TGLayoutPos is represent to the position of a view. UIView provides six extension variables:tg_left, tg_top, tg_bottom, tg_right, tg_centerX, tg_centerY to set view's margin or space distance between self and others. TGLayoutSize is represent to the size of a view. UIView provides two extension variables:tg_width,tg_height to set view's width and height dimension. there are three special TGLayoutSize const object: .wrap, .fill, .average mean: wrap all subviews size, fill in to superview's residual size, average the superview's size.