Discussion on 'didReceiveData' method for HTTP Connection

Question

I created an indenpendent class for HTTP connection. All the connection works fine. The problem is that I find method 'didReceiveData' will be called AFTER the method who call the connection. (method 'didReceiveData' will be called after IBAction 'accept')

- (IBAction)accept:(id)sender {
    [self connect:url];
    //labelStr = ReturnStr; Cannot be written here. 
}

-(void)connect:(NSString *)strURL
{
    NSURLRequest *theRequest=[NSURLRequest requestWithURL:[NSURL URLWithString:strURL]
                cachePolicy:NSURLRequestUseProtocolCachePolicy
               timeoutInterval:60.0];

    NSURLConnection *theConnection=[[NSURLConnection alloc] initWithRequest:theRequest delegate:self];
    if (theConnection) 
    {
     // receivedData is declared as a method instance elsewhere
     receivedData = [[NSMutableData data] retain];
    } 
    else 
    { 
     // inform the user that the download could not be made
    }
}

- (void)connection:(NSURLConnection *)connection didReceiveData:(NSData *)data
{
    // append the new data to the receivedData
    [receivedData appendData:data];
    ReturnStr = [[NSString alloc] initWithData:data encoding:NSASCIIStringEncoding];
}

This will cause a problem that if I want to change the text of a label to the received string, the code cannot be written in IBAction 'accept' but have to be written in method 'didReceiveData' like this:

    MainViewController *mainView = [[MainViewController alloc] initWithNibName:@"MainView" bundle:nil];
    AMEAppDelegate *delegate = [[UIApplication sharedApplication] delegate];
    [delegate.navController pushViewController:mainView animated:YES];
    mainView.labelStr.text = ReturnStr;

A further problem is that the data on MainView will be overwritten if I initialize MainView in 'didReceiveData'. Is it possible for me to change the text of labelStr without initialize MainView?

Answer 1

The problem is that I find method 'didReceiveData' will be called AFTER the method who call the connection. (method 'didReceiveData' will be called after IBAction 'accept')

You're expecting the connection to send you connection:didReceiveData: before you create and connect it?

This will cause a problem that if I want to change the text of a label to the received string, the code cannot be written in IBAction 'accept' but have to be written in method 'didReceiveData' …

Sounds about right. You can't work with something you've received until you've received it.

A further problem is that the data on MainView will be overwritten if I initialize MainView in 'didReceiveData'. Is it possible for me to change the text of labelStr without initialize MainView?

Creating the main view controller and app delegate in your connection:didReceiveData: method seems really late to do that. Do those things earlier, then have connection:didReceiveData: do nothing but set labelStr.text.

BTW, the implementation of connection:didReceiveData: that you show leaks ReturnStr. Remember to release or autorelease what you have allocked.

Answer 2

NSURLConnection and other similar classes are designed to be used asynchronously.

initWithRequest:delegate: returns immediately, and you aren't annoyed with the connection stuff till it sends the delegate methods to its delegate.

Answer 3

If you want your app to wait until the data comes in, use NSURLConnection's sendSynchronousRequest:returningResponse:error: method. Note, however, that the rest of your app will be frozen while this method is being run, and of course the method could take a while if the user has a crappy connection.

Answer 4

Using NSMutableData instead of NSData.