二十三、iOS底层原理-block底层原理

作者: Mjs | 来源:发表于2020-11-13 16:50 被阅读0次

block分类

1. `NSGlobalBlock`

    void(^globalBlock)(void) = ^ {
        NSLog(@"1111111");
    };
    NSLog(@"%@",globalBlock);
.......................
<__NSGlobalBlock__: 0x108fb70b8>

声明一个block，没有对外界变量持有为 __NSGlobalBlock__

位于全局区
在Block内部不使用外部变量，或者只适用静态变量和全局变量。

2. `NSMallocBlock`

    int a = 10;
    void (^ malloBlock)(void) = ^{
        NSLog(@"Cooci - %d",a);
    };

    NSLog(@"%@",malloBlock);
.......................
<__NSMallocBlock__: 0x6000010ec840>

位于堆区
在Block内部使用局部变量或者OC属性，并且赋值给强引用或者Copy修饰。
当block对外界变量持有是，就会从 __NSStackBlock__copy成 __NSMallocBlock__，具体情况下文分析

3. `NSStackBlock`

    int a = 10;
    void (^ malloBlock)(void) = ^{
        NSLog(@"Cooci - %d",a);
    };

    NSLog(@"%@",malloBlock);
.......................
<__NSStackBlock__: 0x7ffee6c473f8>

当block出栈就会销毁的时候为 __NSStackBlock__，block为函数参数是也是 __NSStackBlock__。

位于栈区
与MallocBlock一样，可以在Block内部使用局部变量或者OC属性，但是不能赋值给强引用或者Copy修饰。

block的循环引用

互相持有.png

对象A持有对象B，对象B又持有对象A，相互持有，最终导致两个对象都不能释放。


@interface ViewController ()
@property(nonatomic,copy)void(^blockA)(void);
@property (nonatomic, copy) NSString *name;
@end

@implementation ViewController

- (void)viewDidLoad {
    [super viewDidLoad];
    // 循环引用
    self.name = @"AAA";
    self.blockA = ^{
        NSLog(@"%@",self.name);
    };
    self.blockA();
}
@end

- (void)dealloc{
    NSLog(@"dealloc 来了");
}

在这里，self持有block，block里又对self进行了持有，导致循环引用，dealloc不会打印。

循环引用的解决方案

__weak、__strong

    __weak typeof(self) weakSelf = self;
    self.blockA = ^{
        NSLog(@"%@",weakSelf.name);
    };
    self.blockA();

原来是self->block->self,现在变成了self->block->weakSelf->self，
将block中的self交给weakSelf,weakSelf在block结束时就会释放,我们可以通过CFGetRetainCount((__bridge CFTypeRef)(self))打印当前引用计数，发现self引用计数并没有增加。
但是只用 __weak还会存在问题

    __weak typeof(self) weakSelf = self;
    self.block = ^(void){
        // 时间 - 精力
        // self 的生命周期
        __strong __typeof(weakSelf)strongSelf = weakSelf; // 可以释放 when
        dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)(2 * NSEC_PER_SEC)), dispatch_get_main_queue(), ^{
            NSLog(@"%@",strongSelf.name);
        });
    };
    self.block();

当存在延时运行的时候，block结束时weakSelf还没调用就被释放了。self的生命周期没有得到保存。这个时候就需要__strong对其进行强引用，strongSelf是临时变量，在除了作用空间就会被释放掉。weak-strong 强弱共舞

__block

    __block ViewController *vc = self;
    self.block = ^(void){
        dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)(2 * NSEC_PER_SEC)), dispatch_get_main_queue(), ^{
            NSLog(@"%@",vc.name);
            vc = nil;
        });
    };
    self.block();

我们除了__weak自动释放还可以通过__block手动释放，通过__block 修饰之后可以在block中修改值，通过手动的方式销毁，但这里有个问题，如果block没有调用的话，就不会释放，因为self被block捕获了，没有调用所以无法释放。

3.修改通信模式
因为当前self是被持有的，但我们可以通过通知、代理、传参的方式将self传入。

    self.block = ^(ViewController *vc){
        dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)(2 * NSEC_PER_SEC)), dispatch_get_main_queue(), ^{
            NSLog(@"%@",vc.name);
        });
    };
    self.block(self);

