How can I create a activity indicator in middle of view programmatically?

Change the center of the activity indicator as

activityIndicator.center = self.view.center;

I think i am not able to explain it well.ok lets try again. its a nav based app. i have tableView. every cell creates a detailView. on RootView which is a tableview there is a refresh button. when i click that button it parses the feed again. and it takes some time. for that time program doesnt respond. and parsing is complete it works again. now i need activity indicator for that time. i dont know how to add in xib. bcz when i open main.xib and put activity indicator in RootViewController. it comes infront of whole tableView. now may be i explained well. if not let me know i ll try again.

from what you are saying above the program is not responding during the parsing which is a problem. If the GUI freezes while you are parsing the data you should move that operation to a secondary thread. That way your GUI remains responsive and you will be able to see the activity indicator.

on the main thread you should have something similar to this in order to show the activity indicator:

UIActivityIndicatorView  *av = [[[UIActivityIndicatorView alloc]initWithActivityIndicatorStyle:UIActivityIndicatorViewStyleGray] autorelease];
av.frame = CGRectMake(round((yourView.frame.size.width - 25) / 2), round((yourView.frame.size.height - 25) / 2), 25, 25);
av.tag  = 1;
[yourView addSubview:av];
[av startAnimating];

after the secondary thread is finished, this is the thread where you parse the data, you should call on the main thread something like this to remove the activity indicator:

UIActivityIndicatorView *tmpimg = (UIActivityIndicatorView *)[yourView viewWithTag:1];
[tmpimg removeFromSuperview];

UIActivityIndicatorView *activityIndicator = [[UIActivityIndicatorView alloc] initWithActivityIndicatorStyle:UIActivityIndicatorViewStyleGray];
activityIndicator.alpha = 1.0;
[collectionVwSearch addSubview:activityIndicator];
activityIndicator.center = CGPointMake([[UIScreen mainScreen]bounds].size.width/2, [[UIScreen mainScreen]bounds].size.height/2);
[activityIndicator startAnimating];//to start animating

For Swift

let activityIndicator: UIActivityIndicatorView = UIActivityIndicatorView.init(activityIndicatorStyle: UIActivityIndicatorViewStyle.Gray)
activityIndicator.alpha = 1.0
collectionVwSearch.addSubview(activityIndicator)
activityIndicator.center = CGPointMake(UIScreen.mainScreen().bounds.size.width / 2, UIScreen.mainScreen().bounds.size.height / 2)
activityIndicator.startAnimating()

To stop activityIndicator wirte [activityIndicator stopAnimating] on appropriate place

A UIActivityIndicator generally needs to be placed into a separate thread from the long process (parsing feed) in order to appear.

If you want to keep everything in the same thread, then you need to give the indicator time to appear. This Stackoverflow question addresses placing a delay in the code.

EDIT: Two years later, I think that any time you are using a delay to make something happen you are probably doing it wrong. Here is how I would do this task now:

- (IBAction)refreshFeed:(id)sender {
    //main thread
    UIActivityIndicatorView *spinner = [[UIActivityIndicatorView alloc] initWithActivityIndicatorStyle:UIActivityIndicatorViewStyleWhiteLarge]; [self.view addSubview:spinner];

    //switch to background thread
    dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_BACKGROUND, 0), ^{

        //back to the main thread for the UI call
        dispatch_async(dispatch_get_main_queue(), ^{
            [spinner startAnimating];
        });
        // more on the background thread

        // parsing code code

        //back to the main thread for the UI call
        dispatch_async(dispatch_get_main_queue(), ^{
            [spinner stopAnimating];
        });
    });
}