# Initializing SDK Clients

> \[!CAUTION]
>
> Programmable Chat has been deprecated and is no longer supported. Instead, we'll be focusing on the next generation of chat: Twilio Conversations. Find out more about the [EOL process here](https://www.twilio.com/en-us/changelog/programmable-chat-end-of-life-notice).
>
> If you're starting a new project, please visit the [Conversations Docs](/docs/conversations) to begin. If you've already built on Programmable Chat, please visit our [Migration Guide](/docs/conversations/migrating-chat-conversations) to learn about how to switch.

Initializing Programmable Chat SDKs is an important step to ensure your client is ready to be used by the user. SDKs help put necessary data in place and set up event handlers for new messages and other events.

## Mobile SDKs

Once your Chat Client is fully synchronized at client startup:

* The client is subscribed to events for all User Channels.
* The Messages and Members collections are available for querying.
* The only initially subscribed User in the system is that of the local user, which will never be automatically unsubscribed.

> \[!NOTE]
>
> You must maintain a strong reference to the client object you received, keeping it in scope for the entirety of your usage of the Chat Client.
>
> Before releasing the client it is important to release references to all objects created and returned by this Chat Client (i.e. set all objects to `nil`) and to call the `shutdown` method of the client to ensure proper cleanup of shared resources.

No previously existing Messages are fetched for the client on load. These will be loaded when you call the `getMessages` method to fetch messages on demand. Messages are then cached and updated after loading.

You receive feedback on client startup in two ways:

* You will receive an asynchronous callback from the `create` client method when the client has been successfully created and is being synchronized.
* You will receive an event to the client's listener or delegate via the `synchronizationStatusUpdated` method with a value of `StatusCompleted`. This is your indication the client is ready for business, and all User Channels have been obtained and subscribed to.

## JavaScript

The JavaScript client will retrieve the list of Subscribed Channels that a User is a Member of (or has been invited to) once that user logs into the client.

Some additional details on the JavaScript SDK behavior:

* It **will** subscribe to notifications for changes to the Subscribed Channel list itself
* It **will** subscribe to events from each Subscribed Channel in the list
* It **will** retrieve the `FriendlyName` , `UniqueName`, and `Attributes` for each Subscribed Channel in the list
* It **will not** retrieve any Messages for individual Channels
* It **will** retrieve Member lists for Channels
* It **will not** retrieve nor subscribe to Users linked to Members of Subscribed Channels
* It **will** retrieve a currently logged in User object and subscribe to this User's events

To load Messages for a Subscribed Channel and subscribe to other Channel-level events you will need to load individual Channels manually. More information on this can be found in the [Channels and Messages guide](/docs/chat/channels).

## Knowing When the SDK is Ready for Use

It is important to know when the SDK Client has completed its initialization and is ready for use. Once the client is connected, you can configure all your listeners, event handlers, and other logic.

This manifests slightly differently for each SDK as detailed below:

### JavaScript

The Chat Client is instantiated in one of two ways:

```bash
Twilio.Chat.Client.create(token).then(client => {
    // Use client
});
```

Or asynchronously, waiting for the result:

```bash
let client = await Twilio.Chat.Client.create(token);
// Use client
```

### iOS

First, we initialize the Chat Client. Here we provide an initial Access Token:

```bash
 NSString *token = <token goes here>;
    __weak typeof(self) weakSelf = self;
    [TwilioChatClient chatClientWithToken:token
                                    properties:nil
                                    delegate:<delegate>
                                    completion:^(TCHResult *result, TwilioChatClient *chatClient) { 
                                            weakSelf.client = chatClient;
                                     ... }];
```

The iOS Chat SDK then provides a `TCHClientSynchronizationStatus` delegate callback:

```bash
    - (void)chatClient:(TwilioChatClient *)client
    synchronizationStatusUpdated:(TCHClientSynchronizationStatus)status {
        if (status == TCHClientSynchronizationStatusCompleted) {
            // Client is now ready for business
        }
    }
```

### Android

The Android Chat SDK provides a Listener Interface which you must implement to check the init status and completion of the SDK client.

```bash
    ChatClient.Properties props = new ChatClient.Properties.Builder()
        .createProperties();

    ChatClient.create(context.getApplicationContext(),
      accessToken,
      props,
      new CallbackListener<ChatClient>() {
        @Override
        public void onSuccess(final ChatClient client) {
          // save client for future use here
          client.setListener(new ChatClientListener() {
            @Override
            public void onClientSynchronization(ChatClient.SynchronizationStatus status) {
              if (status == ChatClient.SynchronizationStatus.COMPLETED) {
                // Client is now ready for business, start working
              }
            }
          });
        }
      });
```

## Troubleshooting

Before we get too much deeper into showing how to get your users chatting, it's important to know where to look for logs and additional information if you need it. We have a guide about [Error Handling and Diagnostics](/docs/chat/error-handling-and-diagnostics) and Programmable Chat you may find helpful as you build your Chat integration.

## Access Tokens

All Chat SDK clients need a valid Access Token to be able to authenticate and interact with the Chat Service. The Access Token is generated on your server - you can learn more about how to do that in the [Access Token guide](/docs/chat/create-tokens).

The Access Token returned by your backend must be used when instantiating the SDK Client. You will pass the Token string directly to the SDK Client constructor method.

The SDK also provides a method to update the Access Token, which is used when you need to update the Access Token before expiration. Please see the [Creating Access Tokens guide](/docs/chat/create-tokens) for more information.

**Next:** [Creating Access Tokens](/docs/chat/create-tokens)