GC的笔记本

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block相关的修饰符

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__weak修饰符

__weak修饰对象时和废弃对象时的调用流程

__weak修饰对象时和废弃对象时的调用流程

__block说明符

原始代码

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#import <Foundation/Foundation.h>
int main(int argc, char const *argv[])
{
	__block int a = 1;
	^{
		a ++;
	};
	return 0;
}

经clang编译后

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


struct __Block_byref_a_0 {
  void *__isa;
__Block_byref_a_0 *__forwarding;
 int __flags;
 int __size;
 int a; // 最后一个成员变量相当于源自动变量的成员变量
};

struct __main_block_impl_0 {
  struct __block_impl impl;
  struct __main_block_desc_0* Desc;
  __Block_byref_a_0 *a; // by ref
  __main_block_impl_0(void *fp, struct __main_block_desc_0 *desc, __Block_byref_a_0 *_a, int flags=0) : a(_a->__forwarding) {
    impl.isa = &_NSConcreteStackBlock;
    impl.Flags = flags;
    impl.FuncPtr = fp;
    Desc = desc;
  }
};

static void __main_block_func_0(struct __main_block_impl_0 *__cself) {
  __Block_byref_a_0 *a = __cself->a; // bound by ref

  (a->__forwarding->a) ++;
 }

static void __main_block_copy_0(struct __main_block_impl_0*dst, struct __main_block_impl_0*src) {_Block_object_assign((void*)&dst->a, (void*)src->a, 8/*BLOCK_FIELD_IS_BYREF*/);}

static void __main_block_dispose_0(struct __main_block_impl_0*src) {_Block_object_dispose((void*)src->a, 8/*BLOCK_FIELD_IS_BYREF*/);}

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};

int main(int argc, char const *argv[])
{
  // __block 变成了__Block_byref_a_0结构体类型的自动变量，既在栈上生成的__Block_byref_a_0结构体实例，命名规则:__Block_byref_变量名_出现的顺序
  __attribute__((__blocks__(byref))) __Block_byref_a_0 a = {
    (void*)0,
    (__Block_byref_a_0 *)&a, 
    0, 
    sizeof(__Block_byref_a_0), 
    1 // 初始值为1
  };

 ((void (*)())&__main_block_impl_0((void *)__main_block_func_0, &__main_block_desc_0_DATA, (__Block_byref_a_0 *)&a, 570425344));
 return 0;
}

__block变量和对象

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#import <Foundation/Foundation.h>
int main(int argc, char const *argv[])
{
	__block id obj = [[NSObject alloc] init];
	// ARC下等同 __block id _strong obj = [[NSObject alloc] init];
	return 0;
}
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/*
  当block从栈上复制到堆上时,使用_Block_object_assign函数，持有Block截获的对象
  当堆上的Block被废弃时，使用_Block_object_dispose函数,释放Block截获的对象
*/

// __block变量声明部分
struct __Block_byref_obj_0 {
  void *__isa;
__Block_byref_obj_0 *__forwarding;
 int __flags;
 int __size;
 void (*__Block_byref_id_object_copy)(void*, void*);
 void (*__Block_byref_id_object_dispose)(void*);
 __strong id obj;
};

int main(int argc, char const *argv[])
{
  // __block 变量赋值部分
 __attribute__((__blocks__(byref))) __Block_byref_obj_0 obj = 
 {
  (void*)0,
  (__Block_byref_obj_0 *)&obj, 
  33554432, 
  sizeof(__Block_byref_obj_0), 
  __Block_byref_id_object_copy_131, 
  __Block_byref_id_object_dispose_131, 
  ((NSObject *(*)(id, SEL))(void *)objc_msgSend)((id)((NSObject *(*)(id, SEL))(void *)objc_msgSend)((id)objc_getClass("NSObject"), sel_registerName("alloc")), sel_registerName("init"))};

 return 0;
}