How do I start an Asynchronous NSURLConnection inside an NSOperation?

Question

I want to do an Asynchrous NSURLConnection inside of an NSOperation on a background thread. it is because I\'m doing some very expensive operations on the data as they come

Answer 1

-(void)start
{
  [self willChangeValueForKey:@"isExecuting"];
  _isExecuting = YES;
  [self didChangeValueForKey:@"isExecuting"];
  NSURL* url = [[NSURL alloc] initWithString:@"http://url.to/feed.xml"];
  NSMutableURLRequest* request = [[NSMutableURLRequest alloc] initWithURL:url cachePolicy:NSURLRequestUseProtocolCachePolicy timeoutInterval:20];
  _connection = [[NSURLConnection alloc] initWithRequest:request delegate:self startImmediately:NO]; // ivar
  [request release];
  [url release];
  // Here is the trick
  NSPort* port = [NSPort port];
  NSRunLoop* rl = [NSRunLoop currentRunLoop]; // Get the runloop
  [rl addPort:port forMode:NSDefaultRunLoopMode];
  [_connection scheduleInRunLoop:rl forMode:NSDefaultRunLoopMode];
  [_connection start];
  [rl run];
}

More details can be found here: link

Answer 2

A couple of approaches:

1. Schedule the NSURLConnection in the main run loop, by using the startImmediately parameter of NO, set the run loop, and only then should you start the connection, e.g.:

   urlConnection = [[NSURLConnection alloc] initWithRequest:theRequest delegate:self startImmediately:NO];
   [urlConnection scheduleInRunLoop:[NSRunLoop mainRunLoop] forMode:NSRunLoopCommonModes];
   [urlConnection start];
   

2. Create a dedicated thread for the the connection and schedule the connection in the run loop you create for that thread. See AFURLConnectionOperation.m in AFNetworking source for an example of this.

3. Actually use AFNetworking, which gives you NSOperation based operations that you can add to your queue, and takes care of this run loop stuff for you.

---

So, AFNetworking does something like:

+ (void)networkRequestThreadEntryPoint:(id)__unused object {
    @autoreleasepool {
        [[NSThread currentThread] setName:@"NetworkingThread"];

        NSRunLoop *runLoop = [NSRunLoop currentRunLoop];
        [runLoop addPort:[NSMachPort port] forMode:NSDefaultRunLoopMode];
        [runLoop run];
    }
}

+ (NSThread *)networkRequestThread {
    static NSThread *_networkRequestThread = nil;
    static dispatch_once_t oncePredicate;

    dispatch_once(&oncePredicate, ^{
        _networkRequestThread = [[NSThread alloc] initWithTarget:self
                                                        selector:@selector(networkRequestThreadEntryPoint:)
                                                          object:nil];
        [_networkRequestThread start];
    });

    return _networkRequestThread;
}

So I do something like the following. First I have a few private properties:

@property (nonatomic, readwrite, getter = isExecuting)  BOOL executing;
@property (nonatomic, readwrite, getter = isFinished)   BOOL finished;
@property (nonatomic, weak)   NSURLConnection *connection;

Then the network operation can then do something like:

@synthesize executing = _executing;
@synthesize finished  = _finished;

- (instancetype)init {
    self = [super init];
    if (self) {
        _executing = NO;
        _finished = NO;
    }
    return self;
}

- (void)start {
    if (self.isCancelled) {
        [self completeOperation];
        return;
    }

    self.executing = YES;

    [self performSelector:@selector(startInNetworkRequestThread)
                 onThread:[[self class] networkRequestThread]
               withObject:nil
            waitUntilDone:NO];
}

- (void)startInNetworkRequestThread {
    NSURLConnection *connection = [[NSURLConnection alloc] initWithRequest:self.request
                                                                  delegate:self
                                                          startImmediately:NO];
    [connection scheduleInRunLoop:[NSRunLoop currentRunLoop] forMode:NSRunLoopCommonModes];
    [connection start];

    self.connection = connection;
}

- (void)completeOperation {
    self.executing = NO;
    self.finished = YES;
}

- (void)setFinished:(BOOL)finished {
    if (finished != _finished) {
        [self willChangeValueForKey:@"isFinished"];
        _finished = finished;
        [self didChangeValueForKey:@"isFinished"];
    }
}

- (void)setExecuting:(BOOL)executing {
    if (executing != _executing) {
        [self willChangeValueForKey:@"isExecuting"];
        _executing = executing;
        [self didChangeValueForKey:@"isExecuting"];
    }
}

- (BOOL)isConcurrent {
    return YES;
}

- (BOOL)isAsynchronous {
    return YES;
}

// all of my NSURLConnectionDataDelegate stuff here, for example, upon completion:

- (void)connectionDidFinishLoading:(NSURLConnection *)connection {
    // I call the appropriate completion blocks here, do cleanup, etc. and then, when done:

    [self completeOperation];
}

Answer 3

I know this post is over a year old, but I wanted to add some suggestions for people who may run the same problem trying to create own async network operation. You need to add runloop to operation that runs in background and you should stop it when the operations has finished.

There actually 2 simple options:

Option 1 - using NSRunLoop

NSPort *port        = [NSPort port];
NSRunLoop *runLoop  = [NSRunLoop currentRunLoop];
[runLoop addPort:port forMode:NSDefaultRunLoopMode];
[self.connection scheduleInRunLoop:runLoop forMode:NSDefaultRunLoopMode];
[self.connection start];
[runLoop run];

and you need to stop when the operation is finished:

NSRunLoop *runLoop = [NSRunLoop currentRunLoop];
NSDate *date       = [NSDate distantFuture];
while (!runLoopIsStopped && [runLoop runMode:NSDefaultRunLoopMode beforeDate:date]);

Option 2 - using CF

You need to add

CFRunLoopRun();

when you start operation

and call

CFRunLoopStop(CFRunLoopGetCurrent());

when you finish operation.

Read the following post: CFRunLoopRun() vs [NSRunLoop run]