關(guān)于Block

下文所有的討論都基于ARC

如有錯誤或不嚴(yán)謹(jǐn),請多多賜教.感激不盡

1.什么是Block?

block是"帶有自動變量值的匿名函數(shù)"

2.ARC下三種block都是什么?

__NSGlobalBlock__ -- 類似函數(shù)饮六，位于text段
__NSMallocBlock__ -- 堆區(qū)
__NSStackBlock__ -- 棧區(qū)

3.三種block分別在什么情況下出現(xiàn)?

NSGlobalBlock(遇見情況較少)
沒有引用外部變量的block,此時的block為NSGlobalBlock

NSStackBlock

情況1

assign修飾,并且引用了外部變量

@interface ViewController ()
@property (assign, nonatomic) void(^myBlock1)();
@end
@implementation ViewController

- (void)viewDidLoad {
    [super viewDidLoad];
    int a = 10;
    self.myBlock1 = ^{
        NSLog(@"%d",a);
    };
}
@end

情況2

block做方法參數(shù)

- (void)demo:(void(^)())block {   
    NSLog(@"%@",block);
}
- (void)viewDidLoad {
    [super viewDidLoad];
    [self demo:^{
        NSLog(@"%d",a);
    }];
}

一定是在調(diào)用方法傳遞參數(shù)時實現(xiàn)block才是NSStackBlock,如果事先實現(xiàn)好的block,如
[self demo:self.myBlock1];
這樣傳遞來的block是和self.myBlock的block是同類型的

NSMallocBlock(最常見最常用的)

使用copy修飾并引用了外部變量,或直接聲明并引用了外部變量

@property (copy, nonatomic) void(^myBlock2)();
    self.myBlock2 = ^{
        
        NSLog(@"%d",a);
    };
    或
    void(^myBlock3)() = ^{
        
        NSLog(@"%@",arr);
    };

4.__block的原理是什么?

結(jié)論: 把原本以值傳遞傳遞到block中的變量換成引用傳遞傳遞到block內(nèi)部.
結(jié)論怎么來的,下面開始解釋.
第一個測試代碼

    int myNumA = 10;
    self.myBlock1 = ^{     
        myNumA++;
    };

這種寫法是會報錯的, 為什么報錯?

struct __DemoClass__demo_block_impl_0 {
    struct __block_impl impl;
    struct __DemoClass__demo_block_desc_0* Desc;
    int myNumA;
    __DemoClass__demo_block_impl_0(void *fp, struct __DemoClass__demo_block_desc_0 *desc, int _myNumA, int flags=0) : myNumA(_myNumA) {
        impl.isa = &_NSConcreteStackBlock;
        impl.Flags = flags;
        impl.FuncPtr = fp;
        Desc = desc;
    }
};
static void __DemoClass__demo_block_func_0(struct __DemoClass__demo_block_impl_0 *__cself) {
    int myNumA = __cself->myNumA; // bound by copy
    
    
    NSLog((NSString *)&__NSConstantStringImpl__var_folders_97_s25cw5dj7ss2rf1q7_bchd7w0000gn_T_DemoClass_8ec7ba_mi_0,myNumA);
}

static struct __DemoClass__demo_block_desc_0 {
    size_t reserved;
    size_t Block_size;
} __DemoClass__demo_block_desc_0_DATA = { 0, sizeof(struct __DemoClass__demo_block_impl_0)};

static void _I_DemoClass_demo(DemoClass * self, SEL _cmd) {
    
    int myNumA = 10;
    
    void(*myBlock)() = ((void (*)())&__DemoClass__demo_block_impl_0((void *)__DemoClass__demo_block_func_0, &__DemoClass__demo_block_desc_0_DATA, myNumA));
}

這是編譯成的C++代碼, 所有myNumA都是值傳遞.所以,沒法修改是很顯然的.

