Add CartEventContext and dispatch events when pressing proceed to checkout button

assets/js/base/context/providers/cart-checkout/cart-events/event-emit.ts

@@ -0,0 +1,51 @@
+/**
+ * External dependencies
+ */
+import { useMemo } from '@wordpress/element';
+
+/**
+ * Internal dependencies
+ */
+import {
+	emitterCallback,
+	reducer,
+	emitEvent,
+	emitEventWithAbort,
+	ActionType,
+} from '../../../event-emit';
+
+// These events are emitted when the Cart status is BEFORE_PROCESSING and AFTER_PROCESSING
+// to enable third parties to hook into the cart process
+const EVENTS = {
+	PROCEED_TO_CHECKOUT: 'cart_proceed_to_checkout',
+};
+
+type EventEmittersType = Record< string, ReturnType< typeof emitterCallback > >;
+
+/**
+ * Receives a reducer dispatcher and returns an object with the
+ * various event emitters for the payment processing events.
+ *
+ * Calling the event registration function with the callback will register it
+ * for the event emitter and will return a dispatcher for removing the
+ * registered callback (useful for implementation in `useEffect`).
+ *
+ * @param {Function} observerDispatch The emitter reducer dispatcher.
+ * @return {Object} An object with the various payment event emitter registration functions
+ */
+const useEventEmitters = (
+	observerDispatch: React.Dispatch< ActionType >
+): EventEmittersType => {
+	const eventEmitters = useMemo(
+		() => ( {
+			onProceedToCheckout: emitterCallback(
+				EVENTS.PROCEED_TO_CHECKOUT,
+				observerDispatch
+			),
+		} ),
+		[ observerDispatch ]
+	);
+	return eventEmitters;
+};
+
+export { EVENTS, useEventEmitters, reducer, emitEvent, emitEventWithAbort };

assets/js/base/context/providers/cart-checkout/cart-events/index.tsx

@@ -0,0 +1,78 @@
+/**
+ * External dependencies
+ */
+
+import {
+	createContext,
+	useContext,
+	useReducer,
+	useRef,
+	useEffect,
+} from '@wordpress/element';
+
+/**
+ * Internal dependencies
+ */
+import {
+	useEventEmitters,
+	reducer as emitReducer,
+	emitEventWithAbort,
+	EVENTS,
+} from './event-emit';
+import type { emitterCallback } from '../../../event-emit';
+
+type CartEventsContextType = {
+	// Used to register a callback that will fire when the cart has been processed and has an error.
+	onProceedToCheckout: ReturnType< typeof emitterCallback >;
+	// Used to register a callback that will fire when the cart has been processed and has an error.
+	dispatchOnProceedToCheckout: () => Promise< unknown[] >;
+};
+
+const CartEventsContext = createContext< CartEventsContextType >( {
+	onProceedToCheckout: () => () => void null,
+	dispatchOnProceedToCheckout: () => new Promise( () => void null ),
+} );
+
+export const useCartEventsContext = () => {
+	return useContext( CartEventsContext );
+};
+
+/**
+ * Checkout Events provider
+ * Emit Checkout events and provide access to Checkout event handlers
+ *
+ * @param {Object} props          Incoming props for the provider.
+ * @param {Object} props.children The children being wrapped.
+ */
+export const CartEventsProvider = ( {
+	children,
+}: {
+	children: React.ReactNode;
+} ): JSX.Element => {
+	const [ observers, observerDispatch ] = useReducer( emitReducer, {} );
+	const currentObservers = useRef( observers );
+	const { onProceedToCheckout } = useEventEmitters( observerDispatch );
+
+	// set observers on ref so it's always current.
+	useEffect( () => {
+		currentObservers.current = observers;
+	}, [ observers ] );
+
+	const dispatchOnProceedToCheckout = async () => {
+		return await emitEventWithAbort(
+			currentObservers.current,
+			EVENTS.PROCEED_TO_CHECKOUT,
+			null
+		);
+	};
+
+	const cartEvents = {
+		onProceedToCheckout,
+		dispatchOnProceedToCheckout,
+	};
+	return (
+		<CartEventsContext.Provider value={ cartEvents }>
+			{ children }
+		</CartEventsContext.Provider>
+	);
+};

