improve cache hit by caching nested components hierarchy in one pass

Author: yyx990803

src/server/render.js

@@ -33,16 +33,24 @@ export function createRenderFunction (
         if (cache.has(key)) {
           return write(cache.get(key), next)
         } else {
-          if (!write.caching) {
-            // initialize if not already caching
-            write.caching = true
-            const _next = next
-            next = () => {
-              cache.set(key, write.buffer)
+          write.caching = true
+          const buffer = write.cacheBuffer
+          const bufferIndex = buffer.push('') - 1
+          const _next = next
+          next = () => {
+            const result = buffer[bufferIndex]
+            cache.set(key, result)
+            if (bufferIndex === 0) {
+              // this is a top-level cached component,
+              // exit caching mode.
               write.caching = false
-              write.buffer = ''
-              _next()
+            } else {
+              // parent component is also being cached,
+              // merge self into parent's result
+              buffer[bufferIndex - 1] += result
             }
+            buffer.length = bufferIndex
+            _next()
           }
         }
       }

src/server/write.js

@@ -9,7 +9,7 @@ export function createWriteFunction (
   let stackDepth = 0
   const cachedWrite = (text, next) => {
     if (text && cachedWrite.caching) {
-      cachedWrite.buffer += text
+      cachedWrite.cacheBuffer[cachedWrite.cacheBuffer.length - 1] += text
     }
     const waitForNext = write(text, next)
     if (!waitForNext) {
@@ -27,6 +27,6 @@ export function createWriteFunction (
     }
   }
   cachedWrite.caching = false
-  cachedWrite.buffer = ''
+  cachedWrite.cacheBuffer = []
   return cachedWrite
 }