Blocks

Syntax#

• // Declare as a local variable:

  returnType (^blockName)(parameterType1, parameterType2, …) = ^returnType(argument1, argument2, …) {…};

• // Declare as a property:

  @property (nonatomic, copy, nullability) returnType (^blockName)(parameterTypes);

• // Declare as a method parameter:

  - (void)someMethodThatTakesABlock:(returnType (^nullability)(parameterTypes))blockName;

• // Declare as an argument to a method call:

  [someObject someMethodThatTakesABlock:^returnType (parameters) {…}];

• // Declare as a typedef:

  typedef returnType (^TypeName)(parameterTypes);

  TypeName blockName = ^returnType(parameters) {…};

• // Declare a C function return a block object:

  BLOCK_RETURN_TYPE (^function_name(function parameters))(BLOCK_PARAMETER_TYPE);

Remarks#

Blocks are specified by the Language Specification for Blocks for C, Objective-C, C++ and Objective-C++.

Additionally, the Blocks ABI is defined by the Block Implementation Specification.

Blocks as Method Parameters

- (void)methodWithBlock:(returnType (^)(paramType1, paramType2, ...))name;

Defining and Assigning

A block that performs addition of two double precision numbers, assigned to variable addition:

double (^addition)(double, double) = ^double(double first, double second){
    return first + second;
};

The block can be subsequently called like so:

double result = addition(1.0, 2.0); // result == 3.0

Blocks as Properties

@interface MyObject : MySuperclass

@property (copy) void (^blockProperty)(NSString *string);

@end

When assigning, since self retains blockProperty, block should not contain a strong reference to self. Those mutual strong references are called a “retain cycle” and will prevent the release of either object.

__weak __typeof(self) weakSelf = self;
self.blockProperty = ^(NSString *string) {
    // refer only to weakSelf here.  self will cause a retain cycle
};

It is highly unlikely, but self might be deallocated inside the block, somewhere during the execution. In this case weakSelf becomes nil and all messages to it have no desired effect. This might leave the app in an unknown state. This can be avoided by retaining weakSelf with a __strong ivar during block execution and clean up afterward.

__weak __typeof(self) weakSelf = self;
self.blockProperty = ^(NSString *string) {
    __strong __typeof(weakSelf) strongSelf = weakSelf;
    // refer only to strongSelf here.
    // ...
    // At the end of execution, clean up the reference
    strongSelf = nil;
};

Block Typedefs

typedef double (^Operation)(double first, double second);

If you declare a block type as a typedef, you can then use the new type name instead of the full description of the arguments and return values. This defines Operation as a block that takes two doubles and returns a double.

The type can be used for the parameter of a method:

- (double)doWithOperation:(Operation)operation 
                    first:(double)first 
                   second:(double)second;

or as a variable type:

Operation addition = ^double(double first, double second){
    return first + second;
};

// Returns 3.0
[self doWithOperation:addition
                first:1.0
               second:2.0];

Without the typedef, this is much messier:

- (double)doWithOperation:(double (^)(double, double))operation
                    first:(double)first
                   second:(double)second;

double (^addition)(double, double) = // ...

Blocks as local variables

returnType (^blockName)(parameterType1, parameterType2, ...) = ^returnType(argument1, argument2, ...) {...};    

float (^square)(float) = ^(float x) {return x*x;};

square(5); // resolves to 25
square(-7); // resolves to 49

Here’s an example with no return and no parameters:

NSMutableDictionary *localStatus;
void (^logStatus)() = ^(void){ [MYUniversalLogger logCurrentStatus:localStatus]};

// Insert some code to add useful status information
// to localStatus dictionary 

logStatus(); // this will call the block with the current localStatus