assets/js/base/context/providers/cart-checkout/cart-events/test/index.tsx

@@ -0,0 +1,49 @@
+/**
+ * External dependencies
+ */
+import { useCartEventsContext } from '@woocommerce/base-context';
+import { useEffect } from '@wordpress/element';
+import { render, screen, waitFor } from '@testing-library/react';
+
+/**
+ * Internal dependencies
+ */
+import { CartEventsProvider } from '../index';
+import Block from '../../../../../../blocks/cart/inner-blocks/proceed-to-checkout-block/block';
+
+describe( 'CartEventsProvider', () => {
+	it( 'allows observers to unsubscribe', async () => {
+		const mockObserver = jest.fn().mockReturnValue( { type: 'error' } );
+		const MockObserverComponent = () => {
+			const { onProceedToCheckout } = useCartEventsContext();
+			useEffect( () => {
+				const unsubscribe = onProceedToCheckout( () => {
+					mockObserver();
+					unsubscribe();
+				} );
+			}, [ onProceedToCheckout ] );
+			return <div>Mock observer</div>;
+		};
+
+		render(
+			<CartEventsProvider>
+				<div>
+					<MockObserverComponent />
+					<Block checkoutPageId={ 0 } className="test-block" />
+				</div>
+			</CartEventsProvider>
+		);
+		expect( screen.getByText( 'Mock observer' ) ).toBeInTheDocument();
+		const button = screen.getByText( 'Proceed to Checkout' );
+
+		// Forcibly set the button URL to # to prevent JSDOM error: `["Error: Not implemented: navigation (except hash changes)`
+		button.parentElement?.removeAttribute( 'href' );
+
+		// Click twice. The observer should unsubscribe after the first click.
+		button.click();
+		button.click();
+		await waitFor( () => {
+			expect( mockObserver ).toHaveBeenCalledTimes( 1 );
+		} );
+	} );
+} );

assets/js/base/context/providers/cart-checkout/index.js

@@ -1,6 +1,7 @@
 export * from './payment-events';
 export * from './shipping';
 export * from './checkout-events';
+export * from './cart-events';
 export * from './cart';
 export * from './checkout-processor';
 export * from './checkout-provider';

assets/js/blocks/cart/block.js

@@ -5,11 +5,15 @@ import { __ } from '@wordpress/i18n';
 import { useStoreCart } from '@woocommerce/base-context/hooks';
 import { useEffect } from '@wordpress/element';
 import LoadingMask from '@woocommerce/base-components/loading-mask';
-import { CartProvider, noticeContexts } from '@woocommerce/base-context';
 import { CURRENT_USER_IS_ADMIN } from '@woocommerce/settings';
 import BlockErrorBoundary from '@woocommerce/base-components/block-error-boundary';
 import { translateJQueryEventToNative } from '@woocommerce/base-utils';
 import withScrollToTop from '@woocommerce/base-hocs/with-scroll-to-top';
+import {
+	CartEventsProvider,
+	CartProvider,
+	noticeContexts,
+} from '@woocommerce/base-context';
 import {
 	SlotFillProvider,
 	StoreNoticesContainer,
@@ -85,8 +89,10 @@ const Block = ( { attributes, children, scrollToTop } ) => (
 		<StoreNoticesContainer context={ noticeContexts.CART } />
 		<SlotFillProvider>
 			<CartProvider>
-				<Cart attributes={ attributes }>{ children }</Cart>
-				<ScrollOnError scrollToTop={ scrollToTop } />
+				<CartEventsProvider>
+					<Cart attributes={ attributes }>{ children }</Cart>
+					<ScrollOnError scrollToTop={ scrollToTop } />
+				</CartEventsProvider>
 			</CartProvider>
 		</SlotFillProvider>
 	</BlockErrorBoundary>

assets/js/blocks/cart/inner-blocks/proceed-to-checkout-block/block.tsx

@@ -10,6 +10,8 @@ import { getSetting } from '@woocommerce/settings';
 import { useSelect } from '@wordpress/data';
 import { CART_STORE_KEY, CHECKOUT_STORE_KEY } from '@woocommerce/block-data';
 import { applyCheckoutFilter } from '@woocommerce/blocks-checkout';
+import { isErrorResponse } from '@woocommerce/base-context';
+import { useCartEventsContext } from '@woocommerce/base-context/providers';
 
 /**
  * Internal dependencies
@@ -74,12 +76,22 @@ const Block = ( {
 		arg: { cart },
 	} );
 
