问题显现，见图：

bug02.png

原因详解：

1.a界面隐藏了状态栏，b界面显示状态栏，左滑动，返回到a界面，此时观察发现滑动时，b界面整体上移了20px

解决方案：

定义个变量

 @property (nonatomic, assign) BOOL isViewDidAppear;


- (void)viewWillAppear:(BOOL)animated{
 self.isViewDidAppear = NO;
}

- (void)viewWillDisappear:(BOOL)animated {
[super viewWillDisappear:animated];    
self.isViewDidAppear = NO;
}

界面完全显示时，手动调用prefersStatusBarHidden方法，且强制更新隐藏状态栏状态
- (void)viewDidAppear:(BOOL)animated {
   [super viewDidAppear:animated];
   self.isViewDidAppear = YES;
   [self prefersStatusBarHidden];
   if ([self respondsToSelector:@selector(setNeedsStatusBarAppearanceUpdate)]) {
    [self setNeedsStatusBarAppearanceUpdate];
 }
}

- (BOOL)prefersStatusBarHidden {
   return self.isViewDidAppear;
}