[WIP]feat: keep alive

[xiaohuoni]

Checklist

• npm test passes
• tests are included
• documentation is changed or added
• commit message follows commit guidelines

Description of change

只要route中配置属性keepAlive，就会被缓存

routes: [
    {
      path: '/',
      component: '../layouts/index',
      routes: [
        {
          path: '/',
          component: './index',
        },
        {
          path: '/list',
          component: './list',
          keepAlive: true,
        },
        {
          path: '/item',
          component: './item',
        },
      ],
    },
  ],

在不需要的时候，可以手动调用方法取消,下例的 /list 为上面设置了 keepAlive 的路由 path

import dropByCacheKey from 'umi/dropByCacheKey';
const clearCache = () => {
    dropByCacheKey('/list');
};

测试了添加了layout和不添加layout，都有效

• any feature?
• close [RFC] umi@3 支持Keep Alive 试用方案 #3091

---

This change is 

[xiaohuoni]

@CJY0208 看看

[CJY0208]

很棒！能为 umi 间接做一点贡献，很荣幸

不过需要注意，目前的 CacheRoute 还无法与 react-transition-group 共同工作，启用 keepAlive 的路由将丢失路由动效

[LeoHuiyi]

路由是按目录约定生成的要怎么支持keep alive？

[xiaohuoni]

@LeoHuiyi 别着急，等云谦看看，方案可以的话，我会进一步支持。我们项目中用的都是约定式

[netlify]

Ant Design Pro preview using the current branch umi

Built with commit 5175a22

https://deploy-preview-3084--pro-pre-umijs.netlify.com

[coveralls]

Pull Request Test Coverage Report for Build 6582

• 6 of 8 (75.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.01%) to 41.582%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
packages/umi/src/renderRoutes.js	6	8	75.0%

Totals
Change from base Build 6553:	0.01%
Covered Lines:	4378
Relevant Lines:	10464

---

💛 - Coveralls

[CodeMonkeyLin]

keep alive功能还没添加到umi版本么，我用最新的版本没有效果

[xiaohuoni]

没合并呢

[xiaohuoni]

嵌套路由相互跳转失效

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.

[xiaohuoni]

跑不同，很多问题。
嵌套路由，还有hooks的结合，layout都要考虑。