The open method returns a modal instance,an object with the following properties:

• close(result) – a method that can be used to close a modal,passing a result
• dismiss(reason) – a method that can be used to dismiss a modal,passing a reason
• result – a promise that is resolved when a modal is closed and rejected when a modal is dismissed

这里的重要位置是承诺发生了什么。关闭，承诺被解决 – 本质上，“成功”回调触发。在关闭时，promise被拒绝，因此运行“failure”回调。