iOS __kindof NSArray?

Question

I've been checking out iOS 9 new features as a developer and some of them like the StackView look awesome.

When I went the the header file of UIStackView I saw this:

@property(nonatomic,readonly,copy) NSArray<__kindof UIView *> *arrangedSubviews; 

What is the __kindof on NSArray*. Are we able to specify a type on a NSArray * now?

Little test:

@interface DXTEST () @property (strong, nonatomic) NSMutableArray <__kindof NSString *>  *strings; @end  @implementation DXTEST  - ( instancetype ) init {     self = [super init];      if( self ) {         _strings = [NSMutableArray new];          [_strings addObject:@(1)]; <-- compiler warning wieeeeee     }      return self; }  @end 

Answer 1

Sadly the currently top voted answers are a bit incorrect.

You actually can add any subclass of <T> to a generic collection:

@interface CustomView : UIView @end @implementation CustomView @end  NSMutableArray<UIView *> *views; UIView *view = [UIView new]; CustomView *customView = [CustomView new]; [views addObject:view]; [views addObject:customView];//compiles and runs! 

But when you'll try to retrieve the object, it will be strictly typed as <T> and will require casting:

//Warning: incompatible pointer types initializing  //`CustomView *` with an expression of type `UIView * _Nullable` CustomView *retrivedView = views.firstObject; 

But if you'll add __kindof keyword the returned type will be changed to kindof T and no casting will be required:

NSMutableArray<__kindof UIView *> *views; <...> CustomView *retrivedView = views.firstObject; 

TLDR: Generics in Objective-C can accept subclasses of <T>, __kindof keyword specifies that return value can also be subclass of <T>.