Last and least, but really useful: NSEnumerator (iOS/Xcode)

Apple's documentation for enumeration begins with the familiar for in statement before moving swiftly to the more complex block-based enumeration. This is a leap in complexity and most people are probably mentally exhausted before reaching the final item on the page - 'Using an Enumerator' - which is based around a simple and useful class with only two methods.

This is a great shame because the class, NSEnumerator, is so simple that it can be entirely covered in a brief piece of example code:

 NSArray *anArray = @[@"hello",@"world"];
    NSEnumerator *enumerator = [anArray objectEnumerator];
    id object;
   
    while ((object = [enumerator nextObject])) {

        NSLog(@"Array item: %@",object);
    }
   
    NSDictionary *aDictionary = @{@"hello":@"Weird",@"world":@"Home"};
    NSEnumerator *enumeratorDictionary = [aDictionary keyEnumerator];
   
   
    id objectKey;
   
    while ((objectKey = [enumeratorDictionary nextObject])) {

        NSLog(@"Key: %@",objectKey);
     
       
    }
   
    NSArray *allObjects = [enumeratorDictionary allObjects]; //
    NSLog(@"All objects after enumerator %@", allObjects); // Empty - because the enumator releases each object every time it has been called by nextObject
   
   
    NSEnumerator *enumeratorDictionaryValues = [aDictionary objectEnumerator];
   
    id objectValue;
   
   
    NSArray *allValues = [enumeratorDictionaryValues allObjects];
   
    NSLog(@"All values before enumerator %@", allValues); // Full - because the enumator hasn't called nextObject yet
   
   
    while ((objectValue = [enumeratorDictionaryValues nextObject])) {
     
        // Empty because allObjects have been retrieved
        NSLog(@"Value: %@",allValues);
       
       

    }

And I'm sure it would be useful to many currently using for in statements. Not only this, but it will help to make block-based enumeration clearer as well.

The only warning to heed is Apple's statement that:

"It is not safe to modify a mutable collection while enumerating through it. Some enumerators may currently allow enumeration of a collection that is modified, but this behavior is not guaranteed to be supported in the future."

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