From 0ebf071826c6c33a804bcc8a99f970e856cb12f2 Mon Sep 17 00:00:00 2001
From: Lee Robinson <lrobinson2011@gmail.com>
Date: Thu, 25 Jul 2024 13:56:53 -0500
Subject: [PATCH] Optimistic cart (#1364)

---
 components/cart/actions.ts                    |  6 ++
 components/cart/edit-item-quantity-button.tsx | 31 +++---
 components/cart/modal.tsx                     | 98 ++++++++++++++++---
 3 files changed, 109 insertions(+), 26 deletions(-)

diff --git a/components/cart/actions.ts b/components/cart/actions.ts
index fa2c34d37d..ea6d82dd79 100644
--- a/components/cart/actions.ts
+++ b/components/cart/actions.ts
@@ -4,6 +4,7 @@ import { TAGS } from 'lib/constants';
 import { addToCart, createCart, getCart, removeFromCart, updateCart } from 'lib/shopify';
 import { revalidateTag } from 'next/cache';
 import { cookies } from 'next/headers';
+import { redirect } from 'next/navigation';
 
 export async function addItem(prevState: any, selectedVariantId: string | undefined) {
   let cartId = cookies().get('cartId')?.value;
@@ -81,3 +82,8 @@ export async function updateItemQuantity(
     return 'Error updating item quantity';
   }
 }
+
+export async function redirectToCheckout(formData: FormData) {
+  const url = formData.get('url') as string;
+  redirect(url);
+}
diff --git a/components/cart/edit-item-quantity-button.tsx b/components/cart/edit-item-quantity-button.tsx
index b743ab7042..d3c753dd7a 100644
--- a/components/cart/edit-item-quantity-button.tsx
+++ b/components/cart/edit-item-quantity-button.tsx
@@ -3,32 +3,22 @@
 import { MinusIcon, PlusIcon } from '@heroicons/react/24/outline';
 import clsx from 'clsx';
 import { updateItemQuantity } from 'components/cart/actions';
-import LoadingDots from 'components/loading-dots';
 import type { CartItem } from 'lib/shopify/types';
-import { useFormState, useFormStatus } from 'react-dom';
+import { useFormState } from 'react-dom';
 
 function SubmitButton({ type }: { type: 'plus' | 'minus' }) {
-  const { pending } = useFormStatus();
-
   return (
     <button
       type="submit"
-      onClick={(e: React.FormEvent<HTMLButtonElement>) => {
-        if (pending) e.preventDefault();
-      }}
       aria-label={type === 'plus' ? 'Increase item quantity' : 'Reduce item quantity'}
-      aria-disabled={pending}
       className={clsx(
         'ease flex h-full min-w-[36px] max-w-[36px] flex-none items-center justify-center rounded-full px-2 transition-all duration-200 hover:border-neutral-800 hover:opacity-80',
         {
-          'cursor-not-allowed': pending,
           'ml-auto': type === 'minus'
         }
       )}
     >
-      {pending ? (
-        <LoadingDots className="bg-black dark:bg-white" />
-      ) : type === 'plus' ? (
+      {type === 'plus' ? (
         <PlusIcon className="h-4 w-4 dark:text-neutral-500" />
       ) : (
         <MinusIcon className="h-4 w-4 dark:text-neutral-500" />
@@ -37,7 +27,15 @@ function SubmitButton({ type }: { type: 'plus' | 'minus' }) {
   );
 }
 
-export function EditItemQuantityButton({ item, type }: { item: CartItem; type: 'plus' | 'minus' }) {
+export function EditItemQuantityButton({
+  item,
+  type,
+  optimisticUpdate
+}: {
+  item: CartItem;
+  type: 'plus' | 'minus';
+  optimisticUpdate: any;
+}) {
   const [message, formAction] = useFormState(updateItemQuantity, null);
   const payload = {
     lineId: item.id,
@@ -47,7 +45,12 @@ export function EditItemQuantityButton({ item, type }: { item: CartItem; type: '
   const actionWithVariant = formAction.bind(null, payload);
 
   return (
-    <form action={actionWithVariant}>
+    <form
+      action={async () => {
+        optimisticUpdate({ itemId: payload.lineId, newQuantity: payload.quantity });
+        await actionWithVariant();
+      }}
+    >
       <SubmitButton type={type} />
       <p aria-live="polite" className="sr-only" role="status">
         {message}
diff --git a/components/cart/modal.tsx b/components/cart/modal.tsx
index a308189404..a9ad109a1c 100644
--- a/components/cart/modal.tsx
+++ b/components/cart/modal.tsx
@@ -2,13 +2,16 @@
 
 import { Dialog, Transition } from '@headlessui/react';
 import { ShoppingCartIcon } from '@heroicons/react/24/outline';
+import LoadingDots from 'components/loading-dots';
 import Price from 'components/price';
 import { DEFAULT_OPTION } from 'lib/constants';
-import type { Cart } from 'lib/shopify/types';
+import type { Cart, CartItem } from 'lib/shopify/types';
 import { createUrl } from 'lib/utils';
 import Image from 'next/image';
 import Link from 'next/link';
-import { Fragment, useEffect, useRef, useState } from 'react';
+import { Fragment, useEffect, useOptimistic, useRef, useState } from 'react';
+import { useFormStatus } from 'react-dom';
+import { redirectToCheckout } from './actions';
 import CloseCart from './close-cart';
 import { DeleteItemButton } from './delete-item-button';
 import { EditItemQuantityButton } from './edit-item-quantity-button';
@@ -18,8 +21,58 @@ type MerchandiseSearchParams = {
   [key: string]: string;
 };
 
