iOS中怎么定义一个block

1.作为局部变量

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

2.作为属性

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

3.作为方法参数

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

4.作为方法调用的参数

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

5.作为一个类型

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

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