## 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.

...
11:55:40 I/XmlResultReporter: XML test result file generated at /Users/myusername/workspace/okhttp/android-test/build/outputs/androidTest-results/connected/TEST-pixel3a-Q(AVD) - 10-android-test-.xml. Total tests 13, passed 11, assumption_failure 1, ignored 1,
...
BUILD SUCCESSFUL in 1m 30s
63 actionable tasks: 61 executed, 2 up-to-date

```

    }
    stopwatch.sleepMillis(4250); // #6, back to cold state (warmup period + repay last acquire)
    for (int i = 0; i < 11; i++) {
      limiter.acquire(); // #7, showing off the warmup starting from totally cold
    }

    // make sure the areas (times) remain the same, while permits are different
    assertEvents(
        "R0.00, R1.38, R1.13, R0.88, R0.63, R0.50, R0.50, R0.50", // #1
        "U4.50", // #2

    }
    stopwatch.sleepMillis(4250); // #6, back to cold state (warmup period + repay last acquire)
    for (int i = 0; i < 11; i++) {
      limiter.acquire(); // #7, showing off the warmup starting from totally cold
    }

    // make sure the areas (times) remain the same, while permits are different
    assertEvents(
        "R0.00, R1.38, R1.13, R0.88, R0.63, R0.50, R0.50, R0.50", // #1
        "U4.50", // #2

    * In requests and responses will be represented as a `str` in ISO 8601 format, like: `14:23:55.003`.
* `datetime.timedelta`:
    * A Python `datetime.timedelta`.
    * In requests and responses will be represented as a `float` of total seconds.

// 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

	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 {

        // Word count
        buffer[32] = 10; // 10 words = 20 bytes

        // Parameter words for transaction response
        // Total parameter count (2 bytes)
        buffer[33] = 0x00;
        buffer[34] = 0x00;

        // Total data count (2 bytes)
        buffer[35] = 0x00;
        buffer[36] = 0x00;

        // Reserved (2 bytes)
        buffer[37] = 0x00;

		}
		return
	}
	xls := xlStorage{drivePath: base, diskInfoCache: cachevalue.New[DiskInfo]()}
	xls.diskInfoCache.InitOnce(time.Second, cachevalue.Opts{}, func(ctx context.Context) (DiskInfo, error) {
		return DiskInfo{Total: 1 << 40, Free: 1 << 40}, nil
	})
	weSleep := func() bool { return false }

	got, err := scanDataFolder(t.Context(), nil, &xls, dataUsageCache{Info: dataUsageCacheInfo{Name: bucket}}, getSize, 0, weSleep)
	if err != nil {

		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