Question

I am using ASIHTTPRequest framework, in the document, what are the differences between the 2nd and 3rd example, in usage, advantage and disadvantage?

2nd example (Creating an asynchronous request):

- (IBAction)grabURLInBackground:(id)sender
{
NSURL *url = [NSURL URLWithString:@"http://allseeing-i.com"];
ASIHTTPRequest *request = [ASIHTTPRequest requestWithURL:url];
[request setDelegate:self];
[request startAsynchronous];
}

- (void)requestFinished:(ASIHTTPRequest *)request
{
// Use when fetching text data
NSString *responseString = [request responseString];

// Use when fetching binary data
NSData *responseData = [request responseData];
}

- (void)requestFailed:(ASIHTTPRequest *)request
{
NSError *error = [request error];
}

3rd example (Using blocks)

- (IBAction)grabURLInBackground:(id)sender
{
   NSURL *url = [NSURL URLWithString:@"http://allseeing-i.com"];
   __block ASIHTTPRequest *request = [ASIHTTPRequest requestWithURL:url];
   [request setCompletionBlock:^{
      // Use when fetching text data
      NSString *responseString = [request responseString];

      // Use when fetching binary data
      NSData *responseData = [request responseData];
   }];
   [request setFailedBlock:^{
      NSError *error = [request error];
   }];
   [request startAsynchronous];
}

Answer 1

The blocks in iOS are a part of Concurrent Programming

You use a block when you want to create units of work (that is, code segments) that can be passed around as though they are values. Blocks are usually used for writting a callbacks.

Usually, using blocks do not reflect in different applicatino behaviour. The syntactical difference is that, when using blocks you do not need to define a request delegate or implement delegate methods (such as -requestFinished: and -requestFailed:) for async requests.

One of the advantages is in accessing local method variables in completion block, bacause the function expression in block can reference and can preserve access to local variables (like variable url in your method -grabURLInBackground: or any other local variable defined in your method).

The second adventage is in using nested request calls. For example, you may need to perform a few requests in sequence, and without blocks you will need to define a separate delegate method callback for each request, which may result in reduced readability of your code.

Blocks allow you to write code at the point of invocation that is executed later in the context of the method implementation, which may be very usefull, when you get used to using them.

Some patterns to avoid when using blocks are mentioned in Apple Blocks Programming Topis