我在 Xcode 4 的最终版本中遇到了同样的问题，并通过在使用拖动之前选择 File's Owner Custom Class 解决了它&如下图所示的 drop outlet 功能.

I had the same problem with the final version of Xcode 4 and solved it by selecting the File's Owner Custom Class before using the drag & drop outlet feature like shown on the screenshot below.