基本认识

block本质上是一个封装了函数调用以及函数调用环境的OC对象，内部也包含一个isa指针。

block的底层实现

简单的block底层实现如下：

struct __main_block_impl_0 {
  struct __block_impl impl;
  struct __main_block_desc_0* Desc;
  //构造函数
  __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;
  }
};

struct __block_impl {
  void *isa;
  int Flags;
  int Reserved;
  void *FuncPtr;	//函数指针
};

static struct __main_block_desc_0 {
  size_t reserved;
  size_t Block_size; //内存占用
} __main_block_desc_0_DATA = { 0, sizeof(struct __main_block_impl_0)};

变量捕获-基础数据类型

auto变量

定义如下一个捕获auto类型变量（离开作用域自动销毁）的block：

int age = 10;
void (^block)(void) = ^(){
    NSLog(@"%d",age);
};

将其转换为c++代码：

1 2	int age = 10; void (block)(void) = ((void ()())&__main_block_impl_0((void *)__main_block_func_0, &__main_block_desc_0_DATA, age));

block底层结构为:

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;
  }
};

可见，block在捕获auto变量的方式为值传递。

static变量

定义如下一个捕获static类型变量的block：

static int age = 10;
void (^block)(void) = ^(){
    NSLog(@"%d",age);
};

将其转换为c++代码：

1 2	static int age = 10; void (block)(void) = ((void ()())&__main_block_impl_0((void *)__main_block_func_0, &__main_block_desc_0_DATA, &age));

block底层结构为:

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;
  }
};

可见，block在捕获static变量的方式为指针传递。

全局变量

定义如下一个捕获全局变量的block：

//age此时被定义为全局变量
void (^block)(void) = ^(){
    NSLog(@"%d",age);
};

将其转换为c++代码：

1 2	void (block)(void) = ((void ()())&__main_block_impl_0((void *)__main_block_func_0, &__main_block_desc_0_DATA));

block底层结构为:

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;
  }
};

可见，全局变量可以直接使用，不会被block捕获。

总结

变量捕获-对象类型

当block内部访问了对象类型的auto变量时:

如果block是在栈上，将不会对auto变量产生强引用
如果block被拷贝到堆上:
- 会调用block内部的copy函数
- copy函数内部会调用_Block_object_assign函数
- _Block_object_assign函数会根据auto变量的修饰符（__strong、__weak、__unsafe_unretained）做出相应的操作，形成强引用（retain）或者弱引用
如果block从堆上移除:
- 会调用block内部的dispose函数
- dispose函数内部会调用_Block_object_dispose函数
- _Block_object_dispose函数会自动释放引用的auto变量（release）

此时block的底层相关代码如下：

static struct __main_block_desc_0 {
  size_t reserved;
  size_t Block_size; 
  void (*copy)(struct __main_block_impl_0*, struct __main_block_impl_0*);
  void (*dispose)(struct __main_block_impl_0*);
} __main_block_desc_0_DATA = { 0, sizeof(struct __main_block_impl_0), __main_block_copy_0, __main_block_dispose_0};

static void __main_block_copy_0(struct __main_block_impl_0*dst, struct __main_block_impl_0*src) {
	_Block_object_assign((void*)&dst->person, (void*)src->person, 3/*BLOCK_FIELD_IS_OBJECT*/);
}

static void __main_block_dispose_0(struct __main_block_impl_0*src) 
{
	_Block_object_dispose((void*)src->person, 3/*BLOCK_FIELD_IS_OBJECT*/);
}

block的类型

blcok有三种类型，都继承自NSBlock。可以通过class方法或者isa指针查看其具体类型。

__NSGlobalBlock__ （_NSConcreteGlobalBlock) :没有访问auto变量
__NSStackBlock__ （_NSConcreteStackBlock : 访问了auto变量
__NSMallocBlock__ （_NSConcreteMallocBlock）: __NSStackBlock__调用了copy方法

下面代码的输出结果为：__NSGlobalBlock__

void (^block)(void) = ^(){
    NSLog(@"Hello World");
};
NSLog(@"%@",[block class]);

下面代码ARC环境下的输出结果为：__NSMallocBlock__，MRC的输出结果为：__NSStackBlock__

int age = 10;
void (^block)(void) = ^(){
    NSLog(@"%d",age);
};
NSLog(@"%@",[block class]);

下面代码的输出结果为：__NSStackBlock__

int age = 10;
NSLog(@"%@",[^{
    NSLog(@"%d",age);
} class]);

三种类型block的内存存储以及每一种类型的block调用copy后的结果如下所示:

__block修饰符

底层实现

__block可以用于解决block内部无法修改auto变量值的问题。__block不能修饰全局变量、静态变量（static）。

__block int age = 10;
__block NSObject *obj = [[NSObject alloc] init];

void (^block)(void) = ^{
	obj = nil;
	age = 20;
};

上面的代码会被编译为:

struct __main_block_impl_0 {
  struct __block_impl impl;
  struct __main_block_desc_0* Desc;
  __Block_byref_obj_1 *obj; // by ref
  __Block_byref_age_0 *age; // by ref
  __main_block_impl_0(void *fp, struct __main_block_desc_0 *desc, __Block_byref_age_0 *_age, int flags=0) : age(_age->__forwarding) {
    impl.isa = &_NSConcreteStackBlock;
    impl.Flags = flags;
    impl.FuncPtr = fp;
    Desc = desc;
  }
};

struct __Block_byref_age_0 {
  void *__isa;
__Block_byref_age_0 *__forwarding;
 int __flags;
 int __size;
 int age;
};

struct __Block_byref_obj_1 {
  void *__isa;
__Block_byref_obj_1 *__forwarding;
 int __flags;
 int __size;
 //内存管理
 void (*__Block_byref_id_object_copy)(void*, void*);
 void (*__Block_byref_id_object_dispose)(void*);
 NSObject  *obj;
};

//实际执行的block代码
static void __main_block_func_0(struct __main_block_impl_0 *__cself) {
  __Block_byref_obj_1 *obj = __cself->obj; // bound by ref
  __Block_byref_age_0 *age = __cself->age; // bound by ref

            (obj->__forwarding->obj) = __null;
            (age->__forwarding->age) = 20;
        }