Offline flag logic

packages/gatsby-source-contentful/src/gatsby-node.js

@@ -16,6 +16,27 @@ const restrictedNodeFields = [
   `parent`,
 ]
 
+const cmsCacheFilename = `contentful-result.json`
+
+async function writeFetchCache(programDirectory, fetchResult) {
+  try {
+    await fs.writeJson(`${programDirectory}/.cache/${cmsCacheFilename}`, fetchResult)
+  } catch (err) {
+    console.error(err)
+  }
+}
+
+async function readFetchCache(programDirectory) {
+  try {
+    console.time(`Fetch Contentful data`)
+    console.log(`Using Contentful Offline cache ⚠️`)
+    return await fs.readJson(`${programDirectory}/.cache/${cmsCacheFilename}`)
+  } catch (err) {
+    console.error(err)
+  }
+  return null
+}
+
 exports.setFieldsOnGraphQLNodeType = require(`./extend-node-type`).extendNodeType
 
 /***
@@ -53,17 +74,27 @@ exports.sourceNodes = async (
     ]
   }
 
+  const programDir = store.getState().program.directory
+  let fetchResult
+
+  if (process.env.GATSBY_CONTENTFUL_OFFLINE === `true`) {
+    fetchResult = await readFetchCache(programDir)
+  } else {
+    fetchResult = await fetchData({
+      syncToken,
+      spaceId,
+      accessToken,
+      host,
+    })
+    writeFetchCache(programDir, fetchResult)
+  }
+
   const {
     currentSyncData,
     contentTypeItems,
     defaultLocale,
     locales,
-  } = await fetchData({
-    syncToken,
-    spaceId,
-    accessToken,
-    host,
-  })
+  } = fetchResult
 
   const entryList = normalize.buildEntryList({
     currentSyncData,