您需要添加：

txtField.textAlignment = NSTextAlignmentCenter; // Pre-iOS6 SDK: UITextAlignmentCenter

请注意，这会调整占位符文本的对齐方式以及用户将输入的文字。

Keep in mind, this adjusts both the alignment of the placeholder text as well as the text the user will enter in.

如何垂直居中

由于原始问题已更新，请求如何垂直对齐占位符文本，并且答案隐藏在评论中，以下是如何执行此操作：

Since the original question was updated to request how to vertically align the placeholder text and that answer is buried in the comments, here is how to do that:

txtField.contentVerticalAlignment = UIControlContentVerticalAlignmentCenter;