feat: add posthog adapter example (#1139)

### Description

Add an example for using the `@flags-sdk/posthog` adapter.

### Type of Change

- [ ] New Example
- [ ] Example updates (Bug fixes, new features, etc.)
- [ ] Other (changes to the codebase, but not to examples)

### New Example Checklist

- [ ] 🛫 `npm run new-example` was used to create the example
- [x] 📚 The template wasn't used but I carefuly read the [Adding a new
example](https://github.com/vercel/examples#adding-a-new-example) steps
and implemented them in the example
- [x] 📱 Is it responsive? Are mobile and tablets considered?

Author: joshsny

flags-sdk/posthog/.env.example

@@ -0,0 +1,9 @@
+# For @flags-sdk/posthog
+FLAGS_SECRET=""
+EDGE_CONFIG=""
+NEXT_PUBLIC_POSTHOG_KEY=""
+NEXT_PUBLIC_POSTHOG_HOST="https://us.i.posthog.com"
+
+# Optional: For Flags Explorer
+POSTHOG_PERSONAL_API_KEY=""
+POSTHOG_PROJECT_ID=""

flags-sdk/posthog/.eslintrc.json

@@ -0,0 +1,11 @@
+{
+  "root": true,
+  "extends": "next/core-web-vitals",
+  "rules": {
+    "@typescript-eslint/require-await": "off",
+    "@typescript-eslint/no-misused-promises": "off",
+    "import/order": "off",
+    "camelcase": "off",
+    "no-console": "off"
+  }
+}

flags-sdk/posthog/.gitignore

@@ -0,0 +1,37 @@
+# See https://help.github.com/articles/ignoring-files/ for more about ignoring files.
+
+# dependencies
+/node_modules
+/.pnp
+.pnp.js
+.yarn/install-state.gz
+
+# testing
+/coverage
+
+# next.js
+/.next/
+/out/
+
+# production
+/build
+
+# misc
+.DS_Store
+*.pem
+
+# debug
+npm-debug.log*
+yarn-debug.log*
+yarn-error.log*
+
+# local env files
+.env*.local
+
+# vercel
+.vercel
+
+# typescript
+*.tsbuildinfo
+next-env.d.ts
+.env

flags-sdk/posthog/README.md

@@ -0,0 +1,49 @@
+# PostHog Flags SDK Example
+
+This example uses [PostHog](https://posthog.com) for feature flags with the [Flags SDK](https://flags-sdk.dev) along with the `@flags-sdk/posthog` [PostHog adapter](https://flags-sdk.dev/docs/api-reference/adapters/posthog) and the [Flags Explorer](https://vercel.com/docs/workflow-collaboration/feature-flags/using-vercel-toolbar).
+
+## Demo
+
+[https://flags-sdk-posthog.vercel.app/](https://flags-sdk-posthog.vercel.app/)
+
+## How it works
+
+This demo uses two feature flags in PostHog to control the visibility of two banners on the page.
+Both gates are configured to show/hide each banner 50% of the time.
+
+If you deploy your own and configure the feature flags in PostHog, you can also use the [Flags Explorer](https://vercel.com/docs/workflow-collaboration/feature-flags/using-vercel-toolbar) to enabled/disabled the features.
+
+## Deploy this template
+
+[![Deploy with Vercel](https://vercel.com/button)](https://vercel.com/new/clone?repository-url=https%3A%2F%2Fgithub.com%2Fvercel%2Fexamples%2Ftree%2Fmain%2Fflags-sdk/posthog&env=FLAGS_SECRET&envDescription=The+FLAGS_SECRET+will+be+used+by+the+Flags+Explorer+to+securely+overwrite+feature+flags.+Must+be+32+random+bytes%2C+base64-encoded.+Use+the+generated+value+or+set+your+own.&envLink=https%3A%2F%2Fvercel.com%2Fdocs%2Fworkflow-collaboration%2Ffeature-flags%2Fsupporting-feature-flags%23flags_secret-environment-variable&project-name=posthog-flags-sdk-example&repository-name=posthog-flags-sdk-example)
+
+### Step 1: Link the project
+
+In order to use the Flags Explorer, you need to link the project on your local machine.
+
+```bash
+vercel link
+```
+
+Select the project from the list you just deployed.
+
+### Step 2: Pull all environment variables
+
+This allows the Flags SDK and the Flags Explorer to work correctly, by getting additional metadata.
+
+```bash
+vercel env pull
+```
+
+### Step 3: Create Feature Flags
+
+Head over to [PostHog](posthog.com) and create the feature flags required by this template.
+
+Feature Flags:
+
+- `Summer Sale` with the key `summer_sale`
+- `Free Shipping` with the key `free_delivery`
+
+You can also find the feature flag keys in the `flags.ts` file.
+
+Set both feature flags to rollout to 50% of users.

