create an external webservice and call it in viewdidload - ios

Question

First of all, excuse me for my bad english but I'm french and I'll try my best to be understandable.
So, I'm coding a simple application with this structure :
- viewController class (deal with the UI)
- product class (define the object product)
- ws_product class (contains some functions which get json datas)
What I'm trying to do is to return the products array, that I get after I parsed my json in ws_product, in my viewController. Thanks to this I'll can fill my tableView and my application will no longer be empty !
My actual ws_product is :
#import "WS_Produit.h"
#import "Produit.h"
#import "ViewController.h"
#implementation WS_Produit
- (NSMutableArray *)getProduitsJSON
{
__block NSMutableArray *result;
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^() {
NSLog(#"on passe en async");
NSError *error = nil;
NSData *jsonData = [NSData dataWithContentsOfURL:[NSURL URLWithString:#"the url to load"]];
NSDictionary *produits = [NSJSONSerialization JSONObjectWithData:jsonData options:NSJSONReadingMutableContainers error:&error];
if( error )
{
NSLog(#"%#", [error localizedDescription]);
}
else {
dispatch_sync(dispatch_get_main_queue(), ^(){
NSLog(#"retour en sync");
result = [[NSMutableArray alloc] init];
Produit *tmp;
NSArray *produit = produits[#"produits"];
for ( NSDictionary *property in produit )
{
tmp = [Produit new];
tmp.ref = property[#"ref"];
tmp.name = property[#"name"];
tmp.description = property[#"description"];
tmp.price = property[#"price"];
tmp.imgURL = property[#"imgURL"];
[result addObject:tmp];
NSLog(#"%#", result);
}
});
}
});
NSLog(#"sortie du block");
NSLog(#"%#", result);
return result;
}
#end
My problem is when I'm out of the dispatch_queue my result array is empty so it's useless to return it in my viewController class, what can I do ?

Answer 1

Because you're using dispatch_async, your results array will be returned as empty before it gets filled.
Blocks are exactly what you need. They can be used as callbacks for async methods.
In your viewController, you should pass blocks to your method
[myObject getProduitsJSON:
success:^(NSArray *results){
// Use your results here
// Reload table for example.
}
failure:^(NSError *error){
// Use your error message (show it for example)
}];
So you're method should look like this:
-(void)getProduitsJson:(void(^)(NSArray* results))success failure:(void(^)(NSError* error))failure {
{
NSMutableArray *result = [[NSMutableArray alloc] init];
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^() {
NSError *error = nil;
NSData *jsonData = [NSData dataWithContentsOfURL:[NSURL URLWithString:#"the url to load"]];
NSDictionary *produits = [NSJSONSerialization JSONObjectWithData:jsonData options:NSJSONReadingMutableContainers error:&error];
if(error) {
failure(error);
}else{
// Fill your array
success(result);
}
}
}