> ## Documentation Index
> Fetch the complete documentation index at: https://store.mobilenativefoundation.org/llms.txt
> Use this file to discover all available pages before exploring further.

# Validator

> Allows you to define custom logic to determine whether your local data is still valid or if it needs to be refreshed from the remote source using the Fetcher.

<Info>
  The
  [Validator](https://github.com/MobileNativeFoundation/Store/blob/f9072fc59cc8bfe95cfe008cc8a9ce999301b242/store/src/commonMain/kotlin/org/mobilenativefoundation/store/store5/Validator.kt)
  is optional. If you don't provide one, the
  [Store](https://github.com/MobileNativeFoundation/Store/blob/f9072fc59cc8bfe95cfe008cc8a9ce999301b242/store/src/commonMain/kotlin/org/mobilenativefoundation/store/store5/Store.kt)
  defaults to considering the cached data valid.
</Info>

## **Purpose of the Validator**

* **Data Freshness**: Ensures the data served to your application is up-to-date and meets your application's specific validity criteria.
* **Optimizing Network Usage**: Prevents unnecessary network calls by using valid cached data when appropriate.
* **Consistency Control**: Gives you fine-grained control over when to refresh data, enhancing consistency between the client and server.

## **APIs**

### **Validator**

[Validator](https://github.com/MobileNativeFoundation/Store/blob/f9072fc59cc8bfe95cfe008cc8a9ce999301b242/store/src/commonMain/kotlin/org/mobilenativefoundation/store/store5/Validator.kt) has the following structure:

```kotlin theme={"system"}
interface Validator<Output : Any> {
    suspend fun isValid(item: Output): Boolean
}
```

<ParamField path="Output" type="Any" required>
  The type representing your domain data model. For example,
  if you have a `Store<Int, Post>`, the `Output` is `Post`.
</ParamField>

<ParamField query="isValid(item: Output)" type="Boolean">
  A suspending function that determines whether the given item is still valid.
</ParamField>

## **Data Flow**

<Steps>
  <Step title="Data Retrieval Request" stepNumber="1">
    When your application requests cached data from the [Store](https://github.com/MobileNativeFoundation/Store/blob/f9072fc59cc8bfe95cfe008cc8a9ce999301b242/store/src/commonMain/kotlin/org/mobilenativefoundation/store/store5/Store.kt), it first checks the [Memory Cache](https://github.com/MobileNativeFoundation/Store/blob/f9072fc59cc8bfe95cfe008cc8a9ce999301b242/store/src/commonMain/kotlin/org/mobilenativefoundation/store/store5/MemoryCache.kt) and the [Source of Truth](https://github.com/MobileNativeFoundation/Store/blob/f9072fc59cc8bfe95cfe008cc8a9ce999301b242/store/src/commonMain/kotlin/org/mobilenativefoundation/store/store5/SourceOfTruth.kt) to see if the data is available.

    <Note>
      The
      [Validator](https://github.com/MobileNativeFoundation/Store/blob/f9072fc59cc8bfe95cfe008cc8a9ce999301b242/store/src/commonMain/kotlin/org/mobilenativefoundation/store/store5/Validator.kt)
      only operates on data from the [Source of
      Truth](https://github.com/MobileNativeFoundation/Store/blob/f9072fc59cc8bfe95cfe008cc8a9ce999301b242/store/src/commonMain/kotlin/org/mobilenativefoundation/store/store5/SourceOfTruth.kt)
      and does not validate data coming directly from the network.
    </Note>
  </Step>

  <Step title="Validation Check" stepNumber="2">
    The [Validator](https://github.com/MobileNativeFoundation/Store/blob/f9072fc59cc8bfe95cfe008cc8a9ce999301b242/store/src/commonMain/kotlin/org/mobilenativefoundation/store/store5/Validator.kt)'s `isValid` method is called with the cached data as the parameter.
  </Step>

  <Step title="Validity Determination" stepNumber="3">
    * If `isValid` returns `true`, the [Store](https://github.com/MobileNativeFoundation/Store/blob/f9072fc59cc8bfe95cfe008cc8a9ce999301b242/store/src/commonMain/kotlin/org/mobilenativefoundation/store/store5/Store.kt) considers the cached data valid and servies it to the application.
    * If `isValid` returns `false`, the [Store](https://github.com/MobileNativeFoundation/Store/blob/f9072fc59cc8bfe95cfe008cc8a9ce999301b242/store/src/commonMain/kotlin/org/mobilenativefoundation/store/store5/Store.kt) deems the cached data invalid and proceeds to fetch fresh data from the remote source using the [Fetcher](https://github.com/MobileNativeFoundation/Store/blob/f9072fc59cc8bfe95cfe008cc8a9ce999301b242/store/src/commonMain/kotlin/org/mobilenativefoundation/store/store5/Fetcher.kt).
  </Step>

  <Step title="Data Update" stepNumber="4">
    If new data is fetched, it's stored in the [Memory Cache](https://github.com/MobileNativeFoundation/Store/blob/f9072fc59cc8bfe95cfe008cc8a9ce999301b242/store/src/commonMain/kotlin/org/mobilenativefoundation/store/store5/MemoryCache.kt) and the [Source of Truth](https://github.com/MobileNativeFoundation/Store/blob/f9072fc59cc8bfe95cfe008cc8a9ce999301b242/store/src/commonMain/kotlin/org/mobilenativefoundation/store/store5/SourceOfTruth.kt) for future requests.
  </Step>
</Steps>

## **Implementing a Validator**

You can create a [Validator](https://github.com/MobileNativeFoundation/Store/blob/f9072fc59cc8bfe95cfe008cc8a9ce999301b242/store/src/commonMain/kotlin/org/mobilenativefoundation/store/store5/Validator.kt) using the `Validator.by` factory method:

```kotlin theme={"system"}
val validator = Validator.by { item ->
    // Your custom validation logic here
}
```

### **Examples**

#### **Time-Based Validation**

If your application has data that should be refreshed every 24 hours:

```kotlin theme={"system"}
val validator = Validator.by { item ->
    Clock.System.now() < item.expiresAt
}
```

#### **Versioning**

If your application data model changes and you need to invalidate old cached data:

```kotlin theme={"system"}
val validator = Validator.by { item ->
    item.version == CURRENT_VERSION
}
```

#### **User Authentication**

In cases where authentication tokens expire:

```kotlin theme={"system"}
val validator = Validator.by { item ->
    !item.token.isExpired()
}
```

## **Best Practices**

* **Keep Validation Logic Lightweight**: The `isValid` function should execute quickly to avoid slowing down data retrieval. Complex computations or I/O operations should be avoided.
* **No Side Effects**: The [Validator](https://github.com/MobileNativeFoundation/Store/blob/f9072fc59cc8bfe95cfe008cc8a9ce999301b242/store/src/commonMain/kotlin/org/mobilenativefoundation/store/store5/Validator.kt) should not modify the data or state. It should only assess the validity of the provided item.
* **Decide on Validity Criteria**: Clearly define what makes data valid or invalid in your application's context. This could be based on timestamps, data content, user preferences, or other domain-specific factors.