viewWillAppear、viewWillDisappear

注意一点！！不同的方法调用父类方法的时机也不同！！！

-(void)viewWillAppear:(BOOL)animated
{
    [superviewWillAppear:animated];

    //需要添加的代码
}

-(void)viewWillDisappear:(BOOL)animated
{
    //需要添加的代码

    [super viewWillDisappear:animated];
}