Thread still running of old view - ios

Question

I have an app where there are five screens.
On each screen, in viewDidLoad I am accessing data from server.
On each screen I have next button.
When I go from screen one to screen five (by clicking Next 4 times), in NSLog, I still see the process done by all previous four view controller.
Is there any way, how can I kill those threads?
In short, I don't want to do any process when I go away from that view i.e. if I go from view 3 to 4, I want to stop the task that I was for view 3.
Getting data of earlier views & waiting for that data (which is unwanted) is not good for app, hence I want like what I explained above.
Edit 1
Below is the code I use for reading the data.
.h
#property(nonatomic, retain) NSMutableData *webData;
#property(nonatomic, retain) NSMutableData *data;
Using below I request the data
.m
NSString *myTMainURL = [NSString stringWithFormat:#"http://www.google.com"];
NSURL *url = [NSURL URLWithString:myTMainURL];
NSURLRequest *request = [NSURLRequest requestWithURL:url];
[[NSURLConnection alloc] initWithRequest:request delegate:self];
For reading, below is how I read.
- (void)connection:(NSURLConnection *)connection didReceiveResponse:(NSURLResponse *)response {
NSLog(#"didReceiveResponse");
data = [[NSMutableData alloc] init ];
[webData setLength: 0];
}
- (void)connection:(NSURLConnection *)connection didReceiveData:(NSData *)theData {
NSLog(#"didReceiveData");
[data appendData:theData];
[webData appendData:data];
NSLog(#"didreceveidata leng===%d===%d", [webData length], [data length]);
}
- (void)connectionDidFinishLoading:(NSURLConnection *)connection {
NSLog(#"connectionDidFinishLoading");
NSString *myDataFromLink = [[NSString alloc] initWithBytes: [data mutableBytes] length:[data length] encoding:NSUTF8StringEncoding];
NSLog(#"myDataFromLink====%#--", myDataFromLink);
}

Answer 1

In viewWillDisappear:, send cancel to whatever operation is running.
This, of course, assumes you have a cancelable task/method/operation.
For example, for network requests, if you use NSURLConnection this is the case when you employ the delegate approach. With NSURLConnection's convenient class method sendAsynchronousRequest:queue:completionHandler: this is not possible. Thus, any serious application would use the delegate approach, since a long running asynchronous operation must be cancelable.

Answer 2

You can use NSOperation and cancel the operation when you go to the next view may be in the action of the next button or just in viewWillDisappear: method
Edit
Since You are using NSURLConnection then you can call cancel on the connection in viewWillDisappear:
.h
#property(nonatomic, retain) NSMutableData *webData;
#property(nonatomic, retain) NSMutableData *data;
#property(nonatomic, retain) NSURLConnection *connection;
.m
NSString *myTMainURL = [NSString stringWithFormat:#"http://www.google.com"];
NSURL *url = [NSURL URLWithString:myTMainURL];
NSURLRequest *request = [NSURLRequest requestWithURL:url];
self.connection = [[NSURLConnection alloc] initWithRequest:request delegate:self];
in viewWillDisappear:
[self.connection cancel]

Answer 3

Since you are using NSURLConnection, You can cancel that connection and stop the NSURLRequest which you are using with your NSURLConnection with following code:
-(void)viewDidDisappear
{
[super viewDidDisappear];
[connection cancel]; // Here connection is your NSURLConnection object.
}