Framing rules make it impractical to implement http/2 correctly on a single blocking thread. The flow-control features introduce feedback between reads and writes, requiring writes to acknowledge reads and reads to throttle writes.

In OkHttp we expose a blocking API over a framed protocol. This document explains the code and policy that makes that work.

### Threads

#### Application's calling thread

`OAuth2PasswordBearer` makes **FastAPI** know that it is a security scheme. So it is added that way to OpenAPI.

But `OAuth2PasswordRequestForm` is just a class dependency that you could have written yourself, or you could have declared `Form` parameters directly.

But as it's a common use case, it is provided by **FastAPI** directly, just to make it easier.

///

### Use the form data { #use-the-form-data }

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但接著我們在路徑操作函式（*path operation function*）的參數 `commons` 中取得的是一個 `dict`。

而我們知道，編輯器對 `dict` 無法提供太多輔助（例如自動完成），因為它無法預先知道其中的鍵與值的型別。

我們可以做得更好...

## 什麼算是相依性 { #what-makes-a-dependency }

到目前為止，你看到的相依性都是宣告成函式。

但那不是宣告相依性的唯一方式（雖然那大概是最常見的）。

關鍵在於，相依性應該要是「callable」。

在 Python 中，「**callable**」指的是任何可以像函式一樣被 Python「呼叫」的東西。

因此，如果你有一個物件 `something`（它可能不是函式），而你可以像這樣「呼叫」（執行）它：

    OkHttp used _get_ and _set_ prefixes sporadically which make the API
    inconsistent and awkward to explore.

 *  **OkHttpClient now implements the new `Call.Factory` interface.** This
    interface will make your code easier to test. When you test code that makes
    HTTP requests, you can use this interface to replace the real `OkHttpClient`
    with your own mocks or fakes.

    Original Software, to make, have made, use, practice, sell, and
    offer for sale, and/or otherwise dispose of the Original Software
    (or portions thereof).

    (c) The licenses granted in Sections 2.1(a) and (b) are effective on
    the date Initial Developer first distributes or otherwise makes the
    Original Software available to a third party under the terms of this
    License.

  public Splitter trimResults(CharMatcher trimmer) {
    checkNotNull(trimmer);
    return new Splitter(strategy, omitEmptyStrings, trimmer, limit);
  }

  /**
   * Splits {@code sequence} into string components and makes them available through an {@link
   * Iterator}, which may be lazily evaluated. If you want an eagerly computed {@link List}, use
   * {@link #splitToList(CharSequence)}. Java 8+ users may prefer {@link #splitToStream} instead.

4. Submit a pull request

* There should be tests testing the source example.
* Before the PR is applied, the new tests should fail.
* After applying the PR, the new tests should pass.
* Coverage should stay at 100%.
* If you see the PR makes sense, or we discussed it and considered it should be accepted, you can add commits on top of the PR to tweak it, to add docs, tests, format, refactor, remove extra files, etc.

You will see examples of this in the next chapters, about relational and NoSQL databases, security, etc.

## **FastAPI** compatibility { #fastapi-compatibility }

The simplicity of the dependency injection system makes **FastAPI** compatible with:

* all the relational databases
* NoSQL databases
* external packages
* external APIs
* authentication and authorization systems
* API usage monitoring systems