1.Block 實質(zhì)

通過clang(LLVM編譯器)我們可以把block轉(zhuǎn)換為我們想要的可讀代碼，該代碼是c++源碼，但實際上也就用了struct的結(jié)構(gòu)，本質(zhì)為C語言

clang -rewrite-objc  main.m   //無uikit庫調(diào)用
clang -x objective-c -rewrite-objc -isysroot /Applications/Xcode.app/Contents/Developer/Platforms/iPhoneSimulator.platform/Developer/SDKs/iPhoneSimulator.sdk main.m //含uikit庫調(diào)用

為了進一步弄清Block在底層的代碼邏輯，我們先將函數(shù)簡化如下并運行上面的命令得到main.cpp文件

void (^blk)(void) = ^{
        printf("Block function");
      };
    blk();

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;
  }
};
static void __main_block_func_0(struct __main_block_impl_0 *__cself) {

        printf("Block function");
      }

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, char * argv[]) {

    void (*blk)(void) = ((void (*)())&__main_block_impl_0((void *)__main_block_func_0, &__main_block_desc_0_DATA));
    ((void (*)(__block_impl *))((__block_impl *)blk)->FuncPtr)((__block_impl *)blk);

    return 0;
}

與C++的this和OC的self相同，__cself是__main_block_impl_0的結(jié)構(gòu)體指針，該結(jié)構(gòu)體有兩個成員變量 一個是impl 一個是Desc指針，便于理解我們先將如下代碼簡化

void (*blk)(void) = ((void (*)())&__main_block_impl_0((void *)__main_block_func_0, &__main_block_desc_0_DATA));

struct __main_block_impl_0 tmp = __main_block_impl_0(__main_block_func_0,&__main_block_desc_0_DATA);
struct __main_block_impl_0 *blk = &tmp;

不難看出，將Block語法生成的Block賦給Block類型變量blk，等同于將__main_block_impl_0的結(jié)構(gòu)體實例的指針賦值給變量blk，
再來看blk部分

((void (*)(__block_impl *))((__block_impl *)blk)->FuncPtr)((__block_impl *)blk);

(*blk->impl.FuncPtr)(blk);

這就是簡單的使用函數(shù)指針調(diào)用函數(shù)，由代碼看出Block真是作為參數(shù)進行了傳遞

關(guān)于impl 中impl.isa = &_NSConcreteStackBlock; _NSConcreteStackBlock可以看做class_t的結(jié)構(gòu)體實例，將Block作為OC對象處理時，該類信息放在_NSConcreteStackBlock中，isa 對象指向類 對象的isa指向本類 類的isa指向元類 元類的isa指向根元類 根元類的isa指向自己