Question

I have been learning Objective C on my own for some time already and still don’t quite get the hang of memory management. When should I release properties?

Example, I have a class that will handle 2 (register & updateParticulars) different URLRequest connections. updateParticularsConnection will be performed when registerConnection finishes.

@interface ConnectionViewController : UIViewController {

}
@property (nonatomic, retain) NSURLConnection *registerConnection;
@property (nonatomic, retain) NSURLConnection *updateParticularsConnection;
@property (nonatomic, retain) NSMutableData *responseData;
@property (nonatomic, retain) NSMutableURLRequest *requestURL;

@end

@implementation ConnectionViewController
@synthesize registerConnection, updateParticularsConnection, responseData, requestURL,


(void)performRegistration {
    // other here to prepare the data.
    requestURL = [[NSMutableURLRequest alloc] initWithURL:[NSURL URLWithString:@"myURL"]];
    registerConnection = [[NSURLConnection alloc] initWithRequest:requestURL delegate:self startImmediately:YES];
}

(void)updateParticulars {
    // other here to prepare the data.
     [requestURL setURL:[NSURL URLWithString:@"http:myURL.com"]];
     updateParticularsConnection = [[NSURLConnection alloc] initWithRequest:requestURL delegate:self startImmediately:YES];
}

Handling delegate callbacks

- (void)connection:(NSURLConnection *)connection didFailWithError:(NSError *)error {
    [SVProgressHUD dismissWithError:@"Unable to connect"];
}

- (void)connection:(NSURLConnection *)connection didReceiveData:(NSData *)data {
    if (responseData == nil) {
        responseData = [[NSMutableData alloc] init];
    }
    [responseData appendData:data];
}

- (void)connectionDidFinishLoading:(NSURLConnection *)connection {
    if (connection == registerConnection) {
        NSMutableString *responseString = [[NSString alloc] initWithData:responseData encoding:NSUTF8StringEncoding];
        NSLog(@"Register connection recieved data reads : %@", responseString);
        if ([responseString isEqualToString:@"-1"]) { // error. stop connection
            [self.requestURL release]; // remember to release requestURL since we would not be continuing on.
        }   
        else if ([responseString isEqualToString:@""]) { // error. stop connection

            [self.requestURL release]; //remember to release requestURL since we would not be continuing on.
        }

        else {
            [self updateParticulars]; // perform next connection, updateParticulars

        }       
        responseData = nil; // clear the current stored data in preparation for the next connection.
        [self.registerConnection release];
        [responseString release];
    } // end of definition for register connection

    else if (connection == updateParticularsConnection) {
            // do stuff with data received back here 
        self.responseData = nil;
        [self.requestURL release];
        [self.updateParticularsConnection release];
    }       
}

My question is should I be releasing my properties as soon as I can, which is what I think I am doing now? Or only during the dealloc method? Do advise if I am not doing it right.

Answer 1

you sort of need to take it on a case by case basis. the general answer is “as soon as you are done with it”, unless it is a trivial allocation. for trivial allocations (e.g. NSString * firstName), you can just wait until dealloc or it is replaced (e.g. setFirstName:). this merely simplifies the implementation.

your example is slightly different.

// typically, you will set these to nil right when they 
// have finished and you have grabbed what you need.
// that's pretty common for an async one shot request.
@property (nonatomic, retain) NSURLConnection *registerConnection;
@property (nonatomic, retain) NSURLConnection *updateParticularsConnection;
@property (nonatomic, retain) NSMutableURLRequest *requestURL;

and

// in the context of your program, this allocation could be large.
// in many cases, you'll simply convert it to the destination (if
// it's an image, just turn it into an image without holding onto
// the data representation), then dispose of it.
@property (nonatomic, retain) NSMutableData *responseData;

important: you are handling your instance’s ivars directly in the OP – use the accessors, they will save you a ton of headaches. these are the changes you’d make to write your program using the accessors:

- (void)performRegistration {
    self.requestURL = [[[NSMutableURLRequest alloc] initWithURL:[NSURL URLWithString:@"myURL"]] autorelease]; // use setter and autorelease
    self.registerConnection = [[[NSURLConnection alloc] initWithRequest:requestURL delegate:self startImmediately:YES] autorelease]; // use setter and autorelease
}

- (void)updateParticulars {
    [self.requestURL setURL:[NSURL URLWithString:@"http:myURL.com"]]; // use getter
    self.updateParticularsConnection = [[[NSURLConnection alloc] initWithRequest:requestURL delegate:self startImmediately:YES] autorelease]; // use setter and autorelease
}

- (void)connection:(NSURLConnection *)connection didReceiveData:(NSData *)data {
    if (self.responseData == nil) { // use getter
        self.responseData = [NSMutableData data]; // use setter and autorelease
    }
    [self.responseData appendData:data]; // use getter
}

- (void)connectionDidFinishLoading:(NSURLConnection *)connection {
    if (connection == self.registerConnection) { // use getter
        NSMutableString *responseString = [[NSString alloc] initWithData:self.responseData encoding:NSUTF8StringEncoding]; // use getter
        NSLog(@"Register connection recieved data reads : %@", self.responseString); // use getter
        if ([responseString isEqualToString:@"-1"]) {
            self.requestURL = nil; // use setter
        }
        else if ([responseString isEqualToString:@""]) {
            self.requestURL = nil; // use setter
        }
        else {
            [self updateParticulars];
        }
        self.responseData = nil; // use setter
        self.registerConnection = nil; // use setter
        [responseString release];
    }
    else if (connection == self.updateParticularsConnection) { // use getter
        self.responseData = nil; // use setter
        self.requestURL = nil; // use setter
        self.updateParticularsConnection = nil; // use setter
    }
}