As a local variable（變量）:
returnType (^blockName)(parameterTypes) = ^returnType(parameters) {...};

As a property（屬性）:
@property (nonatomic, copy, nullability) returnType (^blockName)(parameterTypes);

As a method parameter(作為參數):
- (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) {...};