iOS-三种类型Block

• block本质上也是一个OC对象，它内部也有个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是封装函数及其上下文的OC对象

查看block源码：

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

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;
  }
};

// 封装了block执行逻辑的函数
static void __main_block_func_0(struct __main_block_impl_0 *__cself) {

            NSLog((NSString *)&__NSConstantStringImpl__var_folders_2r__m13fp2x2n9dvlr8d68yry500000gn_T_main_c60393_mi_0);
        }

static struct __main_block_desc_0 {
  size_t reserved;
  size_t Block_size;
} __main_block_desc_0_DATA = { 0, sizeof(struct __main_block_impl_0)};
int main(int argc, const char * argv[]) {
    /* @autoreleasepool */ { __AtAutoreleasePool __autoreleasepool;
        // 定义block变量
        void (*block)(void) = &__main_block_impl_0(
                                                   __main_block_func_0,
                                                   &__main_block_desc_0_DATA
                                                   );

        // 执行block内部的代码
        block->FuncPtr(block);
    }
    return 0;
}

第二部分：Block特点自动捕获变量

1

int a=10;
void (^Block)(void) = ^{
    NSLog(@"a:%d",a);
};
a = 20;
Block();

输出值为 a：10
原因：创建block的时候，已经把a的值存储在里面了。

 2

auto int a = 10;
static int b = 25;
void (^Block)(void) = ^{
    NSLog(@"a:%d,num:%d",a,b);
};
a = 20;
b = 11;
Block();

输出结果为：a:10,b:11
愿意：auto变量block访问方式是值传递，static变量block访问方式是指针传递
 clang编译以后的源码如下

int a = __cself->a; // bound by copy
int *b = __cself->b; // bound by copy

NSLog((NSString *)&__NSConstantStringImpl__var_folders_2r__m13fp2x2n9dvlr8d68yry500000gn_T_main_d2875b_mi_0, a, (*b));

int a = 10;
static int b = 25;

block = ((void (*)())&__test_block_impl_0((void *)__test_block_func_0, &__test_block_desc_0_DATA, a, &b));

a = 20;
b = 11;

上述代码可查看 static修饰的变量，是根据指针访问的

第三部分 Block类型

block的类型，取决于isa指针，可以通过调用class方法或者isa指针查看具体类型，最终都是继承自NSBlock类型

• __NSGlobalBlock __ （ _NSConcreteGlobalBlock   0x1）
• __NSStackBlock __ （ _NSConcreteStackBlock  0x7 ）
• __NSMallocBlock __ （ _NSConcreteMallocBlock  0x6）

指针在0x1是全局   0x6堆   0x7栈

int a = 1;
void (^block1)(void) = ^{
    NSLog(@"block1");
};
block1();
void (^block2)(void) = ^{
    NSLog(@"block2:%d",a);
};
block2();
NSLog(@"%@/%@/%@",block1,block2,[^{
    NSLog(@"block3:%d",a);
} class]);

输出结果：
<NSGlobalBlock   0x1887898789899>/<NSMallocBlock 0x697878979>/<NSStackBlock  0x7s87687686>