flags-sdk/posthog/app/.well-known/vercel/flags/route.ts

@@ -0,0 +1,23 @@
+import { getProviderData, createFlagsDiscoveryEndpoint } from 'flags/next'
+import { getProviderData as getPostHogProviderData } from '@flags-sdk/posthog'
+import { mergeProviderData } from 'flags'
+import * as flags from '../../../../flags'
+
+export const dynamic = 'force-dynamic' // defaults to auto
+
+export const GET = createFlagsDiscoveryEndpoint(
+  async () => {
+    return mergeProviderData([
+      // Data declared from Flags in Code
+      getProviderData(flags),
+      // metadata from PostHog API using the default posthog adapter
+      getPostHogProviderData({
+        personalApiKey: process.env.POSTHOG_PERSONAL_API_KEY,
+        projectId: process.env.POSTHOG_PROJECT_ID,
+      }),
+    ])
+  },
+  {
+    secret: process.env.FLAGS_SECRET,
+  }
+)

flags-sdk/posthog/app/[code]/add-to-cart.tsx

@@ -0,0 +1,24 @@
+'use client'
+
+import { useState } from 'react'
+import { useRouter } from 'next/navigation'
+import { addToCart } from '@/lib/actions'
+import { useProductDetailPageContext } from '@/components/utils/product-detail-page-context'
+import { AddToCartButton } from '@/components/product-detail-page/add-to-cart-button'
+
+export function AddToCart() {
+  const router = useRouter()
+  const { color, size } = useProductDetailPageContext()
+  const [isLoading, setIsLoading] = useState(false)
+
+  return (
+    <AddToCartButton
+      isLoading={isLoading}
+      onClick={async () => {
+        setIsLoading(true)
+        await addToCart({ id: 'shirt', color, size, quantity: 1 })
+        router.push('/cart')
+      }}
+    />
+  )
+}

flags-sdk/posthog/app/[code]/cart/order-summary.tsx

@@ -0,0 +1,18 @@
+import { OrderSummarySection } from '@/components/shopping-cart/order-summary-section'
+import { ProceedToCheckout } from './proceed-to-checkout'
+
+export async function OrderSummary({
+  showSummerBanner,
+  freeDelivery,
+}: {
+  showSummerBanner: boolean
+  freeDelivery: boolean
+}) {
+  return (
+    <OrderSummarySection
+      showSummerBanner={showSummerBanner}
+      freeDelivery={freeDelivery}
+      proceedToCheckout={<ProceedToCheckout color={'blue'} />}
+    />
+  )
+}

flags-sdk/posthog/app/[code]/cart/page.tsx

@@ -0,0 +1,33 @@
+import { OrderSummary } from '@/app/[code]/cart/order-summary'
+import { Main } from '@/components/main'
+import { ShoppingCart } from '@/components/shopping-cart/shopping-cart'
+import {
+  productFlags,
+  showFreeDeliveryBannerFlag,
+  showSummerBannerFlag,
+} from '@/flags'
+
+export default async function CartPage({
+  params,
+}: {
+  params: Promise<{ code: string }>
+}) {
+  const { code } = await params
+  const showSummerBanner = await showSummerBannerFlag(code, productFlags)
+  const freeDeliveryBanner = await showFreeDeliveryBannerFlag(
+    code,
+    productFlags
+  )
+
+  return (
+    <Main>
+      <div className="lg:grid lg:grid-cols-12 lg:items-start lg:gap-x-12 xl:gap-x-16">
+        <ShoppingCart />
+        <OrderSummary
+          showSummerBanner={showSummerBanner}
+          freeDelivery={freeDeliveryBanner}
+        />
+      </div>
+    </Main>
+  )
+}

flags-sdk/posthog/app/[code]/cart/proceed-to-checkout.tsx

@@ -0,0 +1,24 @@
+'use client'
+
+import { ProceedToCheckoutButton } from '@/components/shopping-cart/proceed-to-checkout-button'
+
+import { toast } from 'sonner'
+
+export function ProceedToCheckout({ color }: { color: string }) {
+  return (
+    <>
+      <ProceedToCheckoutButton
+        color={color}
+        onClick={() => {
+          // Auto capture will track the event
+          toast('End reached', {
+            className: 'my-classname',
+            description:
+              'The checkout flow is not implemented in this template.',
+            duration: 5000,
+          })
+        }}
+      />
+    </>
+  )
+}

flags-sdk/posthog/app/[code]/layout.tsx

@@ -0,0 +1,45 @@
+import { deserialize, generatePermutations } from 'flags/next'
+import { FlagValues } from 'flags/react'
+import { productFlags, showFreeDeliveryBannerFlag } from '@/flags'
+import { FreeDelivery } from '@/app/free-delivery'
+import { DevTools } from '@/components/dev-tools'
+import { Footer } from '@/components/footer'
+import { Navigation } from '@/components/navigation'
+
+export async function generateStaticParams() {
+  // Returning an empty array here is important as it enables ISR, so
+  // the various combinations stay cached after they first time they were rendered.
+  //
+  // return [];
+
+  // Instead of returning an empty array you could also call generatePermutations
+  // to generate the permutations upfront.
+  const codes = await generatePermutations(productFlags)
+  return codes.map((code) => ({ code }))
+}
+
+export default async function Layout(props: {
+  children: React.ReactNode
+  params: Promise<{
+    code: string
+  }>
+}) {
+  const params = await props.params
+  const values = await deserialize(productFlags, params.code)
+
+  const showFreeDeliveryBanner = await showFreeDeliveryBannerFlag(
+    params.code,
+    productFlags
+  )
+
+  return (
+    <div className="bg-white">
+      <FreeDelivery show={showFreeDeliveryBanner} />
+      <Navigation />
+      {props.children}
+      <FlagValues values={values} />
+      <Footer />
+      <DevTools />
+    </div>
+  )
+}