NSThread with Asynchronous Request

NSThread:

It is class used in Cocoa framework to create a thread.You have to specify a target and selector to create a new thread in objective c. So the selector or method is main entry point of your thread. 

Detail:

What happen once the thread execute all the code in the method (Thread main routine)?

Thread is terminated after execute all the code in the entry(its main routine) routine. So how we can run an asynchronous request on NSThread ?

What is Async request?

It is non blocking request which permit the application to do other task side by side as request is in continue. So it enhance parallelism in the application.


Now question is how we can prepare a asynchronous request in iOS:

- (void) startAsyncRequestWithUrl:(NSString*)urlString{

    assert(urlString != nil && urlString.length > 0);

    NSURL* url = [[NSURL alloc] initWithString:urlString];

    NSURLRequest* urlRequest = [[NSURLRequest alloc] initWithURL:url cachePolicy:NSURLRequestReloadIgnoringCacheData timeoutInterval:60];

    theConnection = [[NSURLConnection alloc] initWithRequest:urlRequest delegate:self];

    [theConnection start];

}

So above code has an example of asynchronous http request in iOS. As we know we need to implement delegate method  for the Url connection. So our main motive is to understand what happen  when we set the above method as main routine or entry point for NSThred then what will happen ?

So for the all the delegates method and complete HTTP connection file you can download from following link:

HttpConnection Link:

Let come back to our question is delegate call for the connection ? If yes then on main thread or the new thread?

So Just try this code ?

 HttpConnection* conn = [[HttpConnection alloc] initWithDelegate:self];

    NSString* urlString = @"https://imagizer.imageshack.us/v2/235x352q90/716/nkm8.jpg";

    [NSThread detachNewThreadSelector:@selector(startAsyncRequestWithUrl:) toTarget:conn withObject:urlString];

Did you get call in delegates ?

It will not come. Delegate will never be called and the obvious reason is thread terminate (which init the request).

So  now try to call by following method:

[conn startAsyncRequestWithUrl:urlString];

instead of that

 [NSThread detachNewThreadSelector:@selector(startAsyncRequestWithUrl:) toTarget:conn withObject:urlString]; 

Delegate will be called :)

Is there any tactic so that it can work on NSThread as well?

Yes 

RunLoop:

A run loop is event processing loop and it associated with each thread. So your main thread also have run loop. If you add any source of event to run loop then it will execute on the respective thread to which that run loop begin.

So our solution also use the same approach.  The basic idea is we need to busy the run loop of thread so that our thread will not terminated.

We need to busy the run loop associated with NSThread so that our thread should not terminate.
So we need to block the run loop for indefinite time until we did not receive the callback
We can do that by using following method:

- (connectionStatus) startAsyncRequestWithUrl:(NSString*)urlString{

    self.status = connectionSetupStage;

    assert(urlString != nil && urlString.length > 0);

    NSURL* url = [[NSURL alloc] initWithString:urlString];

    NSURLRequest* urlRequest = [[NSURLRequest alloc] initWithURL:url cachePolicy:NSURLRequestReloadIgnoringCacheData timeoutInterval:CONNECTIONTIMEOUT];

    theConnection = [[NSURLConnection alloc] initWithRequest:urlRequest delegate:self];

    

    while(self.status != connectionSucess && self.status != connectionFail){

        BOOL isBlockForInput = [[NSRunLoop currentRunLoop] runMode:NSDefaultRunLoopMode beforeDate:[NSDate distantFuture]];

        NSLog(@"Blocked:%d",isBlockForInput);

    }

    return self.status;

}

So we are calling the following run loop method in a loop

[[NSRunLoop currentRunLoop] runMode:NSDefaultRunLoopMode beforeDate:[NSDate distantFuture]]

By calling above method run loop will be trying to process the input source if any input source not available then it will return immediately. So If it return after delay it means it processed some input and print 1 in that case for above example. 

Can we optimize the code here ?

I think if there is no input source then CPU cycle will waste to processing the loop until it does not have any source.

So we can add sleep method here for the thread like given below:
 

 while(self.status != connectionSucess && self.status != connectionFail){

        BOOL isBlockForInput = [[NSRunLoop currentRunLoop] runMode:NSDefaultRunLoopMode beforeDate:[NSDate distantFuture]];

       sleep(1);

        NSLog(@"Blocked:%d",isBlockForInput);

    }

time depend upon the request if request take time you can increase the sleep time.

You can find more detail of this method here :
Detail:

So done with the today topic.
Please mail me if you need more information on this.