Extensions API

The Swft Extensions API allows any WordPress plugin to pass structured data through the checkout session and interact with the checkout frontend.

How it works

Data flows in one direction: WP plugin → Swft API → Cloudflare KV → checkout frontend → WooCommerce order meta.

1. A WP plugin hooks into swft_session_extensions and adds data to the $extensions array
2. When a session is created, the plugin calls POST api.swft.co.uk/sessions with the extensions included in the payload
3. The API stores the extensions in the session record and Cloudflare KV
4. The checkout frontend reads extensions from window.__SWFT_SESSION__.extensions
5. After payment, extensions are written to the WooCommerce order as _swft_ext_{key} meta fields

Available hooks

PHP filters (WordPress side)

For modifying session data before it reaches the Swft API:

Filter	Purpose
swft_session_extensions	Primary extension hook — add arbitrary data to the session
swft_session_line_item	Modify or exclude a single line item
swft_session_line_items	Modify all line items after the loop
swft_session_shipping_methods	Modify available shipping methods
swft_session_customer	Modify pre-filled customer data
swft_session_totals	Modify calculated totals
swft_session_payload	Last-resort: modify the complete payload

For modifying Swft Cart data:

Filter	Purpose
swftcart_cart_data	Modify the full cart object sent to the cart frontend
swftcart_cart_item	Modify a single cart item
swftcart_modules	Override module enable/disable state
swftcart_i18n	Override UI strings
swftcart_announcements	Set announcement bar content
swftcart_theme_vars	Override CSS custom property values
swftcart_delivery_data	Set delivery countdown data
swftcart_upsell_ids	Set product IDs for the upsell module

See PHP Filters for full documentation.

JS events (checkout frontend)

Events fired by both Swft Cart and Swft Checkout as CustomEvent on document:

Event	Description
swftcart:opened	Cart drawer opened
swftcart:closed	Cart drawer closed
swftcart:fab-clicked	FAB icon clicked
swftcart:cart-updated	Cart contents changed
swftcart:checkout	Checkout button clicked (cancelable)
swftcart:item-removed	Item removed from cart
swftcart:item-saved-for-later	Item moved to saved-for-later
swftcart:upsell-added	Upsell product added to cart
swftcart:coupon-applied	Coupon code applied
swftcart:coupon-removed	Coupon code removed
swftcart:cart-shared	Share cart URL copied
swftcart:upsell-shown	Upsell strip rendered
swftcheckout:ready	Checkout app initialised, window.SwftCheckout available
swftcheckout:details-complete	Customer submitted the details step
swftcheckout:payment-complete	Payment succeeded
swftcheckout:upsell-shown	Post-payment upsell rendered

See JS Events for detail shapes and examples.

JavaScript API (checkout frontend)

window.SwftCheckout is available after swftcheckout:ready fires:

• .addOrderRow(row) — inject a custom row into the order summary
• .addCustomField(field) — inject a custom form field
• .getExtension(key) — read data from the session extensions object

See SwftCheckout API for the full reference.

Tutorial

For a complete walkthrough of building a module using all these hooks together, see Building a Module.