringFromTable(FS_ERROR_KEY(code), @"FSError", nil)

3) Now it's easy to show a user friendly error message based on an error code. An example:

UIAlertView *alert = [[UIAlertView alloc] initWithTitle:@"Error" 
            message:FS_ERROR_LOCALIZED_DESCRIPTION(error.code) 
            delegate:nil 
            cancelButtonTitle:@"OK" 
            otherButtonTitles:nil];
[alert show];

User · Answer

I ll try summarize the great answer by Alex and the jlmendezbonini s point  adding a modification that will make everything ARC compatible  so far it s not since ARC will complain since you should return id  which means  any object   but BOOL is not an object type    -  BOOL  endWorldHunger  id largeAmountsOfMonies error  NSError   error          begin feeding the world s children           it s all going well until         if  ohNoImOutOfMonies               sad  we can t solve world hunger  but we can let people know what went wrong             init dictionary to be used to populate error object         NSMutableDictionary  details    NSMutableDictionary dictionary            details setValue   ran out of money  forKey NSLocalizedDescriptionKey              populate the error object with the details         if  error    NULL                    populate the error object with the details               error    NSError errorWithDomain   world  code 200 userInfo details                        we couldn t feed the world s children   return nil  sniffle   sniffle         return NO               wohoo  We fed the world s children  The world is now in lots of debt  But who cares       return YES      Now instead of checking for the return value of our method call  we check whether error is still nil  If it s not we have a problem      initialize NSError object NSError  error   nil     try to feed the world BOOL success    self endWorldHunger smallAmountsOfMonies error  amp error   if   success          inspect error    NSLog         error localizedDescription         otherwise the world has been fed  Wow  your code must rock

User · Answer

Please refer following tutorial   i hope it will helpful for you but prior you have to read documentation of NSError  This is very interesting link i found recently ErrorHandling

User · Answer

Objective-C  NSError  err    NSError errorWithDomain   some domain                                     code 100                                userInfo                                               NSLocalizedDescriptionKey   Something went wrong                                       Swift 3  let error   NSError domain   some domain                         code  100                    userInfo   NSLocalizedDescriptionKey   Something went wrong

User · Answer

Great answer Alex   One potential issue is the NULL dereference  Apple s reference on Creating and Returning NSError objects       details setValue   ran out of money  forKey NSLocalizedDescriptionKey    if  error    NULL           populate the error object with the details      error    NSError errorWithDomain   world  code 200 userInfo details        we couldn t feed the world s children   return nil  sniffle   sniffle return nil

User · Answer

Well  what I usually do is have my methods that could error-out at runtime take a reference to a NSError pointer  If something does indeed go wrong in that method  I can populate the NSError reference with error data and return nil from the method   Example   -  id  endWorldHunger  id largeAmountsOfMonies error  NSError   error          begin feeding the world s children           it s all going well until         if  ohNoImOutOfMonies               sad  we can t solve world hunger  but we can let people know what went wrong             init dictionary to be used to populate error object         NSMutableDictionary  details    NSMutableDictionary dictionary            details setValue   ran out of money  forKey NSLocalizedDescriptionKey              populate the error object with the details          error    NSError errorWithDomain   world  code 200 userInfo details              we couldn t feed the world s children   return nil  sniffle   sniffle         return nil               wohoo  We fed the world s children  The world is now in lots of debt  But who cares       return YES      We can then use the method like this  Don t even bother to inspect the error object unless the method returns nil      initialize NSError object NSError  error   nil     try to feed the world id yayOrNay    self endWorldHunger smallAmountsOfMonies error  amp error   if   yayOrNay          inspect error    NSLog         error localizedDescription         otherwise the world has been fed  Wow  your code must rock    We were able to access the error s localizedDescription because we set a value for NSLocalizedDescriptionKey    The best place for more information is Apple s documentation  It really is good   There is also a nice  simple tutorial on Cocoa Is My Girlfriend

User · Answer

extension NSError       static func defaultError   - gt  NSError           return NSError domain   com app error domain   code  0  userInfo   NSLocalizedDescriptionKey   Something went wrong               which I can use NSError defaultError   whenever I don t have valid error object   let error   NSError defaultError   print error localizedDescription    Something went wrong

User · Answer

Well it s a little bit out of question scope but in case you don t have an option for NSError you can always display the Low level error    NSLog   Error         NSString stringWithUTF8String strerror errno

User · Answer

Another design pattern that I have seen involves using blocks  which is especially useful when a method is being run asynchronously   Say we have the following error codes defined   typedef NS ENUM NSInteger  MyErrorCodes        MyErrorCodesEmptyString   500      MyErrorCodesInvalidURL      MyErrorCodesUnableToReachHost       You would define your method that can raise an error like so   -  void getContentsOfURL  NSString   path success  void    NSString  html  success failure  void    NSError  error  failure       if  path length    0            if  failure                failure  NSError errorWithDomain   com example  code MyErrorCodesEmptyString userInfo nil                      return             NSString  htmlContents                Exercise for the reader  get the contents at that URL or raise another error       if  success            success htmlContents             And then when you call it  you don t need to worry about declaring the NSError object  code completion will do it for you   or checking the returning value  You can just supply two blocks  one that will get called when there is an exception  and one that gets called when it succeeds    self getContentsOfURL   http   google com  success   NSString  html        NSLog   Contents       html     failure   NSError  error        NSLog   Failed to get contents       error       if  error code    MyErrorCodesEmptyString       make sure to check the domain too         NSLog   You must provide a non-empty string

[iphone] How can I use NSError in my iPhone App?

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