From d1c50b0c7e17a8ca6f8bae11169167b51fe2e5df Mon Sep 17 00:00:00 2001
From: cnzgray <cnzgray@qq.com>
Date: Tue, 13 Nov 2018 10:57:13 +0800
Subject: [PATCH] =?UTF-8?q?=E6=8F=90=E6=A1=88:=20=E6=B7=BB=E5=8A=A0?=
 =?UTF-8?q?=E4=B8=80=E4=B8=AA`tryGet`=E6=96=B9=E6=B3=95=E5=88=B0cache?=
 =?UTF-8?q?=E5=B7=A5=E5=85=B7=E4=B8=AD=E3=80=82=20(#1130)?=
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit

在抓取一些博客或者论坛的时候，有时候会系统抓取内容页面作为desc。
大多数的做法是获取子页面的地址，然后判断是否已经缓存结果，如果没有则获取，然后设置到缓存中。
这做法固然没有问题，这里只是提供一种新的思路，以将子页面的抓取逻辑独立出去便于逻辑实现。

使用方法：
```
const loadContent = async (link) => {
    // 执行页面加载，解析内容等操作。
    // 最终返回一个结果，该结果将被缓存。
    return result;
}
const result = await ctx.cache.tryGet(link, async () => loadContent(link), 3 * 60 * 60);
```
---
 middleware/lru-cache.js    | 21 +++++++++++++++++++++
 middleware/redis-cache.js  | 21 +++++++++++++++++++++
 routes/typora/changelog.js | 16 +++-------------
 3 files changed, 45 insertions(+), 13 deletions(-)

diff --git a/middleware/lru-cache.js b/middleware/lru-cache.js
index 4be27b186..838cc45b9 100644
--- a/middleware/lru-cache.js
+++ b/middleware/lru-cache.js
@@ -36,6 +36,27 @@ module.exports = function(options = {}) {
                 memoryCache.set(key, value, maxAge * 1000);
             }
         },
+
+        /**
+         *
+         * try get from cache.
+         * if not exists use `getValue` function to get value, and put into cahche.
+         *
+         * @param key cache key
+         * @param getValueFunc a function to get value. call it when key not exists.
+         * @param maxAge
+         *
+         * @returns {Promise<void>}
+         */
+        tryGet: async function(key, getValueFunc, maxAge) {
+            let v = await this.get(key);
+            if (!v) {
+                v = await getValueFunc();
+                this.set(key, v, maxAge);
+            }
+
+            return v;
+        },
     };
 
     return async function cache(ctx, next) {
diff --git a/middleware/redis-cache.js b/middleware/redis-cache.js
index 0115fe9fa..6ec659ee9 100644
--- a/middleware/redis-cache.js
+++ b/middleware/redis-cache.js
@@ -57,6 +57,27 @@ module.exports = function(options = {}) {
                 await redisClient.setex(key, maxAge, value);
             }
         },
+
+        /**
+         *
+         * try get from cache.
+         * if not exists use `getValue` function to get value, and put into cahche.
+         *
+         * @param key cache key
+         * @param getValueFunc a function to get value. call it when key not exists.
+         * @param maxAge
+         *
+         * @returns {Promise<void>}
+         */
+        tryGet: async function(key, getValueFunc, maxAge) {
+            let v = await this.get(key);
+            if (!v) {
+                v = await getValueFunc();
+                this.set(key, v, maxAge);
+            }
+
+            return v;
+        },
     };
 
     return async function cache(ctx, next) {
diff --git a/routes/typora/changelog.js b/routes/typora/changelog.js
index b848f6b1f..e0309d18d 100644
--- a/routes/typora/changelog.js
+++ b/routes/typora/changelog.js
@@ -14,13 +14,7 @@ module.exports = async (ctx) => {
 
     const $ = cheerio.load(response.data);
 
-    const parseContent = async (link) => {
-        // Check cache
-        const cache = await ctx.cache.get(link);
-        if (cache) {
-            return Promise.resolve(JSON.parse(cache));
-        }
-
+    const loadContent = async (link) => {
         const response = await axios({
             method: 'get',
             url: link,
@@ -36,17 +30,13 @@ module.exports = async (ctx) => {
         // const author = $('.post-meta span').text();
         const html = $('#pagecontainer').html();
 
-        const result = {
+        return {
             title: title,
             link: link,
             guid: link,
             pubDate: pubDate,
             description: html,
         };
-
-        ctx.cache.set(link, JSON.stringify(result), 3 * 60 * 60);
-
-        return result;
     };
 
     const items = await Promise.all(
@@ -55,7 +45,7 @@ module.exports = async (ctx) => {
             .map(async (item) => {
                 const node = $('a', item);
                 const link = node.attr('href');
-                const result = await parseContent(link);
+                const result = await ctx.cache.tryGet(link, async () => loadContent(link), 3 * 60 * 60);
 
                 return Promise.resolve(result);
             })