    __block int myNumA = 10;
    self.myBlock1 = ^{     
        myNumA++;
    };

編譯之后,首先,對myNumA進行一個處理

struct __Block_byref_myNumA_0 {
    void *__isa;
    __Block_byref_myNumA_0 *__forwarding;
    int __flags;
    int __size;
    int myNumA;
};

其中有一個__Block_byref_myNumA_0 *__forwarding;指針.
接下來

struct __DemoClass__demo_block_impl_0 {
    struct __block_impl impl;
    struct __DemoClass__demo_block_desc_0* Desc;
    __Block_byref_myNumA_0 *myNumA; // by ref
    __DemoClass__demo_block_impl_0(void *fp, struct __DemoClass__demo_block_desc_0 *desc, __Block_byref_myNumA_0 *_myNumA, int flags=0) : myNumA(_myNumA->__forwarding) {
        impl.isa = &_NSConcreteStackBlock;
        impl.Flags = flags;
        impl.FuncPtr = fp;
        Desc = desc;
    }
};
static void __DemoClass__demo_block_func_0(struct __DemoClass__demo_block_impl_0 *__cself) {
    __Block_byref_myNumA_0 *myNumA = __cself->myNumA; // bound by ref
    
    
    NSLog((NSString *)&__NSConstantStringImpl__var_folders_97_s25cw5dj7ss2rf1q7_bchd7w0000gn_T_DemoClass_29df03_mi_0,(myNumA->__forwarding->myNumA)++);
}
static void __DemoClass__demo_block_copy_0(struct __DemoClass__demo_block_impl_0*dst, struct __DemoClass__demo_block_impl_0*src) {_Block_object_assign((void*)&dst->myNumA, (void*)src->myNumA, 8/*BLOCK_FIELD_IS_BYREF*/);}

static void __DemoClass__demo_block_dispose_0(struct __DemoClass__demo_block_impl_0*src) {_Block_object_dispose((void*)src->myNumA, 8/*BLOCK_FIELD_IS_BYREF*/);}

static struct __DemoClass__demo_block_desc_0 {
    size_t reserved;
    size_t Block_size;
    void (*copy)(struct __DemoClass__demo_block_impl_0*, struct __DemoClass__demo_block_impl_0*);
    void (*dispose)(struct __DemoClass__demo_block_impl_0*);
} __DemoClass__demo_block_desc_0_DATA = { 0, sizeof(struct __DemoClass__demo_block_impl_0), __DemoClass__demo_block_copy_0, __DemoClass__demo_block_dispose_0};

static void _I_DemoClass_demo(DemoClass * self, SEL _cmd) {
    
    __attribute__((__blocks__(byref))) __Block_byref_myNumA_0 myNumA = {(void*)0,(__Block_byref_myNumA_0 *)&myNumA, 0, sizeof(__Block_byref_myNumA_0), 10};
    
    void(*myBlock)() = ((void (*)())&__DemoClass__demo_block_impl_0((void *)__DemoClass__demo_block_func_0, &__DemoClass__demo_block_desc_0_DATA, (__Block_byref_myNumA_0 *)&myNumA, 570425344));
}

所有對myNumA的引用都變成了__Block_byref_myNumA_0 *myNumA.
在最后兩行,我們聲明的int myNumA = 10變成了__Block_byref_myNumA_0 myNumA = {(void*)0,(__Block_byref_myNumA_0 *)&myNumA, 0, sizeof(__Block_byref_myNumA_0), 10};,傳入進去的是(__Block_byref_myNumA_0 *)&myNumA,很明顯,傳遞的是myNumA的內(nèi)存地址, 所以可以改變變量的值.

5.修改什么樣的變量需要使用__block,什么樣的不需要?

先說結(jié)論:
需要加__block修飾的只有局部變量且非指針變量.
成員變量,屬性,static修飾過得局部變量,局部指針變量都不需要加__block修飾.

解釋原因

1.屬性和成員變量不需要加`__block`

測試代碼:

@interface DemoClass ()
@property (assign, nonatomic) int myNumA;
@end

@implementation DemoClass {
    int myNumB;
}

- (void)demo {
    self.myNumA = 10;
    myNumB = 10;
    void(^myBlock)() = ^{
        self.myNumA = self.myNumA + 1;
        myNumB = myNumB + 1;
    };
    myBlock();
}

編譯后的C++代碼:

