Block的理解

一、block的本质

block本质上也是一个OC对象，它内部也有一个isa指针
block是封装了函数调用以及函数调用环境的OC对象
block内部代码会封装到_block_func_0函数中，函数地址保存在FuncPtr中
执行block内部代码时是通过FuncPtr找到函数地址进行调用

block的底层结构如下：

struct __block_impl {
    void *isa;
    int Flags;
    int Reserved;
    void *FuncPtr;
};

static struct __main_block_desc_0 {
    size_t reserved;
    size_t Block_size;
}

struct __main_block_impl_0 {
    struct __block_impl impl;
    struct __main_block_desc_0* Desc;
    //构造函数（类似于OC的init方法），返回结构体对象
    __main_block_impl_0(void *fp, struct __main_block_desc_0 *desc, 
    int _age, int flags=0) : age(_age) {
        impl.isa = &_NSConcreteStackBlock;
        impl.Flags = flags;
        impl.FuncPtr = fp;
        Desc = desc;
    }
};

二、block的变量捕获机制

为了保证block内部能够正常访问外部的变量，block有个变量捕获机制

block变量捕获.jpg

1、block内部访问auto变量时会将auto变量捕获到block内部，block外部修改auto变量的值并不会影响block内部，所以是值捕获

int main(int argc, const char * argv[]) {
    @autoreleasepool {
        int age = 10;
        void (^block)(void) = ^ {
            NSLog(@"age = %d", age);  // age = 10
        };
        age = 20;
        block();
    }
    return 0;
}

// 底层C++代码
struct __main_block_impl_0 {
  struct __block_impl impl;
  struct __main_block_desc_0* Desc;
  int age;
  __main_block_impl_0(void *fp, struct __main_block_desc_0 *desc, int _age, int flags=0) : age(_age) {
    impl.isa = &_NSConcreteStackBlock;
    impl.Flags = flags;
    impl.FuncPtr = fp;
    Desc = desc;
  }
};

2、block访问static变量时会将auto变量捕获到block内部，block外部修改static变量的值会影响block内部，所以是指针捕获

int main(int argc, const char * argv[]) {
    @autoreleasepool {
        static int age = 10;
        void (^block)(void) = ^ {
            NSLog(@"age = %d", age); // age = 20
        };
        age = 20;
        block();
    }
    return 0;
}

// 底层C++代码
struct __main_block_impl_0 {
  struct __block_impl impl;
  struct __main_block_desc_0* Desc;
  int *age;
  __main_block_impl_0(void *fp, struct __main_block_desc_0 *desc, int *_age, int flags=0) : age(_age) {
    impl.isa = &_NSConcreteStackBlock;
    impl.Flags = flags;
    impl.FuncPtr = fp;
    Desc = desc;
  }
};

3、block访问全局变量时不会将全局变量捕获到block内部，而是直接访问

int age = 10;

int main(int argc, const char * argv[]) {
    @autoreleasepool {
        void (^block)(void) = ^ {
            NSLog(@"age = %d", age); // age = 20
        };
        age = 20;
        block();
    }
    return 0;
}

//底层C++代码
struct __main_block_impl_0 {
  struct __block_impl impl;
  struct __main_block_desc_0* Desc;
  __main_block_impl_0(void *fp, struct __main_block_desc_0 *desc, int flags=0) {
    impl.isa = &_NSConcreteStackBlock;
    impl.Flags = flags;
    impl.FuncPtr = fp;
    Desc = desc;
  }
};

三、block的类型

block类型	环境
NSGlobalBlock	没有访问auto变量
NSStackBlock	访问了auto变量
NSMallocBlock	NSStackBlock调用了copy

四、block的copy

在ARC环境下，编译器会根据情况自动将栈上的block复制到堆上，比如以下情况：
1、block作为函数的返回值时
2、将block赋值给__strong指针时
3、block作为Cocoa API中方法名含有usingBlock的方法参数时，例如

NSArray *array = @[];
[array enumerateObjectsUsingBlock:^(id  _Nonnull obj, 
NSUInteger idx, BOOL * _Nonnull stop) {
            
}];

4、block作为GCD API的方法参数时，例如：

dispatch_after(dispatch_time(DISPATCH_TIME_NOW, 
(int64_t)(1 * NSEC_PER_SEC)), dispatch_get_main_queue(), ^{
            
});

四、解决block的循环引用

1、用__weak、__unsafe_unretained
__weak：不会产生强引用，指向的对象销毁时，会自动让指针置为nil

__weak typeof(self)weakSelf = self;
self.block = ^ {
    NSLog(@"%p", weakSelf);
};

__unsafe_unretained：不会产生强引用，不安全，指向的对象销毁时，指针存储的地址值不变

__unsafe_unretained id weakSelf = self;
self.block = ^ {
    NSLog(@"%p", weakSelf);
};

2、用__block解决（必须要调用block）

__block QLPerson *person = [[QLPerson alloc] init];
person.age = 10;
person.block = ^ {
    NSLog(@"age is %d", person.age);
    person = nil;
};
person.block();