当前self就会当做临时变量压栈进来。

block编译

1.block的基本实现
新建一个文件，实现以下代码

#include "stdio.h"

int main(){

    __block int a = 11;
    void(^block)(void) = ^{
        a++;
        printf("LG_Cooci - %d",a);
    };
    
    // block();
    return 0;
}

通过xclang -arch x86_64 -rewrite-objc block.c将其编译成cpp文件


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("aaaa");
    }


int main(){

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

     ((void (*)(__block_impl *))((__block_impl *)block)->FuncPtr)((__block_impl *)block);
    return 0;
}

可以看到block被转换成了__main_block_impl_0结构体。通过构造方法，block函数被编译成函数，作为参数参入impl.FuncPtr = fp，后面通过block->FuncPtr(block)调用block。block作为参数传入当前函数中，和oc方法一样，作为隐藏参数，可以更好地操作block，可以对block对其进行操作。

__block
我们先传入一个普通的参数a

#include "stdio.h"
int main(){
    int a = 10;
    void(^block)(void) = ^{
        printf("aaaa-%d",a);
    };
     block();
    return 0;
}

clang编译一下


struct __main_block_impl_0 {
  struct __block_impl impl;
  struct __main_block_desc_0* Desc;
  int a;
  __main_block_impl_0(void *fp, struct __main_block_desc_0 *desc, int _a, int flags=0) : a(_a) {
    impl.isa = &_NSConcreteStackBlock;
    impl.Flags = flags;
    impl.FuncPtr = fp;
    Desc = desc;
  }
};
static void __main_block_func_0(struct __main_block_impl_0 *__cself) {
  int a = __cself->a; // bound by copy

        printf("aaaa-%d",a);
    }

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 a = 10;
    void(*block)(void) = ((void (*)())&__main_block_impl_0((void *)__main_block_func_0, &__main_block_desc_0_DATA, a));

     ((void (*)(__block_impl *))((__block_impl *)block)->FuncPtr)((__block_impl *)block);
    return 0;
}

可以看到在编译时就在block中自动生成了相应的变量a，通过构造方法赋值。isa指向为_NSConcreteStackBlock。在方法中直接出去a的值来操作，但是由于是赋值拷贝，所以无法修改外部变量，会造成代码歧义。
现在我们加入__block

int main(){
    __block int a = 10;
    void(^block)(void) = ^{
        a++;
        printf("aaaa-%d",a);
    };
     block();
    return 0;
}

clang编译一下

struct __Block_byref_a_0 {
  void *__isa;
__Block_byref_a_0 *__forwarding;
 int __flags;
 int __size;
 int a;
};

struct __main_block_impl_0 {
  struct __block_impl impl;
  struct __main_block_desc_0* Desc;
  __Block_byref_a_0 *a; // by ref
  __main_block_impl_0(void *fp, struct __main_block_desc_0 *desc, __Block_byref_a_0 *_a, int flags=0) : a(_a->__forwarding) {
    impl.isa = &_NSConcreteStackBlock;
    impl.Flags = flags;
    impl.FuncPtr = fp;
    Desc = desc;
  }
};
static void __main_block_func_0(struct __main_block_impl_0 *__cself) {
  __Block_byref_a_0 *a = __cself->a; // bound by ref

        (a->__forwarding->a)++;
        printf("aaaa-%d",(a->__forwarding->a));
    }
static void __main_block_copy_0(struct __main_block_impl_0*dst, struct __main_block_impl_0*src) {_Block_object_assign((void*)&dst->a, (void*)src->a, 8/*BLOCK_FIELD_IS_BYREF*/);}

static void __main_block_dispose_0(struct __main_block_impl_0*src) {_Block_object_dispose((void*)src->a, 8/*BLOCK_FIELD_IS_BYREF*/);}

static struct __main_block_desc_0 {
  size_t reserved;
  size_t Block_size;
  void (*copy)(struct __main_block_impl_0*, struct __main_block_impl_0*);
  void (*dispose)(struct __main_block_impl_0*);
} __main_block_desc_0_DATA = { 0, sizeof(struct __main_block_impl_0), __main_block_copy_0, __main_block_dispose_0};
int main(){
    __attribute__((__blocks__(byref))) __Block_byref_a_0 a = {
(void*)0,
(__Block_byref_a_0 *)&a, 
0,
 sizeof(__Block_byref_a_0), 
10};
    void(*block)(void) = ((void (*)())&__main_block_impl_0((void *)__main_block_func_0, &__main_block_desc_0_DATA, (__Block_byref_a_0 *)&a, 570425344));
     ((void (*)(__block_impl *))((__block_impl *)block)->FuncPtr)((__block_impl *)block);
    return 0;
}