1.static void __DemoClass__demo_block_func_0(struct __DemoClass__demo_block_impl_0 *__cself) {
        DemoClass *self = __cself->self; // bound by copy
       ① ((void (*)(id, SEL, int))(void *)objc_msgSend)((id)self, sel_registerName("setMyNumA:"), ((int (*)(id, SEL))(void *)objc_msgSend)((id)self, sel_registerName("myNumA")) + 1);
       ②(*(int *)((char *)self + OBJC_IVAR_$_DemoClass$myNumB)) = (*(int *)((char *)self + OBJC_IVAR_$_DemoClass$myNumB)) + 1;
}

2.static void _I_DemoClass_demo(DemoClass * self, SEL _cmd) {

    ((void (*)(id, SEL, int))(void *)objc_msgSend)((id)self, sel_registerName("setMyNumA:"), 10);
    ③ (*(int *)((char *)self + OBJC_IVAR_$_DemoClass$myNumB)) = 10;
    void(*myBlock)() = ((void (*)())&__DemoClass__demo_block_impl_0((void *)__DemoClass__demo_block_func_0, &__DemoClass__demo_block_desc_0_DATA, self, 570425344));
    ((void (*)(__block_impl *))((__block_impl *)myBlock)->FuncPtr)((__block_impl *)myBlock);
}

方法"1."是block的回調(diào)函數(shù).由函數(shù)參數(shù)可以看出來,即使是沒有參數(shù)的block,實際運行的時候也會接受一個__DemoClass__demo_block_impl_0的結(jié)構(gòu)體,結(jié)構(gòu)體代碼如下:

struct __DemoClass__demo_block_impl_0 {
  struct __block_impl impl;
  struct __DemoClass__demo_block_desc_0* Desc;
  DemoClass *self;
  __DemoClass__demo_block_impl_0(void *fp, struct __DemoClass__demo_block_desc_0 *desc, DemoClass *_self, int flags=0) : self(_self) {
    impl.isa = &_NSConcreteStackBlock;
    impl.Flags = flags;
    impl.FuncPtr = fp;
    Desc = desc;
  }
};

從結(jié)構(gòu)體中可以看出,此時的block內(nèi)部可以獲取到self
如①所示,之所以修改屬性不需要__block是因為block中可以獲取到self,而通過self和objc_msgSend就可以調(diào)用該屬性的set和get方法去賦值.
成員變量呢?
如③所示,myNumB = 10被編譯成(*(int *)((char *)self + OBJC_IVAR_$_DemoClass$myNumB)) = 10.
我們可以近似的看成(*(int *))myNumB = 10.可以看出,myNumB的值也是通過修改其內(nèi)存地址里面的內(nèi)容賦值的,而賦值也需要self.
結(jié)合②③很明顯可以看出,成員變量的賦值和訪問也是通過self去賦值的,只不過沒有set和get方法.
2.經(jīng)過static修飾過的局部變量
測試代碼如下:

- (void)demo {
    static int myNumA = 10;
    void(^myBlock)() = ^{
        myNumA = myNumA + 1;
    };
    myBlock();
}

編譯后的C++代碼如下:

struct __DemoClass__demo_block_impl_0 {
  struct __block_impl impl;
  struct __DemoClass__demo_block_desc_0* Desc;
  int *myNumA;
  __DemoClass__demo_block_impl_0(void *fp, struct __DemoClass__demo_block_desc_0 *desc, int *_myNumA, int flags=0) : myNumA(_myNumA) {
    impl.isa = &_NSConcreteStackBlock;
    impl.Flags = flags;
    impl.FuncPtr = fp;
    Desc = desc;
  }
};
static void __DemoClass__demo_block_func_0(struct __DemoClass__demo_block_impl_0 *__cself) {
  int *myNumA = __cself->myNumA; // bound by copy

        (*myNumA) = (*myNumA) + 1;
}

此時的結(jié)構(gòu)體中獲取到的是myNumA的內(nèi)存地址,所以不需要__block修飾.

3.局部指針變量

測試代碼如下:

- (void)demo {
    int a = 10;
    int *myNumA = &a;
    NSMutableArray *arrM = [NSMutableArray arrayWithObjects:@1, @2, nil];
    void(^myBlock)() = ^{
        *myNumA = *myNumA + 1;
        [arrM addObject:@3];
    };
    myBlock();
}