这是setPrompt的
解决方法.它没有动画,所以我称之为
解决方案而不是
解决方案.必须在viewDidAppear中,而不是viewWillAppear.
-(void) viewDidAppear:(BOOL)animated {
[super viewDidAppear:animated];
// This is needed for apple bug with self.navigationItem.prompt
[self.navigationController.navigationBar setNeedsUpdateConstraints];
}