在这里生成了一个__Block_byref_a_0结构体

struct __Block_byref_a_0 {
  void *__isa;
__Block_byref_a_0 *__forwarding;
 int __flags;
 int __size;
 int a;
};

将a封装成相应的对象，通过地址拷贝传递到block函数中去，对其进行操作。

block底层原理

block源码调试
我们现在block声明的地方打上断点，然后Debug Workflow -> Alaways Show Diassembly查看汇编。

block断点调试.png
加他条件断点，可以看到libsystem_blocks.dylib _Block_copy
我们就可以找到block源码进行查看
我们通过源码可以看到所有的参数都来源于Block_layout

struct Block_layout {
    void *isa;
    volatile int32_t flags; // contains ref count
    int32_t reserved;
    BlockInvokeFunction invoke;
    struct Block_descriptor_1 *descriptor; //
    // imported variables
};

flags作为标识码标识了当前的一些标志

// Values for Block_layout->flags to describe block objects
enum {
    BLOCK_DEALLOCATING =      (0x0001),  // runtime 是否正在析构
    BLOCK_REFCOUNT_MASK =     (0xfffe),  // runtime 引用计数的MASK
    BLOCK_NEEDS_FREE =        (1 << 24), // runtime 是否需要释放
    BLOCK_HAS_COPY_DISPOSE =  (1 << 25), // compiler
    BLOCK_HAS_CTOR =          (1 << 26), // compiler: helpers have C++ code
    BLOCK_IS_GC =             (1 << 27), // runtime
    BLOCK_IS_GLOBAL =         (1 << 28), // compiler  是否为全局GLOBAL
    BLOCK_USE_STRET =         (1 << 29), // compiler: undefined if !BLOCK_HAS_SIGNATURE
    BLOCK_HAS_SIGNATURE  =    (1 << 30), // compiler 是否有签名
    BLOCK_HAS_EXTENDED_LAYOUT=(1 << 31)  // compiler
};

Block_descriptor 除了Block_descriptor_1还有Block_descriptor_2、Block_descriptor_3


#define BLOCK_DESCRIPTOR_1 1
struct Block_descriptor_1 {
    uintptr_t reserved;
    uintptr_t size;
};

// 可选
#define BLOCK_DESCRIPTOR_2 1
struct Block_descriptor_2 {
    // requires BLOCK_HAS_COPY_DISPOSE
    BlockCopyFunction copy;
    BlockDisposeFunction dispose;
};

#define BLOCK_DESCRIPTOR_3 1
struct Block_descriptor_3 {
    // requires BLOCK_HAS_SIGNATURE
    const char *signature;
    const char *layout;     // contents depend on BLOCK_HAS_EXTENDED_LAYOUT
};

Block_descriptor_1是一直存在的，而Block_descriptor_2、Block_descriptor_3就是通过flags来决定是否存在

#if 0
static struct Block_descriptor_1 * _Block_descriptor_1(struct Block_layout *aBlock)
{
    return aBlock->descriptor;
}
#endif

static struct Block_descriptor_2 * _Block_descriptor_2(struct Block_layout *aBlock)
{
    if (! (aBlock->flags & BLOCK_HAS_COPY_DISPOSE)) return NULL;
    uint8_t *desc = (uint8_t *)aBlock->descriptor;
    desc += sizeof(struct Block_descriptor_1);
    return (struct Block_descriptor_2 *)desc;
}

static struct Block_descriptor_3 * _Block_descriptor_3(struct Block_layout *aBlock)
{
    if (! (aBlock->flags & BLOCK_HAS_SIGNATURE)) return NULL;
    uint8_t *desc = (uint8_t *)aBlock->descriptor;
    desc += sizeof(struct Block_descriptor_1);
    if (aBlock->flags & BLOCK_HAS_COPY_DISPOSE) {
        desc += sizeof(struct Block_descriptor_2);
    }
    return (struct Block_descriptor_3 *)desc;
}

如果flags显示存在，就找到aBlock->descriptor 通过内存平移去找到Block_descriptor_2、Block_descriptor_3。

通过读取内存查看当前block.png

我们可以通过lldb register read查看x0地址变化，发现在_Block_copy __NSStackBlock__ 变为了__NSMallocBlock__

// Copy, or bump refcount, of a block.  If really copying, call the copy helper if present.
// 栈 -> 堆 研究拷贝
void *_Block_copy(const void *arg) {
    struct Block_layout *aBlock;

    if (!arg) return NULL;
    
