Why I use gcdasyncudpsocket to send data will be blocked

See more:

Objective-C

sockets

iOS

Cocoa

GCDAsyncUdpSocket *sendSocket;
sendSocket = [[GCDAsycUdpSocket alloc] initwithDelegate:self delegateQueue:dispatch_get_main_queue()];
for(int i = 0;i < sort.count; i++)
{ 
   [sendSocket sendData:[sort objectAtIndex:i toHost:@"239.1.1.110" port:46110 Timeout:-1 tag:1];
}

GCDAsyncUdpSocket *sendSocket;
sendSocket = [[GCDAsycUdpSocket alloc] initwithDelegate:self delegateQueue:dispatch_get_main_queue()];
for(int i = 0;i < sort.count; i++)
{ 
   [sendSocket sendData:[sort objectAtIndex:i toHost:@"239.1.1.110" port:46110 Timeout:-1 tag:1];
   [Thread sleepForTimeInterval:0.003f];
}

What I have tried:

I use GCDAsyncSocket to send images in my application.The images are big,so I split its data to many small packets which save in a NSMutablearray called sort.When I use the first block of code,nothing is sent.
Then I add a line of code as second block of code,it worked.However my application is weird.
The point is I want to know the right or better way to send a list of data by GCDAsyncUdpSocket. Any help is appreciated,thanks in advance.