需要进一步帮助使javascript同步生成器处理异步函数

Question

I am a relative newbie in Node JS. I have a requirement to synchronously call an async function (http request()). This article by Kylie Simpson has helped me a lot; especially this code bit was exactly what I needed:

function request(url) {
    // this is where we're hiding the asynchronicity,
    // away from the main code of our generator
    // `it.next(..)` is the generator's iterator-resume
    // call
    makeAjaxCall( url, function(response){
        it.next( response );
    } );
    // Note: nothing returned here!
}

function *main() {
    var result1 = yield request( "http://some.url.1" );
    var data = JSON.parse( result1 );

    var result2 = yield request( "http://some.url.2?id=" + data.id );
    var resp = JSON.parse( result2 );
    console.log( "The value you asked for: " + resp.value );
}

var it = main();
it.next(); // get it all started

But I need to take this one step further: I need to be able pass result1 to another function (processResult() in the example below), which then would call request() if some conditions are met. Something like this:

function request(url) {
    makeAjaxCall( url, function(response){
        it.next( response );
    } );
}

function processResult(resSet) {
    if (resSet.length>100)
        return request("http://some.url.1/offset100");
    else
        write2File(resSet);
}

function *main() {
    var result1 = yield request( "http://some.url.1" );
    processResult(result1)
}

var it = main();
it.next();

But when I attempt to do this, request("http://some.url.1/offset100") does not return any values. Any ideas?

Answer 1

But when I attempt to do this, request("http://some.url.1/offset100") does not return any values.

A generator function executes synchronously like any other function except when pausing it using yield .

Your call to function processResult(resSet){} is a synchronous call that returns to function *main() {} before the async function request(url) {} has completed. You in stead need to keep yielding out the call to the async function request(url){} from inside the generator function. You can do this by restructuring your code this way:

function processResult(resSet) {
    if (resSet.length>100)
        return true;
    else
        write2File(resSet); // assuming that this function is synchronous
        return false; // not necessary as the function in this case returns `undefined` by default which coerces to false - but it illustrates the point
}

function *main() {
    var result1 = yield request( "http://some.url.1" );
    if(processResult(result1)) {
        var offset100 = yield request("http://some.url.1/offset100");
    }

    // do something with offset100

var it = main();
it.next();