我可以使用UIDocumentInteractionController在 Instagram 上成功分享图片，但InstagramCaption密钥无法正常工作。这是我的代码：
self.controller = UIDocumentInteractionController()
// controller.delegate = self
self.controller.URL = fileUri
// HERE, the caption won't appear.
self.controller.annotation = NSDictionary(objectsAndKeys: "my caption", "InstagramCaption")
self.controller.UTI = "com.instagram.exclusivegram"
self.controller.presentOpenInMenuFromRect(CGRectZero, inView: self.view, animated: true)
我做错了什么？