block作为局部变量使用

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

声明一个block属性

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

作为方法参数（形参）

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

作为方法参数（实参）

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

将block声明为变量类型

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