ios - Odd property declaration syntax containing angular brackets <>

Question

I just downloaded FourInARow from 2015 WWDC sample code (https://developer.apple.com/sample-code/wwdc/2015/) and noticed an odd property declaration in file AAPLViewController.m

@property NSArray<NSMutableArray<CAShapeLayer *> *> *chipLayers;

What does it mean?

Answer 1

It is a new addition to Objective-C, called Lightweight Generics. It was introduced in iOS9 / OS X 10.11 in order to enhance interoperability between Swift and Objective-C. As the documentation says:

Objective-C declarations of NSArray, NSSet and NSDictionary types using lightweight generic parameterization are imported by Swift with information about the type of their contents preserved.

For example, consider the following Objective-C property declarations:

@property NSArray<NSDate *>* dates; 
@property NSSet<NSString *>* words; 
@property NSDictionary<KeyType: NSURL *, NSData *>* cachedData;

Here’s how Swift imports them:

var dates: [NSDate]
var words: Set<String> 
var cachedData: [NSURL: NSData]