NSOperationQueue & NSInvocationOperation

1. In your main class’ init method call this:

operationQueue = [[NSOperationQueue alloc]init];
[operationQueue setMaxConcurrentOperationCount:1];

2. Then in your viewWillAppear you can call this:

[self showLoadingIndicators]; //calls a method which presents loading indicators (optional)
[self beginLoadingTwitterData]; // this is the method that fires it all off

3. In your beginLoadingTwitterData you call this:

NSInvocationOperation *operation = [[NSInvocationOperation alloc] initWithTarget:self selector:@selector(synchronousLoadTwitterData) object:nil];
[operationQueue addOperation:operation]; // creates and adds NSOperation to a queue
[operation release];

4. In this synchronousLoadTwitterData is where you do the heavy lifting:

NSDictionary *dict = [[NSDictionary alloc] initWithDictionary:[TwitterHelper fetchInfoForUsername:[twitterIds objectAtIndex:count]]]
// this calls for a method “fetchInfoForUsername” which returns an NSDictionary, but to do so, it connects to the internet using a synchronous URL fetch which would normally hold up the main thread

Later in this method you call:

[self performSelectorOnMainThread:@selector(didFinishLoadingTwitterDataWithResults:) withObject:tempArray waitUntilDone:YES];
//which returns you to the main thread at said selector…

5. In this final method, didFinishLoadingTwitterDataWithResults, which gets called once the operation finishes, you call:

self.persons = (NSArray*)result;
[self hideLoadingIndicators];
[self.tableView reloadData];
//and you can do other things like, receive the results, hide the indicators and reload a tableview.

This is very useful when downloading data from the web.

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