# single

**限制 `Observable` 只有一个元素，否出发出一个 `error` 事件**

![](/files/-MWlwSCQzlyPOQy7LKF-)

**single** 操作符将限制 `Observable` 只产生一个元素。如果 `Observable` 只有一个元素，它将镜像这个 `Observable` 。如果 `Observable` 没有元素或者元素数量大于一，它将产生一个 `error` 事件。


---

# Agent Instructions: Querying This Documentation

If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question.

Perform an HTTP GET request on the current page URL with the `ask` query parameter:

```
GET https://3440217568.gitbook.io/rxswift/decision_tree/single.md?ask=<question>
```

The question should be specific, self-contained, and written in natural language.
The response will contain a direct answer to the question and relevant excerpts and sources from the documentation.

Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.