-export default function CartModal({ cart }: { cart: Cart | undefined }) {
+type NewState = {
+  itemId: string;
+  newQuantity: number;
+};
+
+function reducer(state: Cart | undefined, newState: NewState) {
+  if (!state) {
+    return state;
+  }
+
+  const updatedLines = state.lines.map((item: CartItem) => {
+    if (item.id === newState.itemId) {
+      const singleItemAmount = Number(item.cost.totalAmount.amount) / item.quantity;
+      const newTotalAmount = Number(item.cost.totalAmount.amount) + singleItemAmount;
+      return {
+        ...item,
+        quantity: newState.newQuantity,
+        cost: {
+          ...item.cost,
+          totalAmount: {
+            ...item.cost.totalAmount,
+            amount: newTotalAmount.toString()
+          }
+        }
+      };
+    }
+    return item;
+  });
+
+  const newTotalQuantity = updatedLines.reduce((sum, item) => sum + item.quantity, 0);
+  const newTotalAmount = updatedLines.reduce(
+    (sum, item) => sum + Number(item.cost.totalAmount.amount),
+    0
+  );
+
+  return {
+    ...state,
+    lines: updatedLines,
+    totalQuantity: newTotalQuantity,
+    cost: {
+      ...state.cost,
+      totalAmount: {
+        ...state.cost.totalAmount,
+        amount: newTotalAmount.toString()
+      }
+    }
+  };
+}
+
+export default function CartModal({ cart: initialCart }: { cart: Cart | undefined }) {
   const [isOpen, setIsOpen] = useState(false);
+  const [cart, updateCartItem] = useOptimistic(initialCart, reducer);
   const quantityRef = useRef(cart?.totalQuantity);
   const openCart = () => setIsOpen(true);
   const closeCart = () => setIsOpen(false);
@@ -67,7 +120,6 @@ export default function CartModal({ cart }: { cart: Cart | undefined }) {
             <Dialog.Panel className="fixed bottom-0 right-0 top-0 flex h-full w-full flex-col border-l border-neutral-200 bg-white/80 p-6 text-black backdrop-blur-xl md:w-[390px] dark:border-neutral-700 dark:bg-black/80 dark:text-white">
               <div className="flex items-center justify-between">
                 <p className="text-lg font-semibold">My Cart</p>
-
                 <button aria-label="Close cart" onClick={closeCart}>
                   <CloseCart />
                 </button>
@@ -140,11 +192,19 @@ export default function CartModal({ cart }: { cart: Cart | undefined }) {
                                 currencyCode={item.cost.totalAmount.currencyCode}
                               />
                               <div className="ml-auto flex h-9 flex-row items-center rounded-full border border-neutral-200 dark:border-neutral-700">
-                                <EditItemQuantityButton item={item} type="minus" />
+                                <EditItemQuantityButton
+                                  item={item}
+                                  type="minus"
+                                  optimisticUpdate={updateCartItem}
+                                />
                                 <p className="w-6 text-center">
                                   <span className="w-full text-sm">{item.quantity}</span>
                                 </p>
-                                <EditItemQuantityButton item={item} type="plus" />
+                                <EditItemQuantityButton
+                                  item={item}
+                                  type="plus"
+                                  optimisticUpdate={updateCartItem}
+                                />
                               </div>
                             </div>
                           </div>
@@ -174,12 +234,9 @@ export default function CartModal({ cart }: { cart: Cart | undefined }) {
                       />
                     </div>
                   </div>
-                  <a
-                    href={cart.checkoutUrl}
-                    className="block w-full rounded-full bg-blue-600 p-3 text-center text-sm font-medium text-white opacity-90 hover:opacity-100"
-                  >
-                    Proceed to Checkout
-                  </a>
+                  <form action={redirectToCheckout}>
+                    <CheckoutButton cart={cart} />
+                  </form>
                 </div>
               )}
             </Dialog.Panel>
@@ -189,3 +246,20 @@ export default function CartModal({ cart }: { cart: Cart | undefined }) {
     </>
   );
 }
+
+function CheckoutButton({ cart }: { cart: Cart }) {
+  const { pending } = useFormStatus();
+
+  return (
+    <>
+      <input type="hidden" name="url" value={cart.checkoutUrl} />
+      <button
+        className="block w-full rounded-full bg-blue-600 p-3 text-center text-sm font-medium text-white opacity-90 hover:opacity-100"
+        type="submit"
+        disabled={pending}
+      >
+        {pending ? <LoadingDots className="bg-white" /> : 'Proceed to Checkout'}
+      </button>
+    </>
+  );
+}