什么是Block?
Block 又稱為“塊” 或 “代碼塊”蜀细，作用是用來保存代碼恋昼，保存在其內(nèi)部的代碼塊 如果Block不被調(diào)用 這段代碼就不會(huì)執(zhí)行
在OC中Block的基本格式是這樣的

返回值類型  (^block名)  (參數(shù)類型 和 數(shù)量) = ^(形參 和 數(shù)量){   
   //code 
}队寇；

Block的本質(zhì)上也是一個(gè)OC對(duì)象 它內(nèi)部也有個(gè)isa指針
Block是封裝了函數(shù)調(diào)用以及函數(shù)調(diào)用環(huán)境(比如參數(shù)和返回值)的OC對(duì)象
Block是封裝了函數(shù)及其上下文的OC對(duì)象

1、寫一個(gè)簡單的block：

    @autoreleasepool {
        appDelegateClassName = NSStringFromClass([AppDelegate class]);
        MyBlock myBlock = ^{
        };
        NSLog(@"myBlock:%@",myBlock);//myBlock:<__NSGlobalBlock__: 0x10bfde038>
        myBlock();
    }

轉(zhuǎn)成底層代碼

typedef void(*MyBlock)(void);
//Block的本質(zhì)
struct __block_impl {
  void *isa;//isa指針(Block的類型-->指向Class)
  int Flags;
  int Reserved;
  void *FuncPtr;//封裝代碼塊的地址
};

struct __test_block_impl_0 {
  struct __block_impl impl;
  struct __test_block_desc_0* Desc;
// 構(gòu)造函數(shù)（類似于OC的init方法），返回結(jié)構(gòu)體對(duì)象
  __test_block_impl_0(void *fp, struct __test_block_desc_0 *desc, int flags=0) {
    impl.isa = &_NSConcreteStackBlock;
    impl.Flags = flags;
    impl.FuncPtr = fp;
    Desc = desc;
  }
};
static void __test_block_func_0(struct __test_block_impl_0 *__cself) {

}

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)};
void test(){
    MyBlock myBlock = ((void (*)())&__test_block_impl_0((void *)__test_block_func_0, &__test_block_desc_0_DATA));


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

}

2晴弃、block內(nèi)部捕獲變量

int a_q = 10;//全局變量
static int b_q = 10;//全局靜態(tài)變量
typedef void(^MyBlock)(void);
void test(){
    int a = 10;//局部變量
    static int b = 10;//局部靜態(tài)變量
    MyBlock myBlock = ^{
        NSLog(@"a=%d", a);//a=10
        NSLog(@"b=%d", b);//b=20
        NSLog(@"a_q=%d", a_q);//a_q=20
        NSLog(@"b_q=%d", b_q);//b_q=20
    };
    a = 20;
    b = 20;
    a_q = 20;
    b_q = 20;
    myBlock();
}

轉(zhuǎn)成底層代碼

typedef void(*MyBlock)(void);
//Block的本質(zhì)
struct __block_impl {
  void *isa;//isa指針(Block的類型-->指向Class)
  int Flags;
  int Reserved;
  void *FuncPtr;//封裝代碼塊的地址
};
int a_q = 10;
static int b_q = 10;
typedef void(*MyBlock)(void);

struct __test_block_impl_0 {
  struct __block_impl impl;
  struct __test_block_desc_0* Desc;
  int a;
  int *b;
  __test_block_impl_0(void *fp, struct __test_block_desc_0 *desc, int _a, int *_b, int flags=0) : a(_a), b(_b) {
    impl.isa = &_NSConcreteStackBlock;
    impl.Flags = flags;
    impl.FuncPtr = fp;
    Desc = desc;
  }
};
static void __test_block_func_0(struct __test_block_impl_0 *__cself) {
  int a = __cself->a; // bound by copy
  int *b = __cself->b; // bound by copy

        NSLog((NSString *)&__NSConstantStringImpl__var_folders_r0_rjxg_z6540vblvmqqv4klgsm0000gn_T_main_df2751_mi_0, a);
        NSLog((NSString *)&__NSConstantStringImpl__var_folders_r0_rjxg_z6540vblvmqqv4klgsm0000gn_T_main_df2751_mi_1, (*b));
        NSLog((NSString *)&__NSConstantStringImpl__var_folders_r0_rjxg_z6540vblvmqqv4klgsm0000gn_T_main_df2751_mi_2, a_q);
        NSLog((NSString *)&__NSConstantStringImpl__var_folders_r0_rjxg_z6540vblvmqqv4klgsm0000gn_T_main_df2751_mi_3, b_q);
    }

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)};
void test(){
    int a = 10;
    static int b = 10;
    MyBlock myBlock = ((void (*)())&__test_block_impl_0((void *)__test_block_func_0, &__test_block_desc_0_DATA, a, &b));
    a = 20;
    b = 20;
    a_q = 20;
    b_q = 20;
    ((void (*)(__block_impl *))((__block_impl *)myBlock)->FuncPtr)((__block_impl *)myBlock);
}

