initWithNibName 大多用于VC和Xib连起来 ，延迟加载，等vc显示出来的时候再初始化视图
loadNibNamed 大多是那种view和xib连起来 创建出来的时候所有视图已经初始化好了
when using loadNibNamed:owner:options:, the File's Owner should be NSObject, the main view should be your class type, and all outlets should be hooked up to the view, not the File's Owner
initWithCoder ：initWithNibName 和 loadNibNamed 都会调用
awakeFromNib ：xib文件加载的时候会触发