    // The following would be better done as a switch statement
    //拷贝当前的block防止对外层的影响
    aBlock = (struct Block_layout *)arg;
    if (aBlock->flags & BLOCK_NEEDS_FREE) {
        // latches on high
        latching_incr_int(&aBlock->flags);
        return aBlock;
    }
    else if (aBlock->flags & BLOCK_IS_GLOBAL) {
        return aBlock; // 不需要改变，直接返回
    }
    else { // 栈
        // Its a stack block.  Make a copy.
        struct Block_layout *result =
            (struct Block_layout *)malloc(aBlock->descriptor->size);
        if (!result) return NULL;
        //内存拷贝
        memmove(result, aBlock, aBlock->descriptor->size); // bitcopy first
#if __has_feature(ptrauth_calls)
        // Resign the invoke pointer as it uses address authentication.
        result->invoke = aBlock->invoke;
#endif
        // reset refcount
        result->flags &= ~(BLOCK_REFCOUNT_MASK|BLOCK_DEALLOCATING);    // XXX not needed
        result->flags |= BLOCK_NEEDS_FREE | 2;  // logical refcount 1
        _Block_call_copy_helper(result, aBlock);
        // Set isa last so memory analysis tools see a fully-initialized object.
        result->isa = _NSConcreteMallocBlock;
        return result;
    }
}

block 是怎么对外界变量进行操作的

int main(int argc, char * argv[]) {
    @autoreleasepool {
        __block NSString *a = [NSString stringWithFormat:@"aaa"];
        void(^block)(void) = ^{
            a = @"xxx";
            printf("aaaa-%@",a);
        };
         block();
        
    }
    return UIApplicationMain(argc, argv, nil, NSStringFromClass([AppDelegate class]));
}

clang -x objective-c -rewrite-objc -isysroot /Applications/Xcode.app/Contents/Developer/Platforms/iPhoneSimulator.platform/Developer/SDKs/iPhoneSimulator.sdk main.m


struct __Block_byref_a_0 {
  void *__isa;
__Block_byref_a_0 *__forwarding;
 int __flags;
 int __size;
 void (*__Block_byref_id_object_copy)(void*, void*);
 void (*__Block_byref_id_object_dispose)(void*);
 NSString *a;
};

struct __main_block_impl_0 {
  struct __block_impl impl;
  struct __main_block_desc_0* Desc;
  __Block_byref_a_0 *a; // by ref
  __main_block_impl_0(void *fp, struct __main_block_desc_0 *desc, __Block_byref_a_0 *_a, int flags=0) : a(_a->__forwarding) {
    impl.isa = &_NSConcreteStackBlock;
    impl.Flags = flags;
    impl.FuncPtr = fp;
    Desc = desc;
  }
};
static void __main_block_func_0(struct __main_block_impl_0 *__cself) {
  __Block_byref_a_0 *a = __cself->a; // bound by ref

            (a->__forwarding->a) = (NSString *)&__NSConstantStringImpl__var_folders__2_948tyv6520110qy_phw9x4fw0000gn_T_main_076a95_mi_1;
            printf("aaaa-%@",(a->__forwarding->a));
        }
static void __main_block_copy_0(struct __main_block_impl_0*dst, struct __main_block_impl_0*src) {_Block_object_assign((void*)&dst->a, (void*)src->a, 8/*BLOCK_FIELD_IS_BYREF*/);}

static void __main_block_dispose_0(struct __main_block_impl_0*src) {_Block_object_dispose((void*)src->a, 8/*BLOCK_FIELD_IS_BYREF*/);}

static struct __main_block_desc_0 {
  size_t reserved;
  size_t Block_size;
  void (*copy)(struct __main_block_impl_0*, struct __main_block_impl_0*);
  void (*dispose)(struct __main_block_impl_0*);
} __main_block_desc_0_DATA = { 0, sizeof(struct __main_block_impl_0), __main_block_copy_0, __main_block_dispose_0};
int main(int argc, char * argv[]) {
    /* @autoreleasepool */ { __AtAutoreleasePool __autoreleasepool; 
        __attribute__((__blocks__(byref))) __Block_byref_a_0 a = {(void*)0,(__Block_byref_a_0 *)&a, 33554432, sizeof(__Block_byref_a_0), __Block_byref_id_object_copy_131, __Block_byref_id_object_dispose_131, ((NSString * _Nonnull (*)(id, SEL, NSString * _Nonnull, ...))(void *)objc_msgSend)((id)objc_getClass("NSString"), sel_registerName("stringWithFormat:"), (NSString *)&__NSConstantStringImpl__var_folders__2_948tyv6520110qy_phw9x4fw0000gn_T_main_076a95_mi_0)};
        void(*block)(void) = ((void (*)())&__main_block_impl_0((void *)__main_block_func_0, &__main_block_desc_0_DATA, (__Block_byref_a_0 *)&a, 570425344));
         ((void (*)(__block_impl *))((__block_impl *)block)->FuncPtr)((__block_impl *)block);

