假設有一個複雜、想重覆利用的 UIView subclass。如果用 xib 來設定該 view，一般會寫 code 來載入 nib。
但是，如果要在另一個 xib 或 storyboard 使用這個 custom view 為一個 subview，每次都要回到 code 來處理就變得很不實際。
這個 project 裡，HPLProfileView 要被重覆使用，但本身也要用 xib 來製作。
在 storyboard 裡，拉出普通的 UIView 並把 class 設成 HPLProfileView。位置可用 auto layout 做設定。
HPLProfileView.xib 的 file owner 是 HPLProfileView，這樣才能接 IBOutlet，但最底層的 view class 其實只是單純的 UIView 而非 HPLProfileView (不然下一步會產生 infinite loop)。
在 HPLProfileView -awakeFromNib，手動把 nib 載入加為 subview，並把它用 auto layout 設成跟自己同大。
換句話說，HPLProfileView 本身只是個容器，並統一負責 loadNib 的邏輯。
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