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

# start

Create a session and start Headless Universal Checkout with its `clientToken` and your custom settings.

```kotlin KOTLIN theme={"dark"}
fun start(
    context: Context,
    clientToken: String,
    settings: PrimerSettings? = null,
    checkoutListener: PrimerHeadlessUniversalCheckoutListener? = null,
    uiListener: PrimerHeadlessUniversalCheckoutUiListener? = null
)
```

## Parameters

<Expandable title="Parameters" defaultOpen>
  <ResponseField name="context" type="Context" required>
    Android [Context](https://developer.android.com/reference/android/content/Context.html) of your `Activity` or `Fragment`.
  </ResponseField>

  <ResponseField name="clientToken" type="String" required>
    A client token as returned by the [create client session](/checkout/client-session#create-a-client-session) call.
  </ResponseField>

  <ResponseField name="settings" type="PrimerSettings">
    ⚠️ If no settings are provided, the SDK will initialize with its default settings. <br />
    Use `PrimerSettings` to provide different options based on your use case.

    <Expandable defaultOpen>
      <ResponseField name="paymentHandling" type="PrimerPaymentHandling" post={["Default: PrimerPaymentHandling.AUTO"]}>
        Force the payment handling in the SDK.

        <Expandable>
          <ResponseField name="AUTO">
            By default, the SDK operates in `PrimerPaymentHandling.AUTO`
            mode. In this case the SDK is responsible for creating the payment.
          </ResponseField>

          <ResponseField name="MANUAL">
            You can use the `PrimerPaymentHandling.MANUAL` option
            in order to [handle the payment creation](/checkout/advanced/manual-payment-creation)
            by yourself.
          </ResponseField>
        </Expandable>
      </ResponseField>

      <ResponseField name="locale" type="Locale">
        Force the SDK locale. By default, the locale will be set to the device's
        locale.
      </ResponseField>

      <ResponseField name="paymentMethodOptions" type="PrimerPaymentMethodOptions">
        <Expandable>
          <ResponseField name="redirectScheme" type="String?">
            Sets the deeplink schema used when redirecting back from 3rd party
            applications to your application.
          </ResponseField>

          <ResponseField name="threeDsOptions" type="PrimerThreeDsOptions">
            Sets the 3DS options in the SDK.

            <Expandable defaultOpen title="Properties">
              <ResponseField name="threeDsAppRequestorUrl" type="String?">
                Set the [App link](https://developer.android.com/training/app-links) that's used to call your app after an **out-of-band (OOB)** authentication.
                Supported in 3D Secure protocol versions 2.2.0 and after.
              </ResponseField>
            </Expandable>
          </ResponseField>

          <ResponseField name="googlePayOptions" type="PrimerGooglePayOptions">
            ⚠️ Required when using Google Pay in your integration.

            <Expandable defaultOpen title="Properties">
              <ResponseField name="merchantName" type="String?">
                Set it the merchant name that you want to be shown on the Google Pay screen.
              </ResponseField>

              <ResponseField name="allowedCardNetworks" type="List<String>" post={["Default: listOf('AMEX', 'DISCOVER', 'JCB', 'MASTERCARD', 'VISA')"]}>
                Sets
                the [card networks](https://developers.google.com/pay/api-reference/android/guides/tutorial#supported-card-networks)
                that your application accepts.
              </ResponseField>

              <ResponseField name="buttonStyle" type="GooglePayButtonStyle" post={["Default: GooglePayButtonStyle.BLACK"]}>
                Sets the style of the Google Pay button.

