How does this block return work? - ios

Question

Im trying to interpret this block code:
SCENARIO 1
NSIndexSet* indexes = [[self.orderItems allKeys] indexesOfObjectsPassingTest:^BOOL(id obj, NSUInteger idx, BOOL *stop) {
IODItem* key = obj;
return [searchItem.name isEqualToString:key.name] && searchItem.price == key.price;
}];
Ok so I get allKeys of the self.orderItems dictionary and pass them as id obj to the block {}. Inside the block, that obj is assigned to an IODItem *key. Then both the call isEqualToString returns a BOOL as does the comparator ==. This makes sense to me because the block is of return type BOOL. So how does that fill an NSIndexSet of indexes?
SCENARIO 2
NSArray* keys = [[self.orderItems allKeys] sortedArrayUsingComparator:^NSComparisonResult(id obj1, id obj2) {
IODItem* item1 = (IODItem*)obj1;
IODItem* item2 = (IODItem*)obj2;
return [item1.name compare:item2.name];
}];
So I get allKeys again for that dictionary. I then sortArrayUsingComparator and I pass in the keys as obj1 & obj2? This is confusing. obj1 & obj2 are just keys in the [self.orderItems allKeys]-array?
Then I take those 2 objs and assign each to a different IDOItem. Then I actually return the items1 & 2? This is confusing again. I thought I was filling in an NSArray *keys. Why am i returning 2 things?
SCENARIO 3
// 3 - Enumerate items and add item name and quantity to description
[keys enumerateObjectsUsingBlock:^(id obj, NSUInteger idx, BOOL *stop) {
IODItem* item = (IODItem*)obj;
NSNumber* quantity = (NSNumber*)[self.orderItems objectForKey:item];
[orderDescription appendFormat:#"%# x%#\n", item.name, quantity];
}];
im enumerating through the keys array (which is gotten from SCENARIO 2 actually) and using the key obj and assigning it to an IODItem item. But here is where i got lost...I take the objectForKey item and use it as a quantity? If objectForKey returns the object paired to that key, and the key "item" refers to the id obj, then that id obj is a key from the sorted keys array. So its a key, not a value! Isnt it?

Answer 1

It's not too confusing if you think of blocks as a method with arguments and a return type. That first method is just iterating through the entire set of keys and determining if they pass the test you created or not. If your block returns 'YES' then it is added to the new index set. If the block returns 'NO' then it is ignored. The resulting index set will refer to a complete set of keys that passed the test.
The second method is a way for you to perform a custom sort on an array. Depending on which object should be closer to the beginning of the array, you either return NSOrderedAscending, NSOrderedSame or NSOrderedDescending. This could change based on the criteria you have for the sort. What you have basically done is called the compare: method on the first object's 'name' property. Depending on what data type this is (I'm assuming it's an NSString), your block will call the compare: method from NSString. If it was an NSNumber or another class, it would call that class's compare: method, etc. etc. The compare: method also returns an NSComparisonResult (one of the three options listed above).
Added Scenario 3:
For scenario 3 while enumerating through the 'keys', each 'id obj' is a key in the dictionary, NOT an IODItem as you've coded it. To get the applicable IODItem, you will probably need to do something like this (I'm assuming self.orderItems refers to an NSDictionary object):
__block NSMutableString *orderDescription;
__block NSNumber *quantity;
[keys enumerateObjectsUsingBlock:^(id obj, NSUInteger idx, BOOL *stop) {
NSString* currentKey = (NSString*)obj;
IODItem *item = [self.orderItems objectForKey:currentKey];
quantity = item.quantity;
[orderDescription appendFormat:#"%# x%#\n", item.name, quantity];
}];