Learn about the lifecycle of requests created using Cronet and how to manage
them using the callback methods provided by the library.

## Lifecycle overview

Network requests created using the Cronet Library are represented by the
[`UrlRequest`](https://developer.android.com/develop/connectivity/cronet/reference/org/chromium/net/UrlRequest) class.
The following concepts are important to understand the
[`UrlRequest`](https://developer.android.com/develop/connectivity/cronet/reference/org/chromium/net/UrlRequest) lifecycle:

**States**
:   A state is the particular condition that the request is in at a specific time.
    [UrlRequest](https://developer.android.com/develop/connectivity/cronet/reference/org/chromium/net/UrlRequest) objects created using the Cronet
    Library move through different states in their lifecycle. The request
    lifecycle includes an initial state, and multiple transitional and final
    states.

**`UrlRequest` methods**
:   Clients can call specific methods on
    [`UrlRequest`](https://developer.android.com/develop/connectivity/cronet/reference/org/chromium/net/UrlRequest) objects depending on the
    state. The methods move the request from one state to another.

**`Callback` methods**
:   By implementing methods of the
    [`UrlRequest.Callback`](https://developer.android.com/develop/connectivity/cronet/reference/org/chromium/net/UrlRequest.Callback) class, your
    app can receive updates about the progress of the request. You can implement
    the callback methods to call methods of the
    [`UrlRequest`](https://developer.android.com/develop/connectivity/cronet/reference/org/chromium/net/UrlRequest) object that take the lifecycle
    from a state to another.

The following list describes the flow of the
[`UrlRequest`](https://developer.android.com/develop/connectivity/cronet/reference/org/chromium/net/UrlRequest) lifecycle:

1. The lifecycle is in the **Started** state after your app calls the [`start()`](https://developer.android.com/develop/connectivity/cronet/reference/org/chromium/net/UrlRequest#start()) method.
2. The server could send a redirect response, which takes the flow to the [`onRedirectReceived()`](https://developer.android.com/develop/connectivity/cronet/reference/org/chromium/net/UrlRequest.Callback#onRedirectReceived(org.chromium.net.UrlRequest,%20org.chromium.net.UrlResponseInfo,%20java.lang.String)) method. In this method, you can take one of the following client actions:
   - Follow the redirect using [`followRedirect()`](https://developer.android.com/develop/connectivity/cronet/reference/org/chromium/net/UrlRequest#followRedirect()). This method takes the request back to the **Started** state.
   - Cancel the request using [`cancel()`](https://developer.android.com/develop/connectivity/cronet/reference/org/chromium/net/UrlRequest#cancel()). This method takes the request to the [`onCanceled()`](https://developer.android.com/develop/connectivity/cronet/reference/org/chromium/net/UrlRequest.Callback#onCanceled(org.chromium.net.UrlRequest,%20org.chromium.net.UrlResponseInfo)) method where the app can perform additional operations before the request is moved to the **Canceled** final state.
3. After the app follows all the redirects, the server sends the response headers and the [`onResponseStarted()`](https://developer.android.com/develop/connectivity/cronet/reference/org/chromium/net/UrlRequest.Callback#onResponseStarted(org.chromium.net.UrlRequest,%20org.chromium.net.UrlResponseInfo)) method is called. The request is in the **Waiting for read()** state. The app should call the [`read()`](https://developer.android.com/develop/connectivity/cronet/reference/org/chromium/net/UrlRequest#read(java.nio.ByteBuffer)) method to attempt to read part of the response body. After `read()` is called, the request is in the **Reading** state, where there are the following possible outcomes:
   - The reading action was successful, but there is more data available. The [`onReadCompleted()`](https://developer.android.com/develop/connectivity/cronet/reference/org/chromium/net/UrlRequest.Callback#onReadCompleted(org.chromium.net.UrlRequest,%20org.chromium.net.UrlResponseInfo,%20java.nio.ByteBuffer)) is called and the request is in the **Waiting for read()** state again. The app should call the [`read()`](https://developer.android.com/develop/connectivity/cronet/reference/org/chromium/net/UrlRequest#read(java.nio.ByteBuffer)) method again to continue reading the response body. The app could also stop reading the request by using the [`cancel()`](https://developer.android.com/develop/connectivity/cronet/reference/org/chromium/net/UrlRequest#cancel()) method .
   - The reading action was successful, and there is no more data available. The [`onSucceeded()`](https://developer.android.com/develop/connectivity/cronet/reference/org/chromium/net/UrlRequest.Callback#onSucceeded(org.chromium.net.UrlRequest,%20org.chromium.net.UrlResponseInfo)) method is called and the request is now in the **Succeeded** final state.
   - The reading action failed. The [`onFailed`](https://developer.android.com/develop/connectivity/cronet/reference/org/chromium/net/UrlRequest.Callback#onFailed(org.chromium.net.UrlRequest,%20org.chromium.net.UrlResponseInfo,%20org.chromium.net.CronetException)) method is called and the final state of the request is now **Failed**.

The following diagram shows the lifecycle of a
[`UrlRequest`](https://developer.android.com/develop/connectivity/cronet/reference/org/chromium/net/UrlRequest) object:

![Cronet request lifecycle
diagram](https://developer.android.com/static/images/develop/connectivity/cronet-lifecycle.svg)  

The Cronet request lifecycle

| Legend |   |
|---|---|
| ![Cronet initial state legend icon](https://developer.android.com/static/images/develop/connectivity/cronet-initial-state.svg) initial state | ![Cronet final state legend icon](https://developer.android.com/static/images/develop/connectivity/cronet-final-state.svg) final state |
| ![Cronet transitional state legend icon](https://developer.android.com/static/images/develop/connectivity/cronet-transitional-state.svg) transitional state | ![Cronet callback methods legend icon](https://developer.android.com/static/images/develop/connectivity/cronet-callback-method.svg) callback methods |
| ![Cronet client action legend icon](https://developer.android.com/static/images/develop/connectivity/cronet-client-action.svg) `UrlRequest` methods |   |