NSURLConnection sendAsynchronousRequest:queue:comp

问题:

I am using [NSURLConnection sendAsynchronousRequest:request queue:[NSOperationQueue mainQueue] completionHandler:^(NSURLResponse *response, NSData *data, NSError *error) in my app. By using this my app is terminated in iOS 4.3 but it is working fine in iOS 5.0.

How to use this in iOS 4.3 can any one help me.

回答1:

Here's a full implementation that works for me. Feel free to rename it and add as a category on NSURLConnection, or just add it as a local method in the class you're working in.

-(void)sendAsynchronousRequest:(NSURLRequest*)request queue:(NSOperationQueue*)queue completionHandler:(void(^)(NSURLResponse *response, NSData *data, NSError *error))handler
{
    __block NSURLResponse *response = nil;
    __block NSError *error = nil;
    __block NSData *data = nil;

    // Wrap up synchronous request within a block operation
    NSBlockOperation *blockOperation = [NSBlockOperation blockOperationWithBlock:^{
        data = [NSURLConnection sendSynchronousRequest:request 
                                     returningResponse:&response 
                                                 error:&error];
    }];

    // Set completion block
    // EDIT: Set completion block, perform on main thread for safety
    blockOperation.completionBlock = ^{

        // Perform completion on main queue
        [[NSOperationQueue mainQueue] addOperationWithBlock:^{
            handler(response, data, error);
        }];
    };

    // (or execute completion block on background thread)
    // blockOperation.completionBlock = ^{ handler(response, data, error); };

    // Execute operation
    [queue addOperation:blockOperation];
}

EDIT I had to modify the method because I was making UIKit calls in my completion block (e.g. updating labels etc). So it's actually a bit safer to call completion block on the main thread. (original version commented out)

回答2:

The method you are trying to use is only available on iOS 5. For earlier OSes, consider using

+ (NSData *)sendSynchronousRequest:(NSURLRequest *)request returningResponse:(NSURLResponse **)response error:(NSError **)error

and wrapping it into a new thread to achieve async behavior.

回答3:

Both H2CO3 and Ken Thomases suggestions are right.

In addition, you could take a look at ios4-implementation-of-nsurlconnection-sendasynchronousrequestqueuecompletio.

If you use the main queue as the queue where the completion handler performs, you could use (as Tom suggested) the delegate pattern. To avoid duplicate code, you could use a wrapper on NSURLConnection delegates mechanism.

In the other case, if you want to maintain the async behaviour and you don't want to deal with sync call (as H2CO3 suggested, note that his suggestion is also valid) and the completion handler performs in a different queue, then I suggest you to wrap the async delegate pattern in a NSOperation class. This approach is quite difficult but you can find a good way of do this in Concurrent Operations Demystified (see both the posts).

Hope it helps.