    }
    return UIApplicationMain(argc, argv, __null, NSStringFromClass(((Class (*)(id, SEL))(void *)objc_msgSend)((id)objc_getClass("AppDelegate"), sel_registerName("class"))));
}





static __NSConstantStringImpl __NSConstantStringImpl__var_folders__2_948tyv6520110qy_phw9x4fw0000gn_T_main_076a95_mi_0 __attribute__ ((section ("__DATA, __cfstring"))) = {__CFConstantStringClassReference,0x000007c8,"aaa",3};
static void __Block_byref_id_object_copy_131(void *dst, void *src) {
 _Block_object_assign((char*)dst + 40, *(void * *) ((char*)src + 40), 131);
}
static void __Block_byref_id_object_dispose_131(void *src) {
 _Block_object_dispose(*(void * *) ((char*)src + 40), 131);
}
static __NSConstantStringImpl __NSConstantStringImpl__var_folders__2_948tyv6520110qy_phw9x4fw0000gn_T_main_076a95_mi_1 __attribute__ ((section ("__DATA, __cfstring"))) = {__CFConstantStringClassReference,0x000007c8,"xxx",3};

这是生成的descriptor的，我们在descriptor_2可以看到

static void __main_block_copy_0(struct __main_block_impl_0*dst, struct __main_block_impl_0*src) {_Block_object_assign((void*)&dst->a, (void*)src->a, 8/*BLOCK_FIELD_IS_BYREF*/);}

我们去查看_Block_object_assign源码

void _Block_object_assign(void *destArg, const void *object, const int flags) {
    const void **dest = (const void **)destArg;
   
    switch (os_assumes(flags & BLOCK_ALL_COPY_DISPOSE_FLAGS)) {
     
        case BLOCK_FIELD_IS_OBJECT:
        /*******
        id object = ...;
        [^{ object; } copy];
        ********/
        // objc 指针地址 weakSelf (self)
            // arc 进行处理
        _Block_retain_object(object);
            // 持有
        *dest = object;
        break;

      case BLOCK_FIELD_IS_BLOCK:
        /*******
        void (^object)(void) = ...;
        [^{ object; } copy];
        ********/
            
            // block 被一个 block 捕获

        *dest = _Block_copy(object);
        break;
    
      case BLOCK_FIELD_IS_BYREF | BLOCK_FIELD_IS_WEAK:
      case BLOCK_FIELD_IS_BYREF:
        /*******
         // copy the onstack __block container to the heap
         // Note this __weak is old GC-weak/MRC-unretained.
         // ARC-style __weak is handled by the copy helper directly.
         __block ... x;
         __weak __block ... x;
         [^{ x; } copy];
         ********/
            
        *dest = _Block_byref_copy(object);
        break;
        
      case BLOCK_BYREF_CALLER | BLOCK_FIELD_IS_OBJECT:
      case BLOCK_BYREF_CALLER | BLOCK_FIELD_IS_BLOCK:
        /*******
         // copy the actual field held in the __block container
         // Note this is MRC unretained __block only. 
         // ARC retained __block is handled by the copy helper directly.
         __block id object;
         __block void (^object)(void);
         [^{ object; } copy];
         ********/

        *dest = object;
        break;

      case BLOCK_BYREF_CALLER | BLOCK_FIELD_IS_OBJECT | BLOCK_FIELD_IS_WEAK:
      case BLOCK_BYREF_CALLER | BLOCK_FIELD_IS_BLOCK  | BLOCK_FIELD_IS_WEAK:
        /*******
         // copy the actual field held in the __block container
         // Note this __weak is old GC-weak/MRC-unretained.
         // ARC-style __weak is handled by the copy helper directly.
         __weak __block id object;
         __weak __block void (^object)(void);
         [^{ object; } copy];
         ********/

        *dest = object;
        break;

      default:
        break;
    }
}

通过flag分为以下几种类型

enum {
    // see function implementation for a more complete description of these fields and combinations
    BLOCK_FIELD_IS_OBJECT   =  3,  // id, NSObject, __attribute__((NSObject)), block, ...
    BLOCK_FIELD_IS_BLOCK    =  7,  // a block variable
    BLOCK_FIELD_IS_BYREF    =  8,  // the on stack structure holding the __block variable
    BLOCK_FIELD_IS_WEAK     = 16,  // declared __weak, only used in byref copy helpers
    BLOCK_BYREF_CALLER      = 128, // called from __block (byref) copy/dispose support routines.
};

