Autoresizing

    UIViewAutoresizingNone                 = 0,                        不约束

    UIViewAutoresizingFlexibleLeftMargin   = 1 << 0,         左边灵活(不约束左边)

   UIViewAutoresizingFlexibleBottomMargin = 1 << 5       下边灵活(不约束下边)

    UIViewAutoresizingFlexibleRightMargin  = 1 << 2,        右边灵活(不约束右边)

    UIViewAutoresizingFlexibleTopMargin    = 1 << 3,         上边灵活(不约束上边)

    UIViewAutoresizingFlexibleWidth        = 1 << 1,            宽度灵活(宽度随着父控件的缩放而缩放)

    UIViewAutoresizingFlexibleHeight       = 1 << 4,            高度灵活(高度随着父控件的缩放而缩放)