怎么判断某个 cell 是否显示在屏幕上

简单的方案是监测tableView的滚动事件，然后通过visibleCells属性判断cell是否可见。

• 成为某个视图的子视图时，先移除之前的kVO，然后对视图层级中所有class为UIScrollView的contentSize添加KVO

  - (void)didMoveToSuperview {
    [self unKVO];
    UIView *aView = self.superview;

    while (aView) {
      if ([aView isKindOfClass:[UIScrollView class]]) {
        [self observer:aView forKeyPath:@"contentSize" options:NSKeyValueObservingOptionNew context:(void *)__LINE__];
      }
      aView = aView.superview;
    }
  }

记录KVO了哪些视图的，方便及时移除

  static NSMutableArray *receiverArr;
  static SunTableViewCell *cell;
  - (void)observer:(NSObject *)receiver forKeyPath:(NSString *)keyPath options:(NSKeyValueObservingOptions)options context:(void *)context {
    if (cell && cell != self) {
      return;
    }
    cell = self;
    [receiver addObserver:self forKeyPath:keyPath options:options context:context];
    if (!receiverArr) {
      receiverArr = [NSMutableArray array];
    }
    [receiverArr addObject:receiver];
  }

  - (void)dealloc {
    [self unKVO];
  }

  - (void)unKVO {
    NSLog(@"");
    [receiverArr enumerateObjectsUsingBlock:^(id _Nonnull receiver, NSUInteger idx, BOOL *_Nonnull stop) {
      NSObject *observationInfo = [receiver observationInfo];
      NSArray *_observances = [observationInfo valueForKey:@"_observances"];
      [_observances enumerateObjectsUsingBlock:^(id _Nonnull obj, NSUInteger idx, BOOL *_Nonnull stop) {
        NSObject *observer = [obj valueForKey:@"_observer"];
        if ([observer isEqual:self]) {
          NSString *_property = [obj valueForKeyPath:@"_property._keyPath"];
          [receiver removeObserver:self forKeyPath:_property];
        }
      }];
    }];
  }

打印监测到的值变化

  - (void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary<NSString *, id> *)change context:(void *)context {

    NSLog(@"self:%@ visibleCells:%@", self, [[self tableView] visibleCells]);

    // 判断是否在tableView的可见cell内
    if (![[[self tableView] visibleCells] containsObject:self]) {
      NSLog(@"隐藏");
      self.textLabel.text = @"隐藏";
      NSLog(@"%@", [self tableView]);
    } else{
      NSLog(@"显示");
      self.textLabel.text = @"显示";
      NSLog(@"%@", [self tableView]);
    }
    NSLog(@"self:%@ visibleCells:%@", self, [[self tableView] visibleCells]);
  }