分析：
1纯路、局部變量a：可以看到我們Block內(nèi)部有一個(gè)（int _a）變量,局部變量a直接傳值給_a;（在局部變量中 auto局部變量出了作用域就會(huì)銷毀 為了避免執(zhí)行block時(shí) 變量不存在 所以auto變量在Block內(nèi)部被直接賦值為當(dāng)前值）
2或油、局部靜態(tài)變量b：可以看到我們Block內(nèi)部有一個(gè)（int _b）變量,局部靜態(tài)變量b把b（指針）傳給*_b;（static變量一直存在于內(nèi)存中不必有此擔(dān)憂，所以存儲(chǔ)的是變量指針驰唬，可以隨時(shí)取出最新值顶岸。）
3、全局變量a_q和全局靜態(tài)變量b_q：Block內(nèi)部沒有生成對(duì)應(yīng)的變量（如果是全局變量 Block使用時(shí)不用捕獲 直接訪問 所以得到的也全部都是最新值叫编。）
總結(jié)：局部變量之所以需要捕獲 因?yàn)槲覀兪强绾瘮?shù)使用的 聲明和使用不是在一個(gè)作用域內(nèi)
注意：self這個(gè)變量也是局部變量 也會(huì)被block捕獲,因?yàn)樵贠C中所有的方法編譯后都是自帶兩個(gè)參數(shù) 一個(gè)是實(shí)例對(duì)象 一個(gè)是SEL _cmd
所以Block中如果用到self辖佣，self也會(huì)被捕獲。且Block內(nèi)部的self屬性 指向我們傳進(jìn)去的self
所以 我們也能理解為什么會(huì)造成循環(huán)引用了吧搓逾。

self捕獲

int a_q = 10;
static int b_q = 10;
typedef void(^MyBlock)(void);
- (void)test{
    int a = 10;
    static int b = 10;
    MyBlock myBlock = ^{
        NSLog(@"a=%d", a);//a=10
        NSLog(@"b=%d", b);//b=20
        NSLog(@"a_q=%d", a_q);//a_q=20
        NSLog(@"b_q=%d", b_q);//b_q=20
        NSLog(@"%@",self);
    };
    a = 20;
    b = 20;
    a_q = 20;
    b_q = 20;
    myBlock();
}

轉(zhuǎn)成底層代碼

int a_q = 10;
static int b_q = 10;
typedef void(*MyBlock)(void);

struct __ViewController__test_block_impl_0 {
  struct __block_impl impl;
  struct __ViewController__test_block_desc_0* Desc;
  int a;
  int *b;
  ViewController *self;
  __ViewController__test_block_impl_0(void *fp, struct __ViewController__test_block_desc_0 *desc, int _a, int *_b, ViewController *_self, int flags=0) : a(_a), b(_b), self(_self) {
    impl.isa = &_NSConcreteStackBlock;
    impl.Flags = flags;
    impl.FuncPtr = fp;
    Desc = desc;
  }
};
static void __ViewController__test_block_func_0(struct __ViewController__test_block_impl_0 *__cself) {
  int a = __cself->a; // bound by copy
  int *b = __cself->b; // bound by copy
  ViewController *self = __cself->self; // bound by copy

        NSLog((NSString *)&__NSConstantStringImpl__var_folders_r0_rjxg_z6540vblvmqqv4klgsm0000gn_T_ViewController_16af55_mi_0, a);
        NSLog((NSString *)&__NSConstantStringImpl__var_folders_r0_rjxg_z6540vblvmqqv4klgsm0000gn_T_ViewController_16af55_mi_1, (*b));
        NSLog((NSString *)&__NSConstantStringImpl__var_folders_r0_rjxg_z6540vblvmqqv4klgsm0000gn_T_ViewController_16af55_mi_2, a_q);
        NSLog((NSString *)&__NSConstantStringImpl__var_folders_r0_rjxg_z6540vblvmqqv4klgsm0000gn_T_ViewController_16af55_mi_3, b_q);
        NSLog((NSString *)&__NSConstantStringImpl__var_folders_r0_rjxg_z6540vblvmqqv4klgsm0000gn_T_ViewController_16af55_mi_4,self);
    }
static void __ViewController__test_block_copy_0(struct __ViewController__test_block_impl_0*dst, struct __ViewController__test_block_impl_0*src) {_Block_object_assign((void*)&dst->self, (void*)src->self, 3/*BLOCK_FIELD_IS_OBJECT*/);}

static void __ViewController__test_block_dispose_0(struct __ViewController__test_block_impl_0*src) {_Block_object_dispose((void*)src->self, 3/*BLOCK_FIELD_IS_OBJECT*/);}

static struct __ViewController__test_block_desc_0 {
  size_t reserved;
  size_t Block_size;
  void (*copy)(struct __ViewController__test_block_impl_0*, struct __ViewController__test_block_impl_0*);
  void (*dispose)(struct __ViewController__test_block_impl_0*);
} __ViewController__test_block_desc_0_DATA = { 0, sizeof(struct __ViewController__test_block_impl_0), __ViewController__test_block_copy_0, __ViewController__test_block_dispose_0};

static void _I_ViewController_test(ViewController * self, SEL _cmd) {
    int a = 10;
    static int b = 10;
    MyBlock myBlock = ((void (*)())&__ViewController__test_block_impl_0((void *)__ViewController__test_block_func_0, &__ViewController__test_block_desc_0_DATA, a, &b, self, 570425344));
    a = 20;
    b = 20;
    a_q = 20;
    b_q = 20;
    ((void (*)(__block_impl *))((__block_impl *)myBlock)->FuncPtr)((__block_impl *)myBlock);
}