创建自己的NSError对象

Generating Your Own Errors
In order to create your own NSError objects you’ll need to define your own error domain, which should be of the form:

com.companyName.appOrFrameworkName.ErrorDomain
You’ll also need to pick a unique error code for each error that may occur in your domain, along with a suitable description, which is stored in the user info dictionary for the error, like this:

    NSString *domain = @"com.MyCompany.MyApplication.ErrorDomain";
    NSString *desc = NSLocalizedString(@"Unable to…", @"");
    NSDictionary *userInfo = @{ NSLocalizedDescriptionKey : desc };
 
    NSError *error = [NSError errorWithDomain:domain
                                         code:-101
                                     userInfo:userInfo];
This example uses the NSLocalizedString function to look up a localized version of the error description from a Localizable.strings file, as described in Localizing String Resources.

If you need to pass back an error by reference as described earlier, your method signature should include a parameter for a pointer to a pointer to an NSError object. You should also use the return value to indicate success or failure, like this:

- (BOOL)doSomethingThatMayGenerateAnError:(NSError **)errorPtr;
If an error occurs, you should start by checking whether a non-NULL pointer was provided for the error parameter before you attempt to dereference it to set the error, before returning NO to indicate failure, like this:

- (BOOL)doSomethingThatMayGenerateAnError:(NSError **)errorPtr {
    ...
    // error occurred
    if (errorPtr) {
        *errorPtr = [NSError errorWithDomain:...
                                        code:...
                                    userInfo:...];
    }
    return NO;
}