我们常用的为BLOCK_FIELD_IS_OBJECT和BLOCK_FIELD_IS_BYREF。

BLOCK_FIELD_IS_OBJECT时什么都没做，交给ARC进行处理。
BLOCK_FIELD_IS_BYREF

static struct Block_byref *_Block_byref_copy(const void *arg) {
    
    // Block_byref  结构体
    // 保存一份
    struct Block_byref *src = (struct Block_byref *)arg;

    if ((src->forwarding->flags & BLOCK_REFCOUNT_MASK) == 0) {
        // src points to stack
        struct Block_byref *copy = (struct Block_byref *)malloc(src->size);
        copy->isa = NULL;
        // byref value 4 is logical refcount of 2: one for caller, one for stack
        copy->flags = src->flags | BLOCK_BYREF_NEEDS_FREE | 4;
        
        // 问题 - block 内部 持有的 Block_byref 所持有的对象 同一个
        //拷贝的对象和原有的对象修改的是同一个内存地址
        copy->forwarding = copy; // patch heap copy to point to itself
        src->forwarding = copy;  // patch stack to point to heap copy
        
        copy->size = src->size;

        if (src->flags & BLOCK_BYREF_HAS_COPY_DISPOSE) {
            // Trust copy helper to copy everything of interest
            // If more than one field shows up in a byref block this is wrong XXX
            struct Block_byref_2 *src2 = (struct Block_byref_2 *)(src+1);
            struct Block_byref_2 *copy2 = (struct Block_byref_2 *)(copy+1);
            copy2->byref_keep = src2->byref_keep;
            copy2->byref_destroy = src2->byref_destroy;

            if (src->flags & BLOCK_BYREF_LAYOUT_EXTENDED) {
                struct Block_byref_3 *src3 = (struct Block_byref_3 *)(src2+1);
                struct Block_byref_3 *copy3 = (struct Block_byref_3*)(copy2+1);
                copy3->layout = src3->layout;
            }

            (*src2->byref_keep)(copy, src);
        }
        else {
            // Bitwise copy.
            // This copy includes Block_byref_3, if any.
            memmove(copy+1, src+1, src->size - sizeof(*src));
        }
    }
    // already copied to heap
    else if ((src->forwarding->flags & BLOCK_BYREF_NEEDS_FREE) == BLOCK_BYREF_NEEDS_FREE) {
        latching_incr_int(&src->forwarding->flags);
    }
    
    return src->forwarding;
}

一开始就对当前参数进行拷贝，将拷贝的和原来的forwarding指向同一个地址，这里的byref_keep就是__Block_byref_id_object_copy_131

static void __Block_byref_id_object_copy_131(void *dst, void *src) {
 _Block_object_assign((char*)dst + 40, *(void * *) ((char*)src + 40), 131);
}

dst + 40指向的就是__Block_byref_a_0种的a

struct __Block_byref_a_0 {
  void *__isa;
__Block_byref_a_0 *__forwarding;
 int __flags;
 int __size;
 void (*__Block_byref_id_object_copy)(void*, void*);
 void (*__Block_byref_id_object_dispose)(void*);
 NSString *a;
};

这只_Block_object_assign指向的就是BLOCK_FIELD_IS_OBJECT,对NSObject进行copy。

通过_Block_copy对block进行copy

__block byref 对结构体进行拷贝 _Block_object_assign

对结构体中的对象进行_Block_object_assign

如果参数是block会一层层拷贝下去

二十三、iOS底层原理-block底层原理

block分类

1. `NSGlobalBlock`

2. `NSMallocBlock`

3. `NSStackBlock`

block的循环引用

循环引用的解决方案

block编译

block底层原理

block 是怎么对外界变量进行操作的

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热点阅读

二十三、iOS底层原理-block底层原理

block分类

1. __NSGlobalBlock__

2. __NSMallocBlock__

3. __NSStackBlock__

block的循环引用

循环引用的解决方案

block编译

block底层原理

block 是怎么对外界变量进行操作的

相关文章

网友评论

延伸阅读

深度阅读

栏目导航

热点阅读

1. `NSGlobalBlock`

2. `NSMallocBlock`

3. `NSStackBlock`