Koa handles body asynchronously

  node.js, question

How to realize an asynchronous setting of body ~ ~.

app.use(function*() {
 setTimeout(function(){
 //Set body here
 this.body = '111111'
 }.bind(this),1000)
 });

app.use(function*() {

this.body = yield fun();

});

function fun(){
 
 return function(cb){
 
 setTimeout(function(){
 
 var bodyvalue = "111111",
 
 err = null ;
 
 cb(err,bodyvalue);
 
 },1000);
 
 return ""
 
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The principle of koa callback can only be understood by understanding the implementation of Co framework.