ASIHTTPRequest發送異步請求方法之一

下載ASIHTTPRequest
加入Framework： CFNetwork.framework，SystemConfiguration.framework，MobileCoreServices.framework
__block ASIHTTPRequest *request = [ASIHTTPRequest requestWithURL:sourceURL];
[request setCompletionBlock:^{
    NSLog(@"Zip file downloaded.");
    NSData *data = [request responseData];
    [self processZip:data sourceURL:sourceURL];        
}];
[request setFailedBlock:^{
    NSError *error = [request error];
    NSLog(@"Error downloading zip file: %@", error.localizedDescription);
}];
[request startAsynchronous];