+	const { dispatchOnProceedToCheckout } = useCartEventsContext();
+
 	const submitContainerContents = (
 		<Button
 			className="wc-block-cart__submit-button"
 			href={ filteredLink }
 			disabled={ isCalculating }
-			onClick={ () => setShowSpinner( true ) }
+			onClick={ ( e ) => {
+				dispatchOnProceedToCheckout().then( ( observerResponses ) => {
+					if ( observerResponses.some( isErrorResponse ) ) {
+						e.preventDefault();
+						return;
+					}
+					setShowSpinner( true );
+				} );
+			} }
 			showSpinner={ showSpinner }
 		>
 			{ label }

assets/js/blocks/cart/inner-blocks/proceed-to-checkout-block/test/block.tsx

@@ -1,13 +1,17 @@
 /**
  * External dependencies
  */
-import { render, screen } from '@testing-library/react';
+
+import { render, screen, waitFor } from '@testing-library/react';
 import { registerCheckoutFilters } from '@woocommerce/blocks-checkout';
+import { useCartEventsContext } from '@woocommerce/base-context';
+import { useEffect } from '@wordpress/element';
 
 /**
  * Internal dependencies
  */
 import Block from '../block';
+import { CartEventsProvider } from '../../../../../base/context/providers';
 
 describe( 'Proceed to checkout block', () => {
 	it( 'allows the text to be filtered', () => {
@@ -49,4 +53,37 @@ describe( 'Proceed to checkout block', () => {
 		//@todo When https://github.com/WordPress/gutenberg/issues/22850 is complete use that new matcher here for more specific error message assertion.
 		expect( console ).toHaveErrored();
 	} );
+	it( 'dispatches the onProceedToCheckout event when the button is clicked', async () => {
+		const mockObserver = jest.fn().mockReturnValue( { type: 'error' } );
+		const MockObserverComponent = () => {
+			const { onProceedToCheckout } = useCartEventsContext();
+			useEffect( () => {
+				return onProceedToCheckout( mockObserver );
+			}, [ onProceedToCheckout ] );
+			return <div>Mock observer</div>;
+		};
+
+		render(
+			<CartEventsProvider>
+				<div>
+					<MockObserverComponent />
+					<Block
+						buttonLabel={ 'Proceed to Checkout' }
+						checkoutPageId={ 0 }
+						className="test-block"
+					/>
+				</div>
+			</CartEventsProvider>
+		);
+		expect( screen.getByText( 'Mock observer' ) ).toBeInTheDocument();
+		const button = screen.getByText( 'Proceed to Checkout' );
+
+		// Forcibly set the button URL to # to prevent JSDOM error: `["Error: Not implemented: navigation (except hash changes)`
+		button.parentElement?.removeAttribute( 'href' );
+
+		button.click();
+		await waitFor( () => {
+			expect( mockObserver ).toHaveBeenCalled();
+		} );
+	} );
 } );

assets/js/blocks/mini-cart/mini-cart-contents/block.tsx

@@ -2,6 +2,7 @@
  * External dependencies
  */
 import { DrawerCloseButton } from '@woocommerce/base-components/drawer';
+import { CartEventsProvider } from '@woocommerce/base-context';
 
 /**
  * Internal dependencies
@@ -20,8 +21,10 @@ export const MiniCartContentsBlock = (
 
 	return (
 		<>
-			<DrawerCloseButton />
-			{ children }
+			<CartEventsProvider>
+				<DrawerCloseButton />
+				{ children }
+			</CartEventsProvider>
 		</>
 	);
 };

assets/js/blocks/mini-cart/mini-cart-contents/inner-blocks/mini-cart-checkout-button-block/block.tsx

@@ -5,6 +5,10 @@ import { CHECKOUT_URL } from '@woocommerce/block-settings';
 import Button from '@woocommerce/base-components/button';
 import classNames from 'classnames';
 import { useStyleProps } from '@woocommerce/base-hooks';
+import {
+	isErrorResponse,
+	useCartEventsContext,
+} from '@woocommerce/base-context';
 
 /**
  * Internal dependencies
@@ -24,6 +28,7 @@ const Block = ( {
 	style,
 }: MiniCartCheckoutButtonBlockProps ): JSX.Element | null => {
 	const styleProps = useStyleProps( { style } );
+	const { dispatchOnProceedToCheckout } = useCartEventsContext();
 
 	if ( ! CHECKOUT_URL ) {
 		return null;
@@ -39,6 +44,13 @@ const Block = ( {
 			variant={ getVariant( className, 'contained' ) }
 			style={ styleProps.style }
 			href={ CHECKOUT_URL }
+			onClick={ ( e ) => {
+				dispatchOnProceedToCheckout().then( ( observerResponses ) => {
+					if ( observerResponses.some( isErrorResponse ) ) {
+						e.preventDefault();
+					}
+				} );
+			} }
 		>
 			{ checkoutButtonLabel || defaultCheckoutButtonLabel }
 		</Button>