How to set the static resource server of koa dynamically

I want to request different static files according to the type of client
for example, the same address requested by my mobile rel= and pc http://localhost:3000/images/1.jpg, but the returned image is different

. 
const Koa = require("koa")
const path = require("path")
const static = require("koa-static")
const app = new Koa()

app.use(async (ctx) => {
  const u = ctx.header["user-agent"];
  const isAndroid = u.indexOf("Android") > -1 || u.indexOf("Adr") > -1;
  const isiOS = !!u.match(/\(i[^;]+;( U;)? CPU.+Mac OS X/);
  if(isAndroid || isiOS) {
    app.use(static(path.join(__dirname, "./staticPhone")))
  } else {
    app.use(static(path.join(__dirname, "./staticPc")))
  }
})
app.listen(3000)

but this doesn"t work. Is there a way to request static resources dynamically

Koa2 node.js
Mar.13,2021

because this is just adding middleware by device type.
after judging the device type, determine whether the file you are looking at is a file, and return the file under the path.

wrong way of thinking. App.use actually concatenates predefined methods into a large method call chain, and then processes the response according to the request;
your logic is to change the chain dynamically, which is impossible;

so work around to dynamically change parameters instead of methods by adding if else logic; 

app.use((ctx, next) => {
    if (isMobile) {
        ctx.path = path.resolve(__dirname, 'mobile/static');
    }
    else {
        ctx.path = path.resolve(__dirname, 'normal/static');
    }
    return next();
});

// static middleware
app.use(async()=> {
...
});

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