Cache TOC for later usage in the case of cached file html

[cheng-kang]

Closes #509.

How to reproduce

1. Open any page with id, e.g. https://docsify.js.org/#/plugins?id=full-text-search
2. Navigate (from sidebar) to another page, e.g. http://127.0.0.1:3000/#/write-a-plugin
3. Navigate (from sidebar) to the previous page, https://docsify.js.org/#/plugins

Expecting:

But got:

Issue

Use plugins page as an example

Sub-sidebars are rendered using TOC of the file of current page.

The first time loading plugins.md, compiler compiles the markdown content and caches it [code ref 1].

During the compiling process, TOC of this file is built [code ref 2].

this.toc is then used by compiler.subSidebar to generate sub-sidebars.

When navigating to other pages and back, compiler.compile loads html for plugins.md from cache, which bypasses the step of compiling, which results in no TOC, which results in this issue - no sub-sidebars.

Solution

Add cache for TOC, and use file names as keys to avoid unnecessarily storing whole file content in memory.

Other notes

While debugging this one, found out that SSR version has the same problem. #650

Haven't checked whether this PR fixes both or not.

[QingWei-Li]

Good job