Preview Mode

In the Pages documentation, we talked about how to pre-render a page at build time (Static Generation) using getStaticProps and getStaticPaths.

Static Generation is useful when your pages fetch data from a headless CMS. However, it’s not ideal when you’re writing a draft on your headless CMS and want to preview the draft immediately on your page. You’d want Blitz to render these pages at request time instead of build time and fetch the draft content instead of the published content. You’d want Blitz to bypass Static Generation only for this specific case.

Blitz has the feature called Preview Mode which solves this problem. Here’s an instruction on how to use it.

Step 1. Create and access a preview API route

Take a look at the API Routes documentation first if you’re not familiar with Blitz API Routes.

First, create a preview API route. It can have any name - e.g. app/api/preview.js (or .ts if using TypeScript).

In this API route, you need to call setPreviewData on the response object. The argument for setPreviewData should be an object, and this can be used by getStaticProps (more on this later). For now, we’ll use {}.

const handler = (req, res) => {
  // ...
  res.setPreviewData({})
  // ...
}
export default handler

res.setPreviewData sets some cookies on the browser which turns on the preview mode. Any requests to Blitz containing these cookies will be considered as the preview mode, and the behavior for statically generated pages will change (more on this later).

You can test this manually by creating an API route like below and accessing it from your browser manually:

// A simple example for testing it manually from your browser.
// If this is located at app/api/preview.js, then
// open /api/preview from your browser.
const handler = (req, res) => {
  res.setPreviewData({})
  res.end("Preview mode enabled")
}
export default handler

If you use your browser’s developer tools, you’ll notice that the __prerender_bypass and __next_preview_data cookies will be set on this request.

Securely accessing it from your Headless CMS

In practice, you’d want to call this API route securely from your headless CMS. The specific steps will vary depending on which headless CMS you’re using, but here are some common steps you could take.

These steps assume that the headless CMS you’re using supports setting custom preview URLs. If it doesn’t, you can still use this method to secure your preview URLs, but you’ll need to construct and access the preview URL manually.

First, you should create a secret token string using a token generator of your choice. This secret will only be known by your Blitz app and your headless CMS. This secret prevents people who don’t have access to your CMS from accessing preview URLs.

Second, if your headless CMS supports setting custom preview URLs, specify the following as the preview URL. (This assumes that your preview API route is located at app/api/preview.js.)

https://<your-site>/api/preview?secret=<token>&slug=<path>

<your-site> should be your deployment domain.
<token> should be replaced with the secret token you generated.
<path> should be the path for the page that you want to preview. If you want to preview /posts/foo, then you should use &slug=/posts/foo.

Your headless CMS might allow you to include a variable in the preview URL so that <path> can be set dynamically based on the CMS’s data like so: &slug=/posts/{entry.fields.slug}

Finally, in the preview API route:

Check that the secret matches and that the slug parameter exists (if not, the request should fail).
Call res.setPreviewData.
Then redirect the browser to the path specified by slug. (The following example uses a 307 redirect).

export default async (req, res) => {
  // Check the secret and next parameters
  // This secret should only be known to this API route and the CMS
  if (req.query.secret !== "MY_SECRET_TOKEN" || !req.query.slug) {
    return res.status(401).json({ message: "Invalid token" })
  }

  // Fetch the headless CMS to check if the provided `slug` exists
  // getPostBySlug would implement the required fetching logic to the headless CMS
  const post = await getPostBySlug(req.query.slug)

  // If the slug doesn't exist prevent preview mode from being enabled
  if (!post) {
    return res.status(401).json({ message: "Invalid slug" })
  }

  // Enable Preview Mode by setting the cookies
  res.setPreviewData({})

  // Redirect to the path from the fetched post
  // We don't redirect to req.query.slug as that might lead to open redirect vulnerabilities
  res.writeHead(307, { Location: post.slug })
  res.end()
}

If it succeeds, then the browser will be redirected to the path you want to preview with the preview mode cookies being set.

Step 2. Update `getStaticProps`

The next step is to update getStaticProps to support the preview mode.

If you request a page which has getStaticProps with the preview mode cookies set (via res.setPreviewData), then getStaticProps will be called at request time (instead of at build time).

Furthermore, it will be called with a context object where:

context.preview will be true.
context.previewData will be the same as the argument used for setPreviewData.

export async function getStaticProps(context) {
  // If you request this page with the preview mode cookies set:
  //
  // - context.preview will be true
  // - context.previewData will be the same as
  //   the argument used for `setPreviewData`.
}

We used res.setPreviewData({}) in the preview API route, so context.previewData will be {}. You can use this to pass session information from the preview API route to getStaticProps if necessary.

If you’re also using getStaticPaths, then context.params will also be available.

Fetch preview data

You can update getStaticProps to fetch different data based on context.preview and/or context.previewData.

For example, your headless CMS might have a different API endpoint for draft posts. If so, you can use context.preview to modify the API endpoint URL like below:

export async function getStaticProps(context) {
  // If context.preview is true, append "/preview" to the API endpoint
  // to request draft data instead of published data. This will vary
  // based on which headless CMS you're using.
  const res = await fetch(
    `https://.../${context.preview ? "preview" : ""}`
  )
  // ...
}

That’s it! If you access the preview API route (with secret and slug) from your headless CMS or manually, you should now be able to see the preview content. And if you update your draft without publishing, you should be able to preview the draft.

# Set this as the preview URL on your headless CMS or access manually,
# and you should be able to see the preview.
https://<your-site>/api/preview?secret=<token>&slug=<path>

More Details

Clear the preview mode cookies

By default, no expiration date is set for the preview mode cookies, so the preview mode ends when the browser is closed.

To clear the preview cookies manually, you can create an API route which calls clearPreviewData and then access this API route.

export default (req, res) => {
  // Clears the preview mode cookies.
  // This function accepts no arguments.
  res.clearPreviewData()
  // ...
}

Specify the preview mode duration

setPreviewData takes an optional second parameter which should be an options object. It accepts the following keys:

maxAge: Specifies the number (in seconds) for the preview session to last for.

setPreviewData(data, {
  maxAge: 60 * 60, // The preview mode cookies expire in 1 hour
})

`previewData` size limits

You can pass an object to setPreviewData and have it be available in getStaticProps. However, because the data will be stored in a cookie, there’s a size limitation. Currently, preview data is limited to 2KB.

Works with `getServerSideProps`

The preview mode works on getServerSideProps as well. It will also be available on the context object containing preview and previewData.

Unique per `blitz build`

The bypass cookie value and private key for encrypting the previewData changes when a blitz build is ran, this ensures that the bypass cookie can’t be guessed.

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