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Join a call session using one of two approaches: the quick start method with a session ID, or the advanced flow with manual token generation for more control.

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Overview

The CometChat Calls SDK provides two ways to join a session:
ApproachBest ForComplexity
Join with Session IDMost use cases - simple and straightforwardLow - One method call
Join with TokenCustom token management, pre-generation, cachingMedium - Two-step process
Both approaches require properly configured SessionSettings. The joinSession method returns a Widget? that you must place in your Flutter widget tree to render the call UI.

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Rendering the Call UI

Unlike native platforms that use a container view, the Flutter SDK returns a Widget? from the joinSession call. You must place this widget in your Flutter widget tree to display the call interface: 
Widget? callWidget;

// After joining successfully, place the widget in your tree
if (callWidget != null) {
  // Use it in your build method, e.g.:
  // return Scaffold(body: callWidget!);
}
The returned widget renders the full call UI including video tiles, controls, and participant views. It behaves like any other Flutter widget and can be placed inside a Scaffold, Container, or any layout widget.

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Join with Session ID

The simplest way to join a session. Pass a session ID and the SDK automatically generates the token and joins the call. 
String sessionId = "SESSION_ID";

final sessionSettings = (SessionSettingsBuilder()
    ..setDisplayName("John Doe")
    ..setType(SessionType.video))
  .build();

CometChatCalls.joinSession(
  sessionId: sessionId,
  sessionSettings: sessionSettings,
  onSuccess: (Widget? callWidget) {
    debugPrint("Joined session successfully");
    // Place callWidget in your widget tree to render the call UI
  },
  onError: (CometChatCallsException e) {
    debugPrint("Failed: ${e.message}");
  },
);
ParameterTypeDescription
sessionIdStringUnique identifier for the call session
sessionSettingsSessionSettingsConfiguration for the session
onSuccessFunction(Widget?)Callback returning the call UI widget
onErrorFunction(CometChatCallsException)Callback for error handling
All participants joining the same call must use the same session ID.

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Join with Token

For scenarios requiring more control over token generation, such as pre-generating tokens, implementing custom caching strategies, or managing token lifecycle separately. Step 1: Generate Token Generate a call token for the session. Each token is unique to a specific session and user combination. 
String sessionId = "SESSION_ID";

CometChatCalls.generateToken(
  sessionId: sessionId,
  onSuccess: (CallToken token) {
    debugPrint("Token generated: ${token.token}");
    // Store or use the token
  },
  onError: (CometChatCallsException e) {
    debugPrint("Token generation failed: ${e.message}");
  },
);
ParameterTypeDescription
sessionIdStringUnique identifier for the call session
onSuccessFunction(CallToken)Callback returning the generated token
onErrorFunction(CometChatCallsException)Callback for error handling
Step 2: Join with Token Use the generated token to join the session. This gives you control over when and how the token is used. 
final sessionSettings = (SessionSettingsBuilder()
    ..setDisplayName("John Doe")
    ..setType(SessionType.video))
  .build();

// Use the previously generated token
CometChatCalls.joinSession(
  callToken: generatedToken,
  sessionSettings: sessionSettings,
  onSuccess: (Widget? callWidget) {
    debugPrint("Joined session successfully");
    // Place callWidget in your widget tree to render the call UI
  },
  onError: (CometChatCallsException e) {
    debugPrint("Failed: ${e.message}");
  },
);
ParameterTypeDescription
callTokenCallTokenPreviously generated token object
sessionSettingsSessionSettingsConfiguration for the session
onSuccessFunction(Widget?)Callback returning the call UI widget
onErrorFunction(CometChatCallsException)Callback for error handling
Complete Example 
String sessionId = "SESSION_ID";

// Step 1: Generate token
CometChatCalls.generateToken(
  sessionId: sessionId,
  onSuccess: (CallToken token) {
    // Step 2: Join with token
    final sessionSettings = (SessionSettingsBuilder()
        ..setDisplayName("John Doe")
        ..setType(SessionType.video))
      .build();

    CometChatCalls.joinSession(
      callToken: token,
      sessionSettings: sessionSettings,
      onSuccess: (Widget? callWidget) {
        debugPrint("Joined session successfully");
        // Place callWidget in your widget tree to render the call UI
      },
      onError: (CometChatCallsException e) {
        debugPrint("Failed to join: ${e.message}");
      },
    );
  },
  onError: (CometChatCallsException e) {
    debugPrint("Token generation failed: ${e.message}");
  },
);

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Error Handling

Common errors when joining a session:
Error CodeDescription
ERROR_COMETCHAT_CALLS_SDK_INITSDK not initialized - call init() first
ERROR_AUTH_TOKENUser not logged in or auth token invalid
ERROR_CALL_SESSION_IDSession ID is null or empty
ERROR_CALL_TOKENInvalid or missing call token
ERROR_JSON_EXCEPTIONInvalid session settings or response parsing error