iOS: how to perform a HTTP POST request?

Question

You can use NSURLConnection as follows:

1. Set your NSURLRequest: Use requestWithURL:(NSURL *)theURL to initialise the request.

   If you need to specify a POST request and/or HTTP headers, use NSMutableURLRequest with

   • (void)setHTTPMethod:(NSString *)method
   • (void)setHTTPBody:(NSData *)data
   • (void)setValue:(NSString *)value forHTTPHeaderField:(NSString *)field

2. Send your request in 2 ways using NSURLConnection:

   • Synchronously: (NSData *)sendSynchronousRequest:(NSURLRequest *)request returningResponse:(NSURLResponse **)response error:(NSError **)error

     This returns a NSData variable that you can process.

     IMPORTANT: Remember to kick off the synchronous request in a separate thread to avoid blocking the UI.

   • Asynchronously: (void)start

Don't forget to set your NSURLConnection's delegate to handle the connection as follows:

- (void)connection:(NSURLConnection *)connection didReceiveResponse:(NSURLResponse *)response {
    [self.data setLength:0];
}

- (void)connection:(NSURLConnection *)connection didReceiveData:(NSData *)d {
    [self.data appendData:d];
}

- (void)connection:(NSURLConnection *)connection didFailWithError:(NSError *)error {
    [[[[UIAlertView alloc] initWithTitle:NSLocalizedString(@"Error", @"")
                                 message:[error localizedDescription]
                                delegate:nil
                       cancelButtonTitle:NSLocalizedString(@"OK", @"") 
                       otherButtonTitles:nil] autorelease] show];
}

- (void)connectionDidFinishLoading:(NSURLConnection *)connection {
    NSString *responseText = [[NSString alloc] initWithData:self.data encoding:NSUTF8StringEncoding];

    // Do anything you want with it 

    [responseText release];
}

// Handle basic authentication challenge if needed
- (void)connection:(NSURLConnection *)connection didReceiveAuthenticationChallenge:(NSURLAuthenticationChallenge *)challenge {
    NSString *username = @"username";
    NSString *password = @"password";

    NSURLCredential *credential = [NSURLCredential credentialWithUser:username
                                                             password:password
                                                          persistence:NSURLCredentialPersistenceForSession];
    [[challenge sender] useCredential:credential forAuthenticationChallenge:challenge];
}

EDIT: ASIHTTPRequest has been abandoned by the developer. It's still really good IMO, but you should probably look elsewhere now.

I'd highly recommend using the ASIHTTPRequest library if you are handling HTTPS. Even without https it provides a really nice wrapper for stuff like this and whilst it's not hard to do yourself over plain http, I just think the library is nice and a great way to get started.

The HTTPS complications are far from trivial in various scenarios, and if you want to be robust in handling all the variations, you'll find the ASI library a real help.

I thought I would update this post a bit and say that alot of the iOS community has moved over to AFNetworking after ASIHTTPRequest was abandoned. I highly recommend it. It's a great wrapper around NSURLConnection and allows for asynchronous calls, and basically anything you might need.

Here is an updated answer for iOS7+. It uses NSURLSession, the new hotness. Disclaimer, this is untested and was written in a text field:

- (void)post {
    NSURLSession *session = [NSURLSession sessionWithConfiguration:[NSURLSessionConfiguration defaultSessionConfiguration] delegate:self delegateQueue:nil];
    NSMutableURLRequest *request = [NSMutableURLRequest requestWithURL:[NSURL URLWithString:@"https://example.com/dontposthere"] cachePolicy:NSURLRequestUseProtocolCachePolicy timeoutInterval:60.0];
    // Uncomment the following two lines if you're using JSON like I imagine many people are (the person who is asking specified plain text)
    // [request addValue:@"application/json" forHTTPHeaderField:@"Content-Type"];
    // [request addValue:@"application/json" forHTTPHeaderField:@"Accept"]; 
    [request setHTTPMethod:@"POST"];
    NSURLSessionDataTask *postDataTask = [session dataTaskWithRequest:request completionHandler:^(NSData *data, NSURLResponse *response, NSError *error) {
        NSString *responseString = [[NSString alloc] initWithData:data encoding:NSUTF8StringEncoding];
    }];
    [postDataTask resume];
}

-(void)URLSession:(NSURLSession *)session didReceiveChallenge:(NSURLAuthenticationChallenge *)challenge completionHandler:(void (^)(    NSURLSessionAuthChallengeDisposition disposition, NSURLCredential *credential))completionHandler {
    completionHandler(NSURLSessionAuthChallengeUseCredential, [NSURLCredential credentialForTrust:challenge.protectionSpace.serverTrust]);
}

Or better yet, use AFNetworking 2.0+. Usually I would subclass AFHTTPSessionManager, but I'm putting this all in one method to have a concise example.

- (void)post {
    AFHTTPSessionManager *manager = [[AFHTTPSessionManager alloc] initWithBaseURL:[NSURL URLWithString:@"https://example.com"]];
    // Many people will probably want [AFJSONRequestSerializer serializer];
    manager.requestSerializer = [AFHTTPRequestSerializer serializer];
    // Many people will probably want [AFJSONResponseSerializer serializer];
    manager.responseSerializer = [AFHTTPRequestSerializer serializer];
    manager.securityPolicy.allowInvalidCertificates = NO; // Some servers require this to be YES, but default is NO.
    [manager.requestSerializer setAuthorizationHeaderFieldWithUsername:@"username" password:@"password"];
    [[manager POST:@"dontposthere" parameters:nil success:^(NSURLSessionDataTask *task, id responseObject) {
        NSString *responseString = [[NSString alloc] initWithData:responseObject encoding:NSUTF8StringEncoding];
    } failure:^(NSURLSessionDataTask *task, NSError *error) {
        NSLog(@"darn it");
    }] resume];
}

If you are using the JSON response serializer, the responseObject will be object from the JSON response (often NSDictionary or NSArray).