Can I use Objective-C blocks as properties

Question

Is it possible to have blocks as properties using the standard property syntax   Are there any changes for ARC

User · Answer

For Swift  just use closures  example     In Objective-C    property  copy void   property  copy void   doStuff  void     It s that simple   Here is the actual Apple documentation  which states precisely what to use   Apple doco   In your  h file      Here is a block as a property        Someone passes you a block  You  hold on to it      while you do other stuff  Later  you use the block        The property  doStuff  will hold the incoming block    property  copy void   doStuff  void       Here s a method in your class     When someone CALLS this method  they PASS IN a block of code     which they want to be performed after the method is finished   - void doSomethingAndThenDoThis  void    void  pleaseDoMeLater      We will hold on to that block of code in  doStuff     Here s your  m file    - void doSomethingAndThenDoThis  void    void  pleaseDoMeLater              Regarding the incoming block of code  save it for later      self doStuff   pleaseDoMeLater          Now do other processing  which could follow various paths         involve delays  and so on  Then after everything       self  alldone          - void  alldone           NSLog   Processing finished  running the completion block            Here s how to run the block      if   self doStuff    nil          self doStuff            Beware of out-of-date example code   With modern  2014   systems  do what is shown here  It is that simple

User · Answer

property  nonatomic  copy  void   simpleBlock  void    property  nonatomic  copy  BOOL   blockWithParamter  NSString  input     If you are going to be repeating the same block in several places use a type def  typedef void  MyCompletionBlock  BOOL success  NSError  error    property  nonatomic  MyCompletionBlock completion

User · Answer

Here s an example of how you would accomplish such a task    import  lt Foundation Foundation h gt  typedef int   IntBlock       interface myobj   NSObject       IntBlock compare      property readwrite  copy  IntBlock compare    end   implementation myobj   synthesize compare   -  void dealloc          need to release the block since the property was declared copy   for heap       allocated blocks this prevents a potential leak  for compiler-optimized        stack blocks it is a no-op        Note that for ARC  this is unnecessary  as with all properties  the memory management is handled for you      compare release       super dealloc      end  int main           autoreleasepool           myobj  ob     myobj alloc  init           ob compare                           return rand                       NSLog    i   ob compare                if not ARC          ob release              return 0      Now  the only thing that would need to change if you needed to change the type of compare would be the typedef int   IntBlock     If you need to pass two objects to it  change it to this  typedef int   IntBlock  id  id   and change your block to       id obj1  id obj2        return rand         I hope this helps   EDIT March 12  2012   For ARC  there are no specific changes required  as ARC will manage the blocks for you as long as they are defined as copy  You do not need to set the property to nil in your destructor  either    For more reading  please check out this document  http   clang llvm org docs AutomaticReferenceCounting html

User · Answer

Disclamer  This is not intended to be  the good answer   as this question ask explicitly for ObjectiveC  As Apple introduced Swift at the WWDC14  I d like to share the different ways to use block  or closures  in Swift   Hello  Swift  You have many ways offered to pass a block equivalent to function in Swift    I found three   To understand this I suggest you to test in playground this little piece of code   func test function String - gt  String  - gt  String       return function  test      func funcStyle s String  - gt  String       return  FUNC      s      FUNC    let resultFunc   test funcStyle   let blockStyle  String  - gt  String    s in return  BLOCK      s      BLOCK   let resultBlock   test blockStyle   let resultAnon   test   s String  - gt  String in return  ANON     s      ANON       println resultFunc  println resultBlock  println resultAnon    Swift  optimized for closures  As Swift is optimized for asynchronous development  Apple worked more on closures  The first is that function signature can be inferred so you don t have to rewrite it    Access params by numbers  let resultShortAnon   test  return  ANON      0      ANON       Params inference with naming  let resultShortAnon2   test  myParam in return  ANON     myParam      ANON       Trailing Closure  This special case works only if the block is the last argument  it s called trailing closure  Here is an example  merged with inferred signature to show Swift power   let resultTrailingClosure   test   return  TRAILCLOS      0      TRAILCLOS      Finally   Using all this power what I d do is mixing trailing closure and type inference  with naming for readability   PFFacebookUtils logInWithPermissions permissions        user  error in     if   user            println  Uh oh  The user cancelled the Facebook login          else if  user isNew            println  User signed up and logged in through Facebook          else           println  User logged in through Facebook

