What does this Objective-C dictionary code do?

Question

.h

@property (nonatomic, strong) NSMutableDictionary * products;  //not synthesized in .m

.m

- (void)productsRequest:(SKProductsRequest *)request didReceiveResponse:(SKProductsResponse *)response
{
    NSLog(@"Loaded list of products...");
    _productsRequest = nil;

    NSArray * skProducts = response.products;
    for (SKProduct * skProduct in skProducts)
    {
        IAPProduct * product = _products[skProduct.productIdentifier];
        product.skProduct = skProduct;
        product.availableForPurchase = YES;
    }

    for (NSString * invalidProductIdentifier in response.invalidProductIdentifiers)
    {
        IAPProduct * product = _products[invalidProductIdentifier];
        product.availableForPurchase = NO;
        NSLog(@"Invalid product identifier, removing: %@", invalidProductIdentifier);
    }

    NSMutableArray * availableProducts = [NSMutableArray array];
    for (IAPProduct * product in _products.allValues) {
        if (product.availableForPurchase) {
            [availableProducts addObject:product];
        }
    }

    _completionHandler(YES, availableProducts);
    _completionHandler = nil;      
}

I'm having trouble with the line below:

IAPProduct * product = _products[skProduct.productIdentifier];

I'm thinking we are setting Type IAPProduct *product= NSMutableDictionary[NSArray.productIdentifier];

I thought Array's only have elements (objectAtIndex)? Why is there a dot (.) and the productIdentifier?

Also

Answer 1

You are having trouble understanding this line:

IAPProduct * product = _products[skProduct.productIdentifier];

Lets break it down:

NSString *key = skProduct.productIdentifier;
IAPProduct * product = _products[key];

The 2nd line is modern syntax for:

IAProduct * product = [_products objectForKey:key];

This is the normal way to lookup a value in a dictionary for a given key.

Answer 2

It's not IAPProduct *product= NSMutableDictionary[NSArray.productIdentifier];

The type of skProduct is SKProduct, not NSArray. The fast enumeration for (SKProduct * skProduct in skProducts) loops through all elements in skProducts as SKProduct.

It's (to some extent) like using a loop with a counter doing:

//for (SKProduct * skProduct in skProducts)
for (int i=0; i<skProducts.count; i++)
{
    SKProduct *skProduct = skProduct[i];
    IAPProduct * product = _products[skProduct.productIdentifier];
    product.skProduct = skProduct;
    product.availableForPurchase = YES;
}