1.申明一个本地变量

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

bool (^isEventBlock)(NSInteger num) = ^bool(NSInteger num){
    if (num % 2 == 0)
        return YES;
    else
        return NO;
};

2.申明一个属性

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

@property (nonatomic, copy,  nullability) bool (^evenBlock)(NSInteger num);

3.申明一个函数参数

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

- (void)filterNumsWithEvenBlock:(bool (^)(NSInteger num)) evenBlock;

- (void)filterNumsWithEvenBlock:(bool (^)(NSInteger num)) evenBlock
{
    NSInteger num = 3;
    evenBlock(3);
}

4.作为一个函数调用的参数

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

[self filterNumsWithEvenBlock:^bool (NSInteger num){
    if (num % 2 == 0 )
        return YES;
    else
        return NO;
}];

5.重命名typedef

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

typedef bool (^EvenBlock)(NSInteger num);
EvenBlock evenBlock = ^bool (NSInteger num){
    if (num % 2 == 0)
        return YES;
    else
        return NO;
};