As a local variable（变量）:
returnType (^blockName)(parameterTypes) = ^returnType(parameters) {...};
As a property（属性）:
@property (nonatomic, copy, nullability) returnType (^blockName)(parameterTypes);
As a method parameter(作为参数):
- (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) {...};
