feat(cli): incrementally process edits on markdown files

packages/cli/cli/versions.yml

@@ -1,3 +1,12 @@
+- changelogEntry:
+    - summary: |
+        Improve performance of `fern docs dev` by only reloading the markdown content when 
+        only markdown files are changed, avoiding unnecessary recompilation of the full docs.
+
+      type: fix
+  irVersion: 55
+  version: 0.50.17
+
 - changelogEntry:
     - summary: |
         Improve performance of `fern docs dev` by debouncing across edits to multiple files, 

packages/cli/docs-preview/src/previewDocs.ts

@@ -1,3 +1,4 @@
+import { readFile } from "fs/promises";
 import { v4 as uuidv4 } from "uuid";
 
 import { DocsDefinitionResolver, filterOssWorkspaces } from "@fern-api/docs-resolver";
@@ -13,27 +14,63 @@ import {
     convertDbDocsConfigToRead,
     convertDocsDefinitionToDb
 } from "@fern-api/fdr-sdk";
-import { convertToFernHostAbsoluteFilePath } from "@fern-api/fs-utils";
+import { AbsoluteFilePath, convertToFernHostAbsoluteFilePath, relative } from "@fern-api/fs-utils";
 import { IntermediateRepresentation } from "@fern-api/ir-sdk";
 import { Project } from "@fern-api/project-loader";
 import { convertIrToFdrApi } from "@fern-api/register";
 import { TaskContext } from "@fern-api/task-context";
 
+import { replaceReferencedMarkdown } from "../../docs-markdown-utils/src";
+
 export async function getPreviewDocsDefinition({
     domain,
     project,
-    context
+    context,
+    previousDocsDefinition,
+    editedAbsoluteFilepaths
 }: {
     domain: string;
     project: Project;
     context: TaskContext;
+    previousDocsDefinition?: DocsV1Read.DocsDefinition;
+    editedAbsoluteFilepaths?: AbsoluteFilePath[];
 }): Promise<DocsV1Read.DocsDefinition> {
     const docsWorkspace = project.docsWorkspaces;
     const apiWorkspaces = project.apiWorkspaces;
     if (docsWorkspace == null) {
         throw new Error("No docs workspace found in project");
     }
 
+    if (editedAbsoluteFilepaths != null && previousDocsDefinition != null) {
+        const allMarkdownFiles = editedAbsoluteFilepaths.every(
+            (filepath) => filepath.endsWith(".mdx") || filepath.endsWith(".md")
+        );
+        for (const absoluteFilePath of editedAbsoluteFilepaths) {
+            const relativePath = relative(docsWorkspace.absoluteFilePath, absoluteFilePath);
+            const markdown = (await readFile(absoluteFilePath)).toString();
+            const processedMarkdown = await replaceReferencedMarkdown({
+                markdown,
+                absolutePathToFernFolder: docsWorkspace.absoluteFilePath,
+                absolutePathToMarkdownFile: absoluteFilePath,
+                context
+            });
+
+            const previousValue = previousDocsDefinition.pages[FdrAPI.PageId(relativePath)];
+            if (previousValue == null) {
+                continue;
+            }
+
+            previousDocsDefinition.pages[FdrAPI.PageId(relativePath)] = {
+                markdown: processedMarkdown,
+                editThisPageUrl: previousValue.editThisPageUrl
+            };
+        }
+
+        if (allMarkdownFiles) {
+            return previousDocsDefinition;
+        }
+    }
+
     const fernWorkspaces = await Promise.all(
         apiWorkspaces.map(
             async (workspace) =>

packages/cli/docs-preview/src/runPreviewServer.ts

@@ -126,7 +126,7 @@
     let isReloading = false;
     const RELOAD_DEBOUNCE_MS = 1000;
 
-    const reloadDocsDefinition = async () => {
+    const reloadDocsDefinition = async (editedAbsoluteFilepaths?: AbsoluteFilePath[]) => {
         context.logger.info("Reloading docs...");
         const startTime = Date.now();
         try {
@@ -136,7 +136,9 @@
             const newDocsDefinition = await getPreviewDocsDefinition({
                 domain: `${instance.host}${instance.pathname}`,
                 project,
-                context
+                context,
+                previousDocsDefinition: docsDefinition,
+                editedAbsoluteFilepaths: editedAbsoluteFilepaths
             });
             context.logger.info(`Reload completed in ${Date.now() - startTime}ms`);
             return newDocsDefinition;
@@ -169,6 +171,8 @@
         renameDetection: true
     });
 
+    const editedAbsoluteFilepaths: AbsoluteFilePath[] = [];
+
     // eslint-disable-next-line @typescript-eslint/no-misused-promises
     watcher.on("all", async (event: string, targetPath: string, _targetPathNext: string) => {
         context.logger.info(chalk.dim(`[${event}] ${targetPath}`));
@@ -178,6 +182,8 @@
             return;
         }
 
+        editedAbsoluteFilepaths.push(AbsoluteFilePath.of(targetPath));
+
         // Clear any existing timer
         if (reloadTimer != null) {
             clearTimeout(reloadTimer);
@@ -192,11 +198,13 @@
                     type: "startReload"
                 });
 
-                const reloadedDocsDefinition = await reloadDocsDefinition();
+                const reloadedDocsDefinition = await reloadDocsDefinition(editedAbsoluteFilepaths);
                 if (reloadedDocsDefinition != null) {
                     docsDefinition = reloadedDocsDefinition;
                 }
 
+                editedAbsoluteFilepaths.length = 0;
+
                 sendData({
                     version: 1,
                     type: "finishReload"