///

/// warning

If your application relies on lifespan events, the `AsyncClient` won't trigger these events. To ensure they are triggered, use `LifespanManager` from [florimondmanca/asgi-lifespan](https://github.com/florimondmanca/asgi-lifespan#usage).

///

## Other Asynchronous Function Calls { #other-asynchronous-function-calls }

* **Asynchronous Code**
* **`async` and `await`**
* **Coroutines**

## Asynchronous Code { #asynchronous-code }

Asynchronous code just means that the language 💬 has a way to tell the computer / program 🤖 that at some point in the code, it 🤖 will have to wait for *something else* to finish somewhere else. Let's say that *something else* is called "slow-file" 📝.

Calls are executed in one of two ways:

 * **Synchronous:** your thread blocks until the response is readable.
 * **Asynchronous:** you enqueue the request on any thread, and get [called back](https://square.github.io/okhttp/5.x/okhttp/okhttp3/-callback/) on another thread when the response is readable.

   * asynchronous only if {@link CacheLoader#reload} was overridden with an asynchronous
   * implementation.
   *
   * <p>Returns without doing anything if another thread is currently loading the value for {@code
   * key}. If the cache loader associated with this cache performs refresh asynchronously then this
   * method may return before refresh completes.
   *

   * asynchronous only if {@link CacheLoader#reload} was overridden with an asynchronous
   * implementation.
   *
   * <p>Returns without doing anything if another thread is currently loading the value for {@code
   * key}. If the cache loader associated with this cache performs refresh asynchronously then this
   * method may return before refresh completes.
   *

要讓 `await` 運作，它必須位於支持非同步功能的函式內。為此，只需使用 `async def` 宣告函式：

```Python hl_lines="1"
async def get_burgers(number: int):
    # Do some asynchronous stuff to create the burgers
    return burgers
```

...而不是 `def`:

```Python hl_lines="2"
# This is not asynchronous
def get_sequential_burgers(number: int):
    # Do some sequential stuff to create the burgers
    return burgers
```

Here's a little bit of that history.

## Alternatives { #alternatives }

I have been creating APIs with complex requirements for several years (Machine Learning, distributed systems, asynchronous jobs, NoSQL databases, etc), leading several teams of developers.

As part of that, I needed to investigate, test and use many alternatives.

The history of **FastAPI** is in great part the history of its predecessors.

///

/// warning

如果你的應用仰賴 lifespan 事件，`AsyncClient` 不會觸發這些事件。若要確保它們被觸發，請使用 [florimondmanca/asgi-lifespan](https://github.com/florimondmanca/asgi-lifespan#usage) 的 `LifespanManager`。

///

## 其他非同步函式呼叫 { #other-asynchronous-function-calls }

由於測試函式現在是非同步的，你也可以在測試中呼叫（並 `await`）其他 `async` 函式，和在程式碼其他地方一樣。

/// tip

用户指南介绍了如何为应用添加[自定义中间件](../tutorial/middleware.md)。

以及如何[使用 `CORSMiddleware` 处理 CORS](../tutorial/cors.md)。

本章学习如何使用其它中间件。

## 添加 ASGI 中间件 { #adding-asgi-middlewares }

因为 **FastAPI** 基于 Starlette，且执行 <abbr title="Asynchronous Server Gateway Interface - 异步服务器网关接口">ASGI</abbr> 规范，所以可以使用任意 ASGI 中间件。

中间件不必是专为 FastAPI 或 Starlette 定制的，只要遵循 ASGI 规范即可。

总之，ASGI 中间件是类，并把 ASGI 应用作为第一个参数。

因此，有些第三方 ASGI 中间件的文档推荐以如下方式使用中间件：

```Python

   * CacheLoader#reload}.
   *
   * <p>As the default implementation of {@link CacheLoader#reload} is synchronous, it is
   * recommended that users of this method override {@link CacheLoader#reload} with an asynchronous
   * implementation; otherwise refreshes will be performed during unrelated cache read and write
   * operations.
   *