Using block statement to detect failed file modification - ios

Question

I have a view controller with a tableview and each tableview cell has an editable UILabel in it. Each cell is also associated with an audio file. Whenever the label is edited, I call an NSObject subclass that handles my files to rename the audio file to whatever the label was changed to. I then return the NSURL absolute string to store in core data. My question is, if this process throws an error - such as "file path already exists", how can I get it to show a UIAlertView in my view controller and not my NSObject subclass. I'm guessing I need some type of block that returns either a NSString or an NSError. I don't have much experience with blocks and any help would be appreciated.
Method in my view controller that calls the NSObject subclass to rename the file
recording.audioURL = [self.managedDocument changeFileName:previousPath withNewComponent:textField.text];
This is the method in my NSObject subclass that renames the audio file
-(NSString*) changeFileName:(NSString*) previousPath withNewComponent:(NSString*)newComponenet
{
NSURL * oldURL = [self.url URLByAppendingPathComponent:#"audioFiles"];
oldURL = [oldURL URLByAppendingPathComponent:[previousPath lastPathComponent]];
NSString * trimmedString = [newComponenet stringByReplacingOccurrencesOfString:#" " withString:#""];
NSURL * newURL = [self.url URLByAppendingPathComponent:#"audioFiles"];
newURL = [newURL URLByAppendingPathComponent:trimmedString];
newURL = [newURL URLByAppendingPathExtension:#"m4a"];
NSFileManager * fileManager = [NSFileManager defaultManager];
NSError * err;
BOOL result = [fileManager moveItemAtURL:oldURL toURL:newURL error:&err];
if(!result)
{
NSLog(#"Error: %#", err);
}
return newURL.absoluteString;
}

Answer 1

The most straightforward approach is to follow the example of the NSFileManager method you're using. Output an NSError* pointer indirectly through a by-reference parameter and make the method's direct return value indicate success or failure:
-(NSString*) changeFileName:(NSString*) previousPath withNewComponent:(NSString*)newComponenet error:(NSError**)error
{
NSURL * oldURL = [self.url URLByAppendingPathComponent:#"audioFiles"];
oldURL = [oldURL URLByAppendingPathComponent:[previousPath lastPathComponent]];
NSString * trimmedString = [newComponenet stringByReplacingOccurrencesOfString:#" " withString:#""];
NSURL * newURL = [self.url URLByAppendingPathComponent:#"audioFiles"];
newURL = [newURL URLByAppendingPathComponent:trimmedString];
newURL = [newURL URLByAppendingPathExtension:#"m4a"];
NSFileManager * fileManager = [NSFileManager defaultManager];
BOOL result = [fileManager moveItemAtURL:oldURL toURL:newURL error:error];
if(!result)
return nil;
return newURL.absoluteString;
}
In the caller, check the return value to determine if it failed and, if so, present the error.