RFC: Remove special layouts from v2

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+- Start Date: 2018-04-06
+- RFC PR: (leave this empty)
+- Gatsby Issue: (leave this empty)
+
+# Summary
+
+Gatsby v1 includes a special "layout" component that can wrap page components.
+This RFC proposes removing it and encouraging people to use normal React
+component composition.
+
+[Previous discussion](https://github.com/gatsbyjs/gatsby/issues/3830)
+
+# Basic example
+
+Currently sites might have a global layout components like:
+
+```jsx
+import React from 'react'
+
+export default ({ children }) => (
+  <div>
+    <div>Global navigation</div>
+    {children()}
+  </div>
+)
+```
+
+After this RFC is implemented, people would import the same component into each
+page component e.g.
+
+```jsx
+import React from 'react'
+import Layout from '../components/layout'
+
+export default ({ props }) => (
+  <Layout>
+    <div>Hello world</div>
+  </Layout>
+)
+```
+
+# Motivation
+
+There are several motivations for eliminating the special layout component:
+
+* It's frequently confusing to people used to React as it breaks the normal component composition model
+* It has a number of limitations e.g. we don't support multiple levels of layouts e.g. a global header and footer and then on some pages, a sidebar menu. Currently you have to stuff both levels into a single layout component with a lot of
+conditional logic keyed to pathnames.
+* Communicating between the layout and page components is non-standard and hard to understand
+* It significantly complicates Gatsby's codebase and client runtime
+* It reduces the effectiveness of our new code splitting setup in Gatsby v2 and prevents some potential code splitting/loading improvements
+* It makes Gatsby page transitions work oddly compared to normal React and relatively
+difficult to do well
+
+So it short, it complicates learning and using Gatsby for users as well as
+maintaining our codebase for maintainers — all for no benefit over using normal
+React components. Many people report that they didn't realize the special
+layout components existed so they followed the proposed pattern of creating
+a layout component which they imported into pages. We should be very wary whenever
+we realize we've badly reimplemented a basic React pattern :-)
+
+# Detailed design
+
+V1 layout components are normal components in all respect *except* they, like
+page components, can query for data. People sometimes use this to query data in
+layouts like the site title, etc. for setting site metadata with react-helmet.
+
+Since migrated layout components won't be able to include queries in the
+existing form, I propose adding support to components for "static queries",
+or queries that don't take arguments.
+
+During the planning stages of Gatsby, I considered adding something similar to
+this so that people could query data into any component. Ultimately I decided
+against this as a very common use case for querying is "page template"
+components e.g. a component for blog posts. The query for this page component
+would need to accept variables e.g. the id of the markdown file that's being
+shown on that page.
+
+When we added the layout component, we reused the same graphql syntax and
+pattern as they too needed to include queries.
+
+For static queries in v2 layout components, I propose the following pattern:
+
+```jsx
+import React from 'react'
+import Helmet from 'react-helmet'
+import { StaticQuery } from 'gatsby'
+
+export default class ExampleComponent extends React.Component {
+  render () {
+    <StaticQuery
+      query={graphql`
+        {
+           site {
+             siteMetadata {
+               title
+             }
+           }
+       }
+      `}
+      render={staticData => (
+        <div>
+          <Helmet title={staticData.site.siteMetadata.title} />
+          <h1>Welcome to {staticData.site.siteMetadata.title}!</h1>
+          <p>We hope your stay is pleasant.</p>
+        </div>
+      )}
+    />
+  }
+}
+```
+
+Just like with our current GraphQL implementation, we'd extract the graphql
+queries and run them for you. You'd be able to use fragments, etc. The only
+difference is you can't pass arguments to the query (hence the name,
+`StaticQuery` :-)).
+
+During development, we'd hot-reload changes to the query & underlying data.
+
+Then in production, we'd do a cool optimization. We'd have a babel plugin which
+compiles the above into something that looks like:
+
+```jsx
+import React from 'react'
+import Helmet from 'react-helmet'
+import { StaticData } from 'gatsby'
+import staticData12513 from './staticData12513'
+
+export default class ExampleComponent extends React.Component {
+  render () {
+    <StaticQuery
+      data={staticData12513}
+      render={staticData => (
+        <div>
+          <Helmet title={staticData.site.siteMetadata.title} />
+          <h1>Welcome to {staticData.site.siteMetadata.title}!</h1>
+          <p>We hope your stay is pleasant.</p>
+        </div>
+      )}
+    />
+  }
+}
+```
+
+There's now a little packet of JSON attached to the component! Also, the static file name would be a hash of the query so on the off-chance you reuse a query, webpack 4 would split out the query result module into a new chunk that's shared between components.
+
+It'll also enable new types of components that you can now attach not just markup, styles, and interactivity, but also bits of data. E.g. imagine a SEO component that queries standard data + you can pass in page specific data as well. Or a team member avatar component that has queried the name + image of the 10 team members so you can drop in the component anywhere it's needed. Or a product preview that you can manually add throughout the site as needed. Or a menu component.
+
+# Drawbacks
+
+* Requires implementing the new StaticQuery feature — which isn't trivial — and means
+ more code to maintain.
+* It's a breaking change so we'd need to educate existing users of layouts who'll need to migrate v1 sites.
+ This could involve inventing new patterns to handle more complex cases like
+ i18n
+
+# Alternatives
+
+Keep things as they are. It's not a terrible alternative. Layouts as they exist
+are mediocre but get the job done more-or-less.
+
+# Adoption strategy
+
+* Add how to update to the v2 migration guide
+* Update any relevant docs/tutorials
+
+Fortunately it's very easy to migrate to this new setup. Most people
+can just leave their existing layout component as is (other than making
+`children` a normal prop instead of a function) and import it into their
+pages and make it the top-most component there.
+
+# How we teach this
+
+For layouts, there's nothing to teach as it's just normal React now. Probably
+will want some guides around more sophisticated patterns e.g. i18n.
+
+For `<StaticQuery>`, we'll add documentation to the tutorial + our GraphQL
+concepts page.