declare a block in Objective-C

• As a property:

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

• As a method parameter:

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

• As an argument to a method call:

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

• As a typedef:

typedef returnType (^TypeName)(parameterTypes);
TypeName blockName = ^returnType(parameters) {...};

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