iOS Block实现探究

使用clang的rewrite-objc filename 能够将有block的c代码转换成cpp代码。从中能够看到block的实现。

#include <stdio.h>
int main()
{
  void (^blk)(void) = ^{
    printf("Block\n");
  };
  blk();
  return 0;
}

使用clang rewrite-objc之后会看到block的实现

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;
  }
};

能够看到其实block是一个正常的OC类

来看看block是怎样访问外部变量的

int main()
{
  int dmy = 256;
  int val = 10;
  const char *fmt = "val = %d\n";
  void (^blk)(void) = ^{
    printf(fmt, val);
  };
  return 0;
}

转换以后，能够看到

struct __main_block_impl_0 {
  struct __block_impl impl;
  struct __main_block_desc_0* Desc;
  const char *fmt;
  int val;
  __main_block_impl_0(void *fp, struct __main_block_desc_0 *desc, const char *_fmt, int _val, int flags=0) : fmt(_fmt), val(_val) {
    impl.isa = &_NSConcreteStackBlock;
    impl.Flags = flags;
    impl.FuncPtr = fp;
    Desc = desc;
  }
};

int main()
{
  int dmy = 256;
  int val = 10;
  const char *fmt = "val = %d\n";
  void (*blk)(void) = (void (*)())&__main_block_impl_0((void *)__main_block_func_0, &__main_block_desc_0_DATA, fmt, val);
  return 0;
}

block的变量会被复制进block中

若是当block要改变传入的变量值怎么办？首先看一下全局变量和本地静态变量

int global_val = 1;
static int static_global_val = 2;
int main()
{
  static int static_val = 3;
  void(^blk)(void) = ^{
    global_val *= 1;
    static_global_val *= 2;
    static_val *= 3;
  };
  blk();
  return 0;
}

struct __main_block_impl_0 {
  struct __block_impl impl;
  struct __main_block_desc_0* Desc;
  int *static_val;
  __main_block_impl_0(void *fp, struct __main_block_desc_0 *desc, int *_static_val, int flags=0) : static_val(_static_val) {
    impl.isa = &_NSConcreteStackBlock;
    impl.Flags = flags;
    impl.FuncPtr = fp;
    Desc = desc;
  }
};
static void __main_block_func_0(struct __main_block_impl_0 *__cself) {
  int *static_val = __cself->static_val; // bound by copy

    global_val *= 1;
    static_global_val *= 2;
    (*static_val) *= 3;
  }

因为全局变量是在Data Section中，因此直接能够访问。局部静态变量是经过将其指针传入到block中，block就能够对其值进行修改。

而后看一下__block修饰符变量

int main()
{
  __block int val = 10;
  void (^blk)(void) = ^{val = 1;};
  blk();
  return 0;
}

struct __main_block_impl_0 {
  struct __block_impl impl;
  struct __main_block_desc_0* Desc;
  __Block_byref_val_0 *val; // by ref
  __main_block_impl_0(void *fp, struct __main_block_desc_0 *desc, __Block_byref_val_0 *_val, int flags=0) : val(_val->__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_val_0 *val = __cself->val; // bound by ref
(val->__forwarding->val) = 1;}
static void __main_block_copy_0(struct __main_block_impl_0*dst, struct __main_block_impl_0*src) {_Block_object_assign((void*)&dst->val, (void*)src->val, 8/*BLOCK_FIELD_IS_BYREF*/);}

__block修饰符的变量，会生成一个__Block_byref_val_0的struct，而后经过访问其__forwarding来访问val值。由于Block有多是在stack或者heap中，因此用__forwarding来访问。之因此会将__block单独生成一个struct是由于可能该变量会被多个block使用。

Block分三种类型

0) NSConcreteStackBlock    --stack

1) NSConcreteGlobalBlock   --data area

2) NSConcreteMallocBlock   --heap

自动copy block

当开启ARC时，在某些状况编译器会自动copy block，从stack到heap。

typedef int （^blk_t)(int);
blk_t func(int rate)
{
  return ^(int count){return rate * count;};
}

blk_t func(int rate)
{
  blk_t tmp = &__func_block_impl_0(
    __func_block_func_0, &__func_block_desc_0_DATA, rate);

  tmp = objc_retainBlock(tmp);
  return objc_autoreleaseReturnValue(tmp);
}

有些状况，编译器是没法检测是否应该copy block： 

当block做为参数传递到方法或函数中。

可是，若是该方法或函数在内部copy，就不用手动再copy：

0）cocoa framework方法， 有usingBlock

1) GCD API

- (id) getBlockArray
{
  int val = 10;
  return [[NSArray alloc] initWithObjects:
     [^{NSLog(@"blk0:%d", val);} copy],
     [^{NSLog(@"blk1:%d", val);} copy], nil];
}

__forwarding

当block从stack copy到 heap中时，block中用到的__block也会copy到heap中，而且copy到heap中的block拥有该__block。

__block int val = 0;
void (^blk)(void) = [^{++val;} copy];

++val;

blk();

NSLog(@"%d", val);

在block和外的++val都会变成 ++(val.__forwarding->val);

当block copy到heap中后， stack中的__forwarding会指向heap中的__block, heap中的__forwarding会指向本身的__block值，这样保证了__forwarding指向的是同一个变量值。

Block何时会copy到heap中

0）对block调用copy方法。

1）block做为一个函数的返回值。 编译器自动copy

2）赋值给id或block type class 有__strong 修饰符的成员变量。   编译器自动copy

3）usingBlock, GCD API。  在函数内copy

何时用该copy block？

0） block是函数返回值

1） block赋值给id或block type class 有__strong 修饰符的成员变量。 

2）3）usingBlock, GCD API。  在函数内copy