feat($markdown): code snippet hmr (close #1309) (#1358)

packages/@vuepress/markdown/lib/snippet.js

@@ -1,7 +1,27 @@
-const { fs } = require('@vuepress/shared-utils')
+const { fs, path } = require('@vuepress/shared-utils')
 
 module.exports = function snippet (md, options = {}) {
+  const fence = md.renderer.rules.fence
   const root = options.root || process.cwd()
+
+  md.renderer.rules.fence = (...args) => {
+    const [tokens, idx, , { loader }] = args
+    const token = tokens[idx]
+    const { src } = token
+    if (src) {
+      if (loader) {
+        loader.addDependency(src)
+      }
+      if (fs.existsSync(src)) {
+        token.content = fs.readFileSync(src, 'utf8')
+      } else {
+        token.content = 'Not found: ' + src
+        token.info = ''
+      }
+    }
+    return fence(...args)
+  }
+
   function parser (state, startLine, endLine, silent) {
     const CH = '<'.charCodeAt(0)
     const pos = state.bMarks[startLine] + state.tShift[startLine]
@@ -25,14 +45,13 @@ module.exports = function snippet (md, options = {}) {
     const end = state.skipSpacesBack(max, pos)
     const rawPath = state.src.slice(start, end).trim().replace(/^@/, root)
     const filename = rawPath.split(/[{\s]/).shift()
-    const content = fs.existsSync(filename) ? fs.readFileSync(filename).toString() : 'Not found: ' + filename
     const meta = rawPath.replace(filename, '')
 
     state.line = startLine + 1
 
     const token = state.push('fence', 'code', 0)
     token.info = filename.split('.').pop() + meta
-    token.content = content
+    token.src = path.resolve(filename)
     token.markup = '```'
     token.map = [startLine, startLine + 1]