iphone - asynchronous upload and download operation freezes the UI - iphone

Question

In my app , I want to achieve multiple uploads and downloads using Websocket protocol.
For that purpose , I have made class called Engine , Upload and Download
Engine - creates a operationQueue and maximumConcurrentOperation is 3 .
Upload and Download - subclass of NSOperation and override the main method . In main method , both the class have following statements
Engine.m
-(id)init
{
if(self = [super init])
{
opQueue = [[NSOperationQueue alloc] init];
opQueue.maximumConcurrentOperationCount = 3 ;
}
}
-(void)startUploadWithPath:(NSString *)file
{
Upload *up = [[Upload alloc] initWithPath:file];
[opQueue addOperation:up];
}
-(void)startDownloadWithPath:(NSString *)file
{
Download *down = [[Upload alloc] initWithPath:file];
[opQueue addOperation:down];
}
Upload.m and Download.m
-(void)main
{
//start transfering data to server
while(workCompleted != YES)
{
// wait()
}
// when work completed
//insertIntoDB();
}
Problem : When I try to upload first video , then it works fine . Immidiately if I try to upload second video , then also it works fine . When I try to upload third video(i.e when I select video from gallery and video compression has not began yet) , the application freeze . The application becomes active only when one of the two previous video upload is completed .
Why does the application freeze even if maxConcurrentOperationCount is 3 ?
Thanks in advance.

Answer 1

try uploading videos in parallel, for example like this:
[NSThread detachNewThreadSelector:#selector(inParallel) toTarget:self withObject:nil];
...
-(void)inParallel {
NSLog(#"occurs in parallel");
}
this should not freeze the UI but it is worth trying