                <Expandable defaultOpen title="GooglePayButtonStyle">
                  <ResponseField name="BLACK">
                    Style of button to be used on light theme.
                  </ResponseField>

                  <ResponseField name="WHITE">
                    Style of button to be used on dark theme.
                  </ResponseField>
                </Expandable>
              </ResponseField>

              <ResponseField name="captureBillingAddress" type="Boolean" post={["Default: false"]}>
                Sets whether user's billing address should be captured from Google Pay.
              </ResponseField>

              <ResponseField name="existingPaymentMethodRequired" type="Boolean" post={["Default: false"]}>
                If set to `true`, this specifies that Google Pay can only be used for payments if the user's Google
                Pay wallet already contains allowed payment methods.
              </ResponseField>

              <ResponseField name="shippingAddressParameters" type="PrimerGoogleShippingAddressParameters" post={["Default: null"]}>
                Indicates whether the shipping address should be collected during the checkout
                process. Note that setting at least one shipping method in Checkout. Shipping
                module is necessary

                <Expandable defaultOpen title="PrimerGoogleShippingAddressParameters">
                  <ResponseField name="phoneNumberRequired" type="Boolean" post={["Default: false"]}>
                    Sets whether the phone number should also be collected.
                  </ResponseField>
                </Expandable>
              </ResponseField>

              <ResponseField
                name="requireShippingMethod"
                type="Boolean"
                post={[
"Default: false"
]}
              >
                Indicates whether selecting a shipping method is required during the checkout process.
                At least one shipping option should be configured in order to use this option.

                This should be set to `true` if the `shippingAddressParameters` are also used.
              </ResponseField>

              <ResponseField name="emailAddressRequired" type="Boolean" post={["Default: false"]}>
                Indicates whether the email address should be collected during the checkout
                process.
              </ResponseField>

              <ResponseField name="buttonOptions" type="GooglePayButtonOptions">
                A configuration class for customizing the appearance of the Google Pay button.

                <Expandable defaultOpen title="Properties">
                  <ResponseField name="buttonType" type="Int">
                    Google Pay Button type as described in the official [documentation](https://developers.google.com/pay/api-reference/android/guides/brand-guidelines).
                  </ResponseField>

                  <ResponseField name="buttonTheme" type="Int">
                    Google Pay button style as described in the official [documentation](https://developers.google.com/pay/api-reference/android/guides/brand-guidelines#personalization)
                  </ResponseField>
                </Expandable>
              </ResponseField>
            </Expandable>
          </ResponseField>

          <ResponseField name="klarnaOptions" type="PrimerKlarnaOptions">
            ⚠️ Required when using Klarna in your integration.

            <Expandable defaultOpen title="Properties">
              <ResponseField name="recurringPaymentDescription" type="String?">
                Set the payment description that will be shown on the Klarna screen.
              </ResponseField>

              <ResponseField name="webViewTitle" type="String?" post={["Default: Klarna"]}>
                ⚠️ This option is `Deprecated`.
                Sets the toolbar title of the Activity displaying Klarna views.
              </ResponseField>
            </Expandable>
          </ResponseField>

          <ResponseField name="stripeOptions" type="PrimerStripeOptions">
            ⚠️ Required when using Stripe ACH in your integration.

            <Expandable defaultOpen title="Properties">
              <ResponseField name="publishableKey" type="String?">
                Set the [Stripe publishable key](https://docs.stripe.com/keys#obtain-api-keys).
              </ResponseField>

              <ResponseField name="mandateData" type="MandateData?">
                Data used for mandate in Drop-in

                <Expandable>
                  <ResponseField name="TemplateMandateData">
                    Data for the mandate allowing you to specify the name of the merchant that
                    would be used in a predefined mandate template.

                    <Expandable defaultOpen title="Properties">
                      <ResponseField name="merchantName" type="String">
                        The name of the merchant.
                      </ResponseField>
                    </Expandable>
                  </ResponseField>

                  <ResponseField name="FullMandateData">
                    Data for the mandate allowing you to specify the string resource pointing
                    to the full mandate.

                    <Expandable defaultOpen title="Properties">
                      <ResponseField name="value" type="@StringRes Int">
                        The string resource pointing to the full mandate.
                      </ResponseField>
                    </Expandable>
                  </ResponseField>

                  <ResponseField name="FullMandateStringData">
                    Data for the mandate allowing you to specify the full mandate as a string.

