如果我們在網上下載的低版本開發的iOS源代碼，而我們的iOS SDK又是高版本的時候，就會有Base SDK Missing的問題纏身，icech在網上搜集了兩個解決辦法：

方法1：老外的方法，網上流傳廣泛。IPhone Base SDK missing

   1. Load your project
   2. From the menu, select Project > Edit Project Settings...
   3. Under Architecture > Base SDK, choose one of the available device options: iPhone 3.2 or iPhone 4.0. If you are iPhone-only, 4.0 is the way to go.
   4. Close that window.
   5. From the menu, select Project > Edit Active Target "YourTarget"
   6. Under Architecture > Base SDK, choose one of the available device or simulator options: iPhone 3.2 or iPhone 4.0.
   7. If you want to target previous iOS versions, then in that same window, under Deployment > iPhone OS Deployment Target, select the lowest version you want to support. Note that support for 2.x versions through the app store is deprecated. See the "Readiness Checklist" quote above.

方法2：看起來和方法1沒什么區別，呵呵

解決辦法就是Project->Edit Project Settings->Build標簽頁中的將默認的Base SDK從3.2版本選擇成現有的iOS SDK 4.2。可這樣依舊不能解決問題。關鍵還有一個步驟，Project->Set Active Target，選擇成你現在的工程。盡管你會看到，Project->Set Active Target中只有你現在工程的名稱，并且已經打鉤了，但你仍然要用鼠標點擊一下。