Add support to preserve app context on mounted Koa app

[lbdremy]

[jonathanong]

can you describe what the problem is?

[rdshoep]

I think this PR can resolve the problem: mount app use signed cookies will catch error.

ps:

1. mounted app.keys ignored
2. https://github.com/koajs/koa/issues/203

[lbdremy]

Hi,

I have done this change to keep the context given to the app, not the context of the app on which it is mounted.

const app = new Koa();
app.context.config = { hello: 'sir' };

app.use((ctx) => {
 ctx.config // is set to { hello: 'sir'} and not { hi: 'sir'};
});

const root = new Koa();
root.context.config = { hi: 'sir'};
root.mount('/api', app, true);

maybe a more explicit api for the options will be nice, something like

root.mount('/api',app, { preserveContext: true }); // keepContext ?

[rdshoep]

I have forked the repo, and use preserve instead of preserveContext.

https://github.com/smartmiting/koa-mount/tree/preserve

It supports:

1. preserved context
2. preserved app props(subdomainOffset, keys for cookies)

If you agree, I will make a new PR.

[z-vr]

Thanks @lbdremy, this is a very useful feature (could you possibly fix the linting problem which makes CI fail? @jonathanong it'd be great if you could merge and release this PR please.

If anyone comes across this issue, here's a fork: https://www.npmjs.com/package/@zavr/koa-mount

[lvl99]

Has this PR aged like a fine whisky? 🥃