iOS block 变量捕获(auto 、 static以及全局变量)

block本质 block是封装了函数调用以及函数调用环境的 OC对象

__block的作用

编译器会将__block变量包装成一个对象,可以用于解决block内部无法修改auto变量值的问题
__block不能修饰全局变量,静态变量(static)

1.在ARC环境下,编译器会根据情况自动将栈上的block复制到堆区,比如以下情况

block作为函数返回值时 return ^{ } ;
将block赋值给__strong指针时 MBlock bolck = ^{ };
block作为cocoa API中方法名含有useingBlock的方法参数时

[array enumerateObjectsUsingBlock:^(id  _Nonnull obj, NSUInteger idx, BOOL * _Nonnull stop) {
    }] ;

block作为GCD API的方法参数时

 dispatch_once(&onceToken, ^{
     });

block一旦没有进行copy操作,就不会在堆上,就无法控制block的生命周期,使用时注意循环引用
block在修饰NSMutableArray,如果只是修改数组内的对象(比如addObject:),不需要添加__block

截屏2020-08-18 上午10.45.40.png

截屏2020-08-17 下午5.50.11.png

先看一个block捕获外部auto变量age，通过命令clang xxx.m -rewrite-objc -o xxx.cpp或者xcrun -sdk iphoneos clang -arch arm64 -rewrite-objc -fobjc-arc -fobjc-runtime=ios-9.0.0 XX.m 将.m文件转成 c++

OC代码

int c = 10 ;
static int d = 10 ;
void test(){
    auto int a = 10 ;
    static int b = 10 ;
    void (^rBlock)(void) = ^{
        NSLog(@"a = %d , b = %d  , c = %d , d= %d",a,b,c,d) ;
    } ;
    a = 20 ;
    b = 20 ;
    rBlock() ;
}

int main(int argc, char * argv[]) {
    @autoreleasepool {
        test() ;
    }
    return  0;
}

C++代码

int c = 10 ;
static int d = 10 ;

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_rg_dnyfk4m119n4q8vj670_shn00000gn_T_main_34442b_mi_0,a,(*b),c,d) ;
    }

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(){

    auto int a = 10 ;
    static int b = 10 ;
    void (*rBlock)(void) = ((void (*)())&__test_block_impl_0((void *)__test_block_func_0, &__test_block_desc_0_DATA, a, &b)) ;

    a = 20 ;
    b = 20 ;

    ((void (*)(__block_impl *))((__block_impl *)rBlock)->FuncPtr)((__block_impl *)rBlock) ;
}

可以看出上面捕获的auto变量有些差别，auto变量被捕获时直接传入的age的值，而static变量被捕获时传入的是age的地址（&age），由于auto变量出了当前作用域内存就会被销毁，所以需要将auto变量的值捕获住，static变量是一直存在内存中，但出了作用域就访问不了啦，所以只需要捕获static变量的内存地址就可以了

全局变量是不会被捕获的，因为全局变量在哪里都可以访问，不需要进行捕获

所以得出的结论是：局部变量是一定会被捕获，全局变量不会被捕获；

局部变量的捕获方式：auto变量是值捕获，static是地址捕获

@implementation KLPerson

-(void)test{
    
    void(^rBlock)(void) = ^{
        
        NSLog(@"self === %@ , name = %@",self,self.rName) ;
    };
    
    rBlock();
}

- (instancetype)initWithName:(NSString*)name
{
    self = [super init];
    if (self) {
        self.rName = name ;
    }
    return self;
}
@end

struct __KLPerson__test_block_impl_0 {
  struct __block_impl impl;
  struct __KLPerson__test_block_desc_0* Desc;
  KLPerson *self;
  __KLPerson__test_block_impl_0(void *fp, struct __KLPerson__test_block_desc_0 *desc, KLPerson *_self, int flags=0) : self(_self) {
    impl.isa = &_NSConcreteStackBlock;
    impl.Flags = flags;
    impl.FuncPtr = fp;
    Desc = desc;
  }
};
static void __KLPerson__test_block_func_0(struct __KLPerson__test_block_impl_0 *__cself) {
  KLPerson *self = __cself->self; // bound by copy


        NSLog((NSString *)&__NSConstantStringImpl__var_folders_rg_dnyfk4m119n4q8vj670_shn00000gn_T_KLPerson_ce4a47_mi_0,self,((NSString *(*)(id, SEL))(void *)objc_msgSend)((id)self, sel_registerName("rName"))) ;
    }
static void __KLPerson__test_block_copy_0(struct __KLPerson__test_block_impl_0*dst, struct __KLPerson__test_block_impl_0*src) {_Block_object_assign((void*)&dst->self, (void*)src->self, 3/*BLOCK_FIELD_IS_OBJECT*/);}

static void __KLPerson__test_block_dispose_0(struct __KLPerson__test_block_impl_0*src) {_Block_object_dispose((void*)src->self, 3/*BLOCK_FIELD_IS_OBJECT*/);}

static struct __KLPerson__test_block_desc_0 {
  size_t reserved;
  size_t Block_size;
  void (*copy)(struct __KLPerson__test_block_impl_0*, struct __KLPerson__test_block_impl_0*);
  void (*dispose)(struct __KLPerson__test_block_impl_0*);
} __KLPerson__test_block_desc_0_DATA = { 0, sizeof(struct __KLPerson__test_block_impl_0), __KLPerson__test_block_copy_0, __KLPerson__test_block_dispose_0};

static void _I_KLPerson_test(KLPerson * self, SEL _cmd) {

    void(*rBlock)(void) = ((void (*)())&__KLPerson__test_block_impl_0((void *)__KLPerson__test_block_func_0, &__KLPerson__test_block_desc_0_DATA, self, 570425344));

    ((void (*)(__block_impl *))((__block_impl *)rBlock)->FuncPtr)((__block_impl *)rBlock);
}


static instancetype _Nonnull _I_KLPerson_initWithName_(KLPerson * self, SEL _cmd, NSString * _Nonnull name) {
    self = ((KLPerson *(*)(__rw_objc_super *, SEL))(void *)objc_msgSendSuper)((__rw_objc_super){(id)self, (id)class_getSuperclass(objc_getClass("KLPerson"))}, sel_registerName("init"));
    if (self) {
        ((void (*)(id, SEL, NSString * _Nonnull))(void *)objc_msgSend)((id)self, sel_registerName("setRName:"), (NSString * _Nonnull)name) ;
    }
    return self;
}