Is it a good practice to extend NSError - ios

Question

Sorry for asking this question. I know in java we are extending Exception class for custom exceptions. But I don't see any scenarios for that in objective c.
So my question, Is it a good practice to extend NSError and introducing custom errors? If so when we should extend NSError class. I checked documentation for this too. But I can't see overriding notes for NSError.

Answer 1

While I agree that you shouldn't subclass NSError, it is very useful to put categories on it, and I do this regularly. For example, say your system often posts errors that come from some JSON block. I'd find it very convenient to create a category like:
#interface NSError (MyErrors)
// Construct an NSError from data in JSON.
// Include full JSON response in the userInfo
+ (NSError *)myErrorWithJSON:(JSON *)json;
// Parse the text out of the user info
- (NSString *)myErrorMessage;
// The full JSON error block as a string
- (NSString *)myErrorJSON;
// BOOLs can be helpful sometimes, or you could return an enum for use in a switch.
- (BOOL)myIsNetworkError;
- (BOOL)myIsAuthError;
#end
I often write little helpers to construct NSError more simply, construct the userinfo the way I want, and the pull data back out of the userinfo without callers needing to know its internal representation. I find this to be a very good form of data-hiding, and encourages the use of more descriptive messages.
Similarly, even for smaller projects, I often create a +myErrorWithCode:localizedDescription: category method. I know my domain, so I usually don't need to pass that, and this makes it a lot easier to set the NSLocalizedDescription key in the user info. Again, this encourages better errors by making them easier to create, and makes it easier to change the implementation details of your error handling.

Answer 2

I've never seen it done and that's because NSError is already very versatile. It allows the type of error to be defined by setting the domain and code properties and allows arbitrary additional information to be attached within the userInfo dictionary.
So, no, it's not good practice.

Answer 3

In the documentation is written that it is ok to subclass:
Applications may choose to create subclasses of NSError, for example,
to provide better localized error strings by overriding
localizedDescription.
In my case I am working with OSStatus which is Int32. NSError constructor supports only Int. So I need to subclass it to support OSSStatus.

Answer 4

It's not a bad idea to extend NSError.
I also have made a category on NSError for my own use. I would like to share it with you.
(1) Make a strings file to define all the error codes:
/* Following are general error messgaes those we can show to user
regarding to Internet connection and request. You can add more
codes. */
"-1001" = "Connection time out";
"-1003" = "Cannot find Host";
"-1004" = "Cannot connect to Host";
"-1005" = "Server is temporarily down";
"-1009" = "The Internet connection appears to be offline";
"-1012" = "Authentication failed";
"2000" = "This is a custom error message"; // custom created error code
/* Always describe unknow error with whatever you want in
except case (i.e. except error codes). If not mentioned
the following line, still you will get message "Unknown error" */
"Unknown error" = "Network error occured";
(2) Make a category on NSError, let say "NSError+ErrorInfo":
#interface NSError (ErrorInfo)
-(NSString *)userDescription;
#end
(3) Define it:
#define ERROR_KEY(code) [NSString stringWithFormat:#"%d",code]
#define ERROR_LOCALIZED_DESCRIPTION(code) NSLocalizedStringFromTable(ERROR_KEY(code),#"Errors",nil)
#implementation NSError (ErrorInfo)
-(NSString *)userDescription
{
NSString *errorDescrption = NSLocalizedStringFromTable(ERROR_KEY(self.code),#"Errors",nil);
if (!errorDescrption || [errorDescrption isEqual:ERROR_KEY(self.code)]){
return NSLocalizedStringFromTable(#"Unknown error",#"Errors",nil);;
}
else{
return ERROR_LOCALIZED_DESCRIPTION(self.code);
}
return nil;
}
#end
(4) Make use of it:
NSError *yourError; // This can be any NSError object you get
yourError = [NSError errorWithDomain:#"yourDomain" code:2000 userInfo:details]; // Just for test
NSLog(#"%#",[yourError userDescription]);