Block底層原理（一）

（一）block的本質，是一個結構體

（二）捕獲變量
第一種：沒有參數，在block中打印，無需捕獲

        void (^block)(void) = ^{
            NSLog(@"Hello, World!");
        };

編譯成c++代碼後（xcrun -sdk phones clang -arch arm64 -rewrite-objc 要編譯的文件）

        // 定義block變量
        void (*block)(void) = &__main_block_impl_0(
                                                   __main_block_func_0,
                                                   &__main_block_desc_0_DATA
                                                   );

        // 執行block內部的代碼
        block->FuncPtr(block);

struct __main_block_impl_0 {
  struct __block_impl impl;
  struct __main_block_desc_0* Desc;
  // 構造函數（類似於OC的init方法），返回結構體對象
  __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;
  }
};

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

// 封裝了block執行邏輯的函數
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)};

（二）block內有參數傳入(無捕獲)

        void (^block)(int, int) = ^(int a, int b){
            NSLog(@"Hello, World! - %d %d", a, b);
        };

        block(10, 20);

在c++中如何展示

 void (*block)(int, int) = ((void (*)(int, int))&__main_block_impl_0((void *)__main_block_func_0, &__main_block_desc_0_DATA));

        block->FuncPtr(block, 10, 20);

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;
  }
};

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

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)};

static void __main_block_func_0(struct __main_block_impl_0 *__cself, int a, int b) {

            NSLog((NSString *)&__NSConstantStringImpl__var_folders_2r__m13fp2x2n9dvlr8d68yry500000gn_T_main_87bc8b_mi_0, a, b);
        }

（三）block內部訪問了局部變量
( int age = 10,被capture，值傳遞； static int height = 10;被capture，指針傳遞)

  void test()
{
    int age = 10;
    static int height = 10;
    
    block = ^{
        // age的值捕獲進來（capture）
        NSLog(@"age is %d, height is %d", age, height);
    };
    
    age = 20;
    height = 20;
}

       test();
       block();
       

在c++內部如何顯示

   test();
  ((void (*)(__block_impl *))((__block_impl *)block)->FuncPtr)((__block_impl *)block);

void test()
{
    int age = 10;
    static int height = 10;

    block = ((void (*)())&__test_block_impl_0((void *)__test_block_func_0, &__test_block_desc_0_DATA, age, &height));

    age = 20;
    height = 20;
}

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;
  }
};

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

static struct __test_block_desc_0 {
  size_t reserved;
  size_t Block_size;
} __test_block_desc_0_DATA = { 0, sizeof(struct __test_block_impl_0)};

block的類型：NSGlobalBlock NSMallocBlock NSStackBlock
三種

int main(int argc, const char * argv[]) {
    @autoreleasepool {
        int a = 10;
        
        // 堆：動態分配內存,需要程序員申請申請，也需要程序員自己管理內存
        void (^block1)(void) = ^{
            NSLog(@"Hello");
        };
        
        int age = 10;
        void (^block2)(void) = ^{
            NSLog(@"Hello - %d", age);
        };
        
        NSLog(@"%@ %@ %@", [block1 class], [block2 class], [^{
            NSLog(@"%d", age);
        } class]);
    }
    return 0;
}
打印結果如圖
![block1未訪問任何變量，爲__NSGlobalBlock__ ；block訪問了局部變量，本爲stack，但在ARC下，自動copy變爲malloc
](https://img-blog.csdnimg.cn/20181215221333568.png)

以下爲總結：
|訪問了auto變量| stack |經過copy變成malloc
|沒有訪問auto變量|global|經過copy，無任何改變
| copy stack類型的block 後|變爲malloc |再copy後，retaincount+1