监听tableView 的滑动

[self.tableView addObserver: self forKeyPath: @"contentOffset" options: NSKeyValueObservingOptionNew context: nil];
- (void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary *)change context:(void *)context
{
    CGFloat offset = self.tableView.contentOffset.y;
    CGFloat delta = offset / 64.f + 1.f;
    delta = MAX(0, delta);
    self.navigationController.navigationBar.alpha = MIN(1, delta);
    
    
    NSLog(@"%lf",offset);
    
}