                    <Expandable defaultOpen title="Properties">
                      <ResponseField name="value" type="String">
                        The full mandate.
                      </ResponseField>
                    </Expandable>
                  </ResponseField>
                </Expandable>
              </ResponseField>
            </Expandable>
          </ResponseField>
        </Expandable>
      </ResponseField>

      <ResponseField name="uiOptions" type="PrimerUIOptions">
        Sets the different UI options in the SDK.

        <Expandable defaultOpen title="Properties">
          <ResponseField name="isInitScreenEnabled" type="Boolean" post={["Default: true"]}>
            Set to `false` to hide the loading screen before the Universal Checkout or the Vault Manager.
          </ResponseField>

          <ResponseField name="isSuccessScreenEnabled" type="Boolean" post={['Default: true']}>
            Set to `false` to hide the screen after a successful payment, or tokenization on the vault flow.
          </ResponseField>

          <ResponseField name="isErrorScreenEnabled" type="Boolean" post={["Default: true"]}>
            Set to `false` to hide the error screen when an error occurs.
          </ResponseField>

          <ResponseField name="dismissalMechanism" type="DismissalMechanism" post={["Default: GESTURES"]}>
            Set the mechanism for dismissing Universal Checkout. Options are:

            * `GESTURES`: The dialog can be dismissed by tapping outside or by swiping down.
            * `CLOSE_BUTTON`: A close button is provided, allowing users to dismiss the dialog manually.
          </ResponseField>

          <ResponseField name="theme" type="PrimerTheme">
            Set a custom theme for Primer SDK.
          </ResponseField>

          <ResponseField name="cardFormUIOptions" type="PrimerCardFormUIOptions?">
            Configure the card form UI options.

            <Expandable title="Properties" defaultOpen>
              <ResponseField name="payButtonAddNewCard" type="Boolean">
                When set to true, the Drop-In's card form pay button will show "Add new card"; otherwise
                it will show "Pay \$x.xx".
              </ResponseField>
            </Expandable>
          </ResponseField>
        </Expandable>
      </ResponseField>

      <ResponseField name="debugOptions" type="PrimerDebugOptions">
        Sets the different debug options in the SDK.

        <Expandable defaultOpen title="Properties">
          <ResponseField name="is3DSSanityCheckEnabled" type="Boolean" post={["Default: false"]}>
            Sets whether the security warnings returned when performing 3DS are ignored.
          </ResponseField>
        </Expandable>
      </ResponseField>

      <ResponseField name="clientSessionCachingEnabled" type="Boolean" post={["Default: false"]}>
        <Warning>
          Before enabling the flag to true it's recommended to reach out to Primer
          support for additional verification since there are edge cases where it's not
          advised.
        </Warning>

        Indicates whether client session caching is enabled.

        When set to `true`, responses from the server will be cached on the client side, allowing for faster subsequent
        access to the same data within the cache duration. When set to `false`, every request to the server will be
        processed without utilizing any client-side cache, ensuring that the client always receives the most up-to-date data.
      </ResponseField>

      <ResponseField name="apiVersion" type="PrimerApiVersion" post={["Default: PrimerApiVersion.V2_4"]}>
        Indicates the API version to use when interacting with the Primer backend. Options are:

        * `PrimerApiVersion.V2_4` - will use ApiVersion 2.4
        * `PrimerApiVersion.LATEST` - will use the latest available ApiVersion version

        Note that this is optional, if a value is not supplied, ApiVersion `V2.4` will be used by default.
      </ResponseField>
    </Expandable>
  </ResponseField>

  <ResponseField name="checkoutListener" type="PrimerHeadlessUniversalCheckoutListener">
    Set Primer SDK's [Headless Checkout Listener](/sdk/android/v2.x.x/primer-headless-checkout/listeners/PrimerHeadlessUniversalCheckoutListener) and implement the required methods.
  </ResponseField>

  <ResponseField name="uiListener" type="PrimerHeadlessUniversalCheckoutUiListener">
    Set Primer SDK's [Headless Checkout Ui Listener](/sdk/android/v2.x.x/primer-headless-checkout/listeners/PrimerHeadlessUniversalCheckoutUiListener).
  </ResponseField>
</Expandable>