## An app with callbacks { #an-app-with-callbacks }

Let's see all this with an example.

Imagine you develop an app that allows creating invoices.

These invoices will have an `id`, `title` (optional), `customer`, and `total`.

The user of your API (an external developer) will create an invoice in your API with a POST request.

Then your API will (let's imagine):

* Send the invoice to some customer of the external developer.

이는 **FastAPI**가 다음과 유사한 본문을 기대한다는 것을 의미합니다:

```JSON
{
    "name": "Foo",
    "description": "The pretender",
    "price": 42.0,
    "tax": 3.2,
    "tags": ["rock", "metal", "bar"],
    "image": {
        "url": "http://example.com/baz.jpg",
        "name": "The Foo live"
    }
}
```

다시 한번, **FastAPI**를 사용하여 해당 선언을 함으로써 얻는 것은:

* 중첩 모델도 편집기 지원(자동완성 등)
* 데이터 변환

## Um aplicativo com callbacks

Vamos ver tudo isso com um exemplo.

Imagine que você tem um aplicativo que permite criar faturas.

Essas faturas terão um `id`, `title` (opcional), `customer` e `total`.

O usuário da sua API (um desenvolvedor externo) criará uma fatura em sua API com uma solicitação POST.

Então sua API irá (vamos imaginar):

* Enviar uma solicitação de pagamento para o desenvolvedor externo.

	DB.Create(&users)

	rows, err := DB.Select("name, count(*) as total").Table("users").Group("name").Having("name IN ?", []string{users[0].Name, users[1].Name}).Rows()
	if err != nil {
		t.Fatalf("got error %v", err)
	}

	defer rows.Close()
	for rows.Next() {
		var name string
		var total int64
		rows.Scan(&name, &total)

		if name == users[0].Name && total != 3 {

// disk usage per path.
// The above means that any added/deleted fields are incompatible.
//
// The above means that any added/deleted fields are incompatible.
//
//msgp:tuple DiskInfo
type DiskInfo struct {
	Total      uint64
	Free       uint64
	Used       uint64
	UsedInodes uint64
	FreeInodes uint64
	Major      uint32
	Minor      uint32
	NRRequests uint64
	FSType     string
	RootDisk   bool
	Healing    bool

		size = diskAssumeUnknownSize
	}

	var available uint64
	var total uint64
	var nDisks int
	for _, disk := range di {
		if disk == nil || disk.Total == 0 {
			// Disk offline, no inodes or something else is wrong.
			continue
		}
		nDisks++
		total += disk.Total
		available += disk.Total - disk.Used
	}

	if nDisks < len(di)/2 || nDisks <= 0 {
		var errs []error

        @CsvSource({ "1048576, 524288, 8, 512, 4294967296, 2147483648", // 4GB total, 2GB free
                "2097152, 1048576, 8, 512, 8589934592, 4294967296", // 8GB total, 4GB free
                "134217728, 67108864, 8, 512, 549755813888, 274877906944", // 512GB total, 256GB free
                "1, 1, 1, 1, 1, 1", // Minimal values
                "0, 0, 1, 1, 0, 0" // Zero allocation units

     * Returns the similar document hash.
     *
     * @return The similar document hash.
     */
    public abstract String getSimilarDocHash();

    /**
     * Returns the track total hits.
     *
     * @return The track total hits.
     */
    public String getTrackTotalHits() {
        return null;
    }

    /**
     * Returns the min score.
     *
     * @return The min score.
     */

    }

    /**
     * Concatenates multiple byte arrays into a single array.
     */
    private static byte[] concat(byte[]... arrays) {
        int total = 0;
        for (byte[] a : arrays) {
            total += a.length;
        }
        byte[] res = new byte[total];
        int off = 0;
        for (byte[] a : arrays) {
            System.arraycopy(a, 0, res, off, a.length);
            off += a.length;
        }

  /** True if this web socket failed and the listener has been notified. */
  private var failed = false

  /** Total number of pings sent by this web socket. */
  private var sentPingCount = 0

  /** Total number of pings received by this web socket. */
  private var receivedPingCount = 0

  /** Total number of pongs received by this web socket. */
  private var receivedPongCount = 0