User · Answer

Of course you could use blocks as properties  But make sure they are declared as  property copy     For example     typedef void  TestBlock  void     interface SecondViewController   UIViewController  property  nonatomic  copy  TestBlock block   end   In MRC  blocks capturing context variables are allocated in stack  they will be released when the stack frame is destroyed  If they are copied  a new block will be allocated in heap  which can be executed later on after the stack frame is poped

User · Answer

Hello  Swift  Complementing what  Francescu answered   Adding extra parameters   func test function String - gt  String  param1 String  param2 String  - gt  String       return function  test  param1   param2     func funcStyle s String  - gt  String       return  FUNC      s      FUNC    let resultFunc   test funcStyle   parameter 1    parameter 2    let blockStyle  String  - gt  String    s in return  BLOCK      s      BLOCK   let resultBlock   test blockStyle   parameter 1    parameter 2    let resultAnon   test   s String  - gt  String in return  ANON     s      ANON      parameter 1    parameter 2     println resultFunc  println resultBlock  println resultAnon

User · Answer

You can follow the format below and can use the testingObjectiveCBlock property in the class   typedef void   testingObjectiveCBlock  NSString  errorMsg     interface MyClass   NSObject  property  nonatomic  strong  testingObjectiveCBlock testingObjectiveCBlock   end   For more info have a look here

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For posterity   completeness s sake    Here are two FULL examples of how to implement this ridiculously versatile  way of doing things    Robert s answer is blissfully concise and correct  but here I want to also show ways to actually  define  the blocks     interface       ReusableClass   NSObject  property  nonatomic copy  CALayer   layerFromArray  NSArray     end   implementation  ResusableClass static  NSString const   privateScope     Touch my monkey     -  CALayer     NSArray    layerFromArray         return  CALayer  NSArray  array           CALayer  returnLayer   CALayer layer         for  id thing in array                 returnLayer doSomethingCrazy                returnLayer setValue privateScope                          forKey   anticsAndShenanigans                      return list            end   Silly  Yes   Useful  Hells yeah   Here is a different   more atomic  way of setting the property   and a class that is ridiculously useful      interface      CALayoutDelegator   NSObject  property  nonatomic strong  void  layoutBlock  CALayer     end   implementation CALayoutDelegator -  id  init       return self   super init              self setLayoutBlock    CALayer layer             for  CALayer  sub in layer sublayers               sub someDefaultLayoutRoutine                self   nil    -  void  layoutSublayersOfLayer  CALayer  layer      self layoutBlock   self layoutBlock layer    nil        end   This illustrates setting the block property via the accessor  albeit inside init  a debatably dicey practice    vs the first example s  nonatomic   getter  mechanism   In either case    the  hardcoded  implementations can always be overwritten  per instance   a l      CALayoutDelegator  littleHelper   CALayoutDelegator new  littleHelper layoutBlock     CALayer layer      layer sublayers do   id sub    sub somethingElseEntirely          someLayer layoutManager   littleHelper    Also   if you want to add a block property in a category    say you want to use a Block instead of some old-school target   action  action      You can just use associated values to  well   associate the blocks   typedef    void  NSControlActionBlock  NSControl      interface       NSControl             ActionBlocks   property  copy  NSControlActionBlock  actionBlock      end  implementation  NSControl             ActionBlocks   -  NSControlActionBlock  actionBlock           use the  getter  method s selector to store retrieve the block      return  objc getAssociatedObject self   cmd       -  void  setActionBlock  NSControlActionBlock ab        objc setAssociatedObject     save  copy  the block associatively  as categories can t synthesize Ivars      self   selector actionBlock  ab  OBJC ASSOCIATION COPY       self target   self                      set self as target  where you call the block      self action    selector doItYourself      this is where it s called    -  void  doItYourself        if  self actionBlock  amp  amp  self target    self  self actionBlock self      end   Now  when you make a button  you don t have to set up some IBAction drama    Just associate the work to be done at creation        button actionBlock     NSControl thisButton           doc open    thisButton setEnabled NO         This pattern can be applied OVER and OVER to Cocoa API s   Use properties to bring the relevant parts of your code closer together  eliminate convoluted delegation paradigms  and leverage the power of objects beyond that of just acting as dumb  containers

[ios] Can I use Objective-C blocks as properties?

@property (copy)void

Here is the actual Apple documentation, which states precisely what to use:

Beware of out-of-date example code.

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