objective c - iOS __kindof NSArray? -

i've been checking out ios 9 new features developer , of them stackview awesome.

when went the header file of uistackview saw this:

@property(nonatomic,readonly,copy) nsarray<__kindof uiview *> *arrangedsubviews; 

what __kindof on nsarray*. able specify type on 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 

are able specify type on nsarray * now?

yes, through objective-c's new lightweight generics. in example provided, have property of type nsarray, which accept elements uiviews.

now, can specified follows (without __kindof).

@property(nonatomic,readonly,copy) nsarray<uiview *> *arrangedsubviews; 

and in case, array accept objects class uiview, not objects subclasses of uiview. __kindof declaration marks array's generic type 1 can accept both instances of uiview class, , instances of of uiview's subclasses.

edit:

i've removed bulk of original answer since mistakenly thought specifying type of array prevent inserting objects of incorrect type, , not case. (thanks artem abramov pointing out. please see answer below additional details)

objective-c's generics seem exist give type information when accessing elements of generic collection. example, consider following code adds uiview , uiimageview nsmutablearray<uiview *>. both objects inserted array without complaints compilers or runtime, when try access elements, you're warned compiler if type of variable other generic type of array (uiview), if it's 1 of uiview's subclasses.

nsmutablearray<uiview *> *subviews = [[nsmutablearray alloc] init];  [subviews addobject:[[uiview alloc] init]]; // works [subviews addobject:[[uiimageview alloc] init]]; // works  uiview *sameview = subviews[0]; // works uiimageview *sameimageview = subviews[1]; // incompatible pointer types initializing 'uiimageview *' expression of type 'uiview *'  nslog(@"%@", nsstringfromclass([sameview class])); // uiview nslog(@"%@", nsstringfromclass([sameimageview class])); // uiimageview 

now, produces compile time warning, not crash @ runtime. key difference between , same example array's generic type marked __kindof, compiler won't complain if try access ones of elements, , store result in variable who's type uiview or 1 of subclasses.

nsmutablearray<__kindof uiview *> *subviews = [[nsmutablearray alloc] init];  [subviews addobject:[[uiview alloc] init]]; // works [subviews addobject:[[uiimageview alloc] init]]; // works  uiview *sameview = subviews[0]; // no problem uiimageview *sameimageview = subviews[1]; // no complaints now!  nslog(@"%@", nsstringfromclass([sameview class])); // uiview nslog(@"%@", nsstringfromclass([sameimageview class])); // uiimageview