Download and Parse Data in Background Thread - ios

Question

I have a UITableView that gets its data from an array. Populating that array, however, requires downloading and parsing large chucks of data from the Web. That being the case, I'd like perform those operations in a background thread. Here's what I've got so far:
#interface MyClass()
#property (nonatomic, strong) NSArray *model;
#end
#implementation MyClass
- (void) getData {
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
NSData *data = [NSData dataWithContentsOfURL:[NSURL URLWithString:SOME_URL]];
if (data) {
NSMutableArray *arr = [NSMutableArray array];
//Populate arr with data just fetched, which can take a while
dispatch_async(dispatch_get_main_queue(), ^{
//THIS IS THE STEP I AM UNSURE ABOUT. SHOULD I DO:
self.model = arr;
//OR
self.model = [NSArray arrayWithArray:arr];
//OR
self.model = [arr copy];
//OR
//something else?
});
}
});
}
#end
Thank you!

Answer 1

// you can use any string instead "mythread"
dispatch_queue_t backgroundQueue = dispatch_queue_create("com.mycompany.myqueue", 0);
dispatch_async(backgroundQueue, ^{
// Send Request to server for Data
NSData *data = [NSData dataWithContentsOfURL:[NSURL URLWithString:SOME_URL]];
dispatch_async(dispatch_get_main_queue(), ^{
// Receive Result here for your request and perform UI Updation Task Here
if ([data length] > 0) {
// if you receive any data in Response, Parse it either (XML or JSON) and reload tableview new data
}
});
});

Answer 2

You should do:
self.model = arr;
The reference to self calls the setter, which will release any previous references in that variable and then add a reference count to arr so it doesn't go out of scope. If you were accessing the ivar directly you would do:
[model release];
model = [arr retain];

Answer 3

Take a look at this link Understanding dispatch_async and this https://developer.apple.com/library/ios/documentation/General/Conceptual/ConcurrencyProgrammingGuide/OperationQueues/OperationQueues.html
You should add DISPATCH_QUEUE_PRIORITY_BACKGROUND instead of DISPATCH_QUEUE_PRIORITY_DEFAULT to run this on the background.
By using DISPATCH_QUEUE_PRIORITY_DEFAULT you just made your task be classified as a normal task. If you have changed it to higher or lower priority, the queue would have run it before or after some other tasks, respectively.