iOS block以及变量捕获

Block是什么

Block也是一个OC对象，内部也有isa指针。

扩展：

instance对象的isa指向class对象
class对象的isa指向meta-class对象
meta-class对象的isa指向基类的meta-class对象

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

2020-11-09 22:24:30.042560+0800 Interview01-Block的本质[1489:30598] __NSGlobalBlock__
2020-11-09 22:24:30.043005+0800 Interview01-Block的本质[1489:30598] __NSGlobalBlock
2020-11-09 22:24:30.043103+0800 Interview01-Block的本质[1489:30598] NSBlock
2020-11-09 22:24:30.043163+0800 Interview01-Block的本质[1489:30598] NSObject

Block的类型

__NSGlobalBlock__(_NSConcreteGlobalBlock)(存放在数据区) （不访问auto变量的block 即便是访问了static局部变量或者全局变量）

__NSStackBlock__(_NSConcreteStackBlock)(存放在栈区系统管理内存)（访问了auto变量：auto变量就是局部变量）

__NSMallocBlock__(_NSConcreteMallocBlock) (存放在堆区程序员管理内存) (__NSStackBlock__调用了copy)

不同类型变量的捕获方式

1、局部变量和block

使用局部变量的block

int main(int argc, const char * argv[]) {@autoreleasepool {int age = 10;block = ^{// age的值捕获进来（capture）NSLog(@"age is %d, ", age);};age = 20;block();}return 0;              
}

clang编译后的底层代码

struct __test_block_impl_0 {struct __block_impl impl;struct __test_block_desc_0* Desc;int age;__test_block_impl_0(void *fp, struct __test_block_desc_0 *desc, int _age, int flags=0) : age(_age) {impl.isa = &_NSConcreteStackBlock;impl.Flags = flags;impl.FuncPtr = fp;Desc = desc;}
};

可以看到，在block的内部多了成员变量_age，并且在构造这个block时，会把age的值直接传进去，几_age=age，可以看出来这时是block捕获了局部变量，并且进行了值传递。

2、静态局部变量

int main(int argc, const char * argv[]) {@autoreleasepool {// auto：自动变量，离开作用域就销毁auto int age = 10;static int height = 10;void (^block)(void) = ^{// age的值捕获进来（capture）NSLog(@"age is %d, height is %d", age, height); // 10  20};age = 20;height = 20;block();}return 0;
}

编译后的结果：

struct __test_block_impl_0 {struct __block_impl impl;struct __test_block_desc_0* Desc;int age;int *height;__test_block_impl_0(void *fp, struct __test_block_desc_0 *desc, int _age, int *_height, int flags=0) : age(_age), height(_height) {impl.isa = &_NSConcreteStackBlock;impl.Flags = flags;impl.FuncPtr = fp;Desc = desc;}
};//定义的时候
block = ((void (*)())&__test_block_impl_0((void *)__test_block_func_0, &__test_block_desc_0_DATA, age, &height));
//执行的时候
static void __test_block_func_0(struct __test_block_impl_0 *__cself) {int age = __cself->age; // bound by copyint *height = __cself->height; // bound by copyNSLog((NSString *)&__NSConstantStringImpl__var_folders_2r__m13fp2x2n9dvlr8d68yry500000gn_T_main_d2875b_mi_0, age, (*height));}

可以看出来age传递的是值，但是静态局部变量height传递的是指针。

3、全局变量和block

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

编译后：

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 flags=0) {impl.isa = &_NSConcreteStackBlock;impl.Flags = flags;impl.FuncPtr = fp;Desc = desc;}
};
//调用函数的时候
static void __test_block_func_0(struct __test_block_impl_0 *__cself) {NSLog((NSString *)&__NSConstantStringImpl__var_folders_2r__m13fp2x2n9dvlr8d68yry500000gn_T_main_d2875b_mi_0, age_, height_);}

可以看到全局变量没有捕获到block内部，是直接访问的，所以每次访问都能获得最新值。

未完待续

参考文档：

block ios 深入理解 ios __block的作用_imking的技术博客_51CTO博客
iOS OC基础知识笔记_ios oc y-CSDN博客