NSURLSessionDataTask doesn't update my table - objective-c

Question

I have a class for get xml and parse it.
+ (instancetype)sharedRssNewsLoader
{
static BGMRssNewsLoader *sharedRssNewsLoader = nil;
static dispatch_once_t onceToken;
dispatch_once(&onceToken, ^{
sharedRssNewsLoader = [[self alloc] initPrivate];
});
return sharedRssNewsLoader;
}
- (instancetype)initPrivate
{
self = [super init];
//did the superclass's designated initializer succeed?
if (self)
{
self.rssNewsItems = [[NSMutableArray alloc] init];
NSURLSessionConfiguration *config = [NSURLSessionConfiguration defaultSessionConfiguration];
_session = [NSURLSession sessionWithConfiguration:config
delegate:nil
delegateQueue:nil];
//RSS call
[self fetchFeed];
}
return self;
}
- (void)fetchFeed
{
NSString *requestString = #"http://xxx...";
NSURL *url = [NSURL URLWithString:requestString];
NSURLRequest *req = [NSURLRequest requestWithURL:url];
NSURLSessionDataTask *dataTask = [self.session dataTaskWithRequest:req
completionHandler:^(NSData *data, NSURLResponse *response, NSError *error){
self.rssXmlString = [[NSString alloc] initWithData:data encoding:NSUTF8StringEncoding];
DDLogVerbose(#"rssXmlString: %#", self.rssXmlString);
dispatch_sync(dispatch_get_main_queue(), ^{
[self parseXML:self.rssXmlString];
});}
];
[dataTask resume];
}
//parse XML
- (void)parseXML:(NSString *)source
{
NSError *error = nil;
DDXMLDocument *theDocument = [[DDXMLDocument alloc] initWithXMLString:source
options:0
error:&error];
NSArray *xmlItems = [theDocument nodesForXPath:#"//item"
error:&error];
for(DDXMLElement *itemElement in xmlItems)
{
NSString *itemTitleValue = [[[itemElement elementsForName:#"title"] lastObject] stringValue];
NSString *itemDescriptionValue = [[[itemElement elementsForName:#"title"] lastObject] stringValue];
BGMRssNewsEntry *rssEntry = [[BGMRssNewsEntry alloc] initRssNewsEntryWithTitle:itemTitleValue
rssText:itemDescriptionValue
rssUrl:nil
rssDate:nil
isRssRead:NO];
[self.rssNewsItems addObject:rssEntry];
}
}
//if a programmer calls [[BGMItemsStore alloc] init], let him know the error of his ways
- (instancetype) init
{
#throw [NSException exceptionWithName:#"Singleton"
reason:#"You are wrong! Use +[BBGMRssNewsLoader sharedRssNewsLoader]"
userInfo:nil];
return nil;
}
I would like to show result in the table. So in the other class I do:
- (id)initWithStyle:(UITableViewStyle)style
{
self = [super initWithStyle:style];
if (self)
{
//custom initialization
//initialization sharedRssNewsLoader with rssItems array and RSS call
[BGMRssNewsLoader sharedRssNewsLoader];
}
return self;
}
But my table is absolutely clear. I know that I need to switch to main_queue when I am working with NSURLSessionDataTask, I did it. I think that I mess with thread :(

Answer 1

That BGMRssNewsLoader is asynchronously loading the data and in parseXML you are adding entries to the self.rssNewsItems object. But I don't see where it tells the table to reload itself when this asynchronous request done. If your table view's data source tries to immediately retrieve the rssNewsItems array, it will likely be empty until the asynchronous request is complete.
Generally, when you're done loading the data, you'd call [self.tableView reloadData] (either by supplying the tableview as a parameter during the instantiation of the BGMRssNewsLoader, or by having it post a notification that the table view controller is observing, or by supplying a completion block parameter in which you'd perform reloadData).
I'd also suggest temporarily adding a breakpoint or log statement where you add objects to the rssNewsItems array, just to make sure that that the XML feed was successfully retrieved and parsed.
For example, I might retire fetch and define a method like so:
- (void)fetchFeedWithCompletionBlock:(void (^)(NSArray *items, NSError *error))completion
{
NSString *requestString = #"http://xxx...";
NSURL *url = [NSURL URLWithString:requestString];
NSURLRequest *req = [NSURLRequest requestWithURL:url];
NSURLSessionDataTask *dataTask = [self.session dataTaskWithRequest:req completionHandler:^(NSData *data, NSURLResponse *response, NSError *error){
self.rssXmlString = [[NSString alloc] initWithData:data encoding:NSUTF8StringEncoding];
DDLogVerbose(#"rssXmlString: %#", self.rssXmlString);
dispatch_sync(dispatch_get_main_queue(), ^{
[self parseXML:self.rssXmlString];
if (completion) {
completion(self.rssNewsItems, nil);
}
});
}];
[dataTask resume];
}
I would, by the way, not have the initPrivate method initiate the fetch. You want the caller to be able to supply whatever completion block it wants.
I'd then have the tableview's controller do something like:
[[BGMRssNewsLoader sharedRssNewsLoader] fetchFeedWithCompletionBlock:^(NSArray *items, NSError *error) {
// use the items array and/or error
[self.tableView reloadData];
}];
Clearly, the fetchFeedWithCompletionBlock should actually detect errors and call the completion block with the appropriate NSError (so the table view can decide how it wants to present the error ... the singleton shouldn't be doing anything with the UI itself), but hopefully this illustrates the idea of a network object that provides a completion block to allow the caller to specify what it wants to do when the asynchronous request is complete.