Block（二）

Block 到底是什么？

它就是里面存储了指向定义Block时的代码块的函数指针，以及Block 外部上下文变量信息的结构体。

1.循环引用

1. weak strong dance

-(void)viewDidLoad{

  [super viewDidLoad];

   __weak typeof(self) weakSelf = self;

   self.block = ^{
     
      __strong typeof(self ) strongSelf = weakSelf;
       
      dispatch_after(dispatch_time(DISPATCH_TIME_NOW,(int64_t)(2 * NSEC_PER_SEC)),dispatch_get_main_queue(),^{

             NSLog(@"%@",strongSelf.name);
      });
 
  };
  self.block();

}

2. self -> block - >vc (nil) -> self

-(void)viewDidLoad{

  [super viewDidLoad];

   __block ViewController *vc = self;

   self.block = ^{
     
      __strong typeof(self ) strongSelf = weakSelf;
       
      dispatch_after(dispatch_time(DISPATCH_TIME_NOW,(int64_t)(2 * NSEC_PER_SEC)),dispatch_get_main_queue(),^{

             NSLog(@"%@",vc.name);
      });
 
 }
 self.block();

};

-(void)viewDidLoad{

  [super viewDidLoad];

   self.block = ^(ViewController * vc){
     
      __strong typeof(self ) strongSelf = weakSelf;
       
      dispatch_after(dispatch_time(DISPATCH_TIME_NOW,(int64_t)(2 * NSEC_PER_SEC)),dispatch_get_main_queue(),^{

             NSLog(@"%@",vc.name);
             vc = nil;

      });
 
 }
 self.block(self);

}

2. 例题

例1

三次打印均有值，因为Block内部对otherObject 进行了持有，它有引用计数为2，
当otherObject = nil 时，Block 内部仍然对其持用，它的引用计数为1

stuct block{

  __strong NSObject * otherObject = 外部变量值;

  function{

    NSLog(........);
  }

}

NSobject  *otherObject  = [NSObject alloc]init];
NSlog(otherObject %p  & otherObject %p ,otherObject, &otherObject);

blk = ^{

  NSlog(otherObject %p   & otherObject %p ,otherObject, &otherObject);
}

blk();

otherObject = nil;

blk();

例2

block 没有对otherObject 进行持有，引用计数为1
所以当otherObject = nil 后，对象释放，引用计数为0，所以打印没值

stuct block{

  __weak NSObject * otherObject = 外部变量值;

  function{
     __strong typeof(weakObject) strongObject = weakObject;
     NSLog(........);
  }

}

NSobject  *otherObject  = [NSObject alloc]init];
NSlog(otherObject %p  & otherObject %p ,otherObject, &otherObject);


__weak typeof(otherObject) weakObject  = otherObject;
blk = ^{

  __strong typeof(weakObject) strongObject = weakObject;
  NSlog(otherObject %p    & otherObject %p , strongObject, & strongObject);
}

blk();

otherObject = nil;

blk();

例 3

Block 内部弱引用外部otherObject，但Block内部函数强引用，所以在内部执行休眠3秒后，依然能打印值，但otherObject = nil 后，休眠5秒，再执行block ，因为对象已经释放，所以没有打印。

stuct block{

  __weak NSObject * otherObject = 外部变量值;

  function{
     __strong typeof(weakObject) strongObject = weakObject;
     NSLog(........);
     sleep(3);
     NSLog(........);

  }

}

NSobject  *otherObject  = [NSObject alloc]init];

__weak typeof(otherObject) weakObject  = otherObject;
blk = ^{

  __strong typeof(weakObject) strongObject = weakObject;

  NSlog(otherObject %p    & otherObject %p , strongObject, & strongObject);

  sleep(3);
  NSlog(otherObject %p    & otherObject %p , strongObject, & strongObject);
}

blk();

otherObject = nil;
sleep(5);
blk();

例 4

Block 内部只对Self 进行了强持有，但成员变量并没有强持有，所以object = nil 后，对象释放，打印无内容。

stuct block{

  typeOf(self ) *tmpSelf = self;

  function{

    NSLog(........);
  }

}

//成员变量
object  = [NSObject alloc]init];

  NSlog(object %p    &object %p , object, & object);

blk = ^{

   NSlog(object %p    &object %p , object, & object);

}

blk();
object = nil;
blk();

例5

__block char key = 0; //key 在栈区

objc_setAssociatedObjec(self, &key, @1, OBJC_ASSOCIATION_ASSIGN);

void(^block)(void) = ^(){

  objc_setAssociatedObjec(self, &key, @2, OBJC_ASSOCIATION_ASSIGN);

} ///堆区

id m = objc_getAssociatedObjec(self, &key);

block();

id n = objc_getAssociatedObject(self,&key);

NSLog(@"m = %@, n = %@",m,n);