Q.js - Using deferred | 易学教程

问题

How do I get the value of the text from the example below?

Q.js has an example on using Deferred:

var deferred = Q.defer();
FS.readFile("foo.txt", "utf-8", function (error, text) {
    if (error) {
        deferred.reject(new Error(error));
    } else {
        deferred.resolve(text);
    }
});
return deferred.promise;

In this case, there is a node async function being used. What I want to do is get the value of text from the deferred.promise being returned. When I console.log(deferred.promise) I get this:

{ promiseSend: [Function], valueOf: [Function] }

What am I doing wrong (as I just copy/pasted the example from here: https://github.com/kriskowal/q#using-deferreds) or what else do I need to do to actually get that text from the file?

I am aware that node.js has a synchronous version of the call above - my goal is to understand how deferred works with this library.

回答1:

You can get the value via the .then() method of a Promise:

function read() {
    // your snippet here...
}

read().then(function (text) {
    console.log(text);
});

Also, error handlers can be passed either as a 2nd argument to .then() or with the .fail() method:

read().fail(function (err) {
    console.log(err);
});

回答2:

See https://github.com/kriskowal/q#adapting-node

Can be rewritten in a nodejs-like:

var read = Q.nfcall(FS.readFile, FS, "foo.txt", "utf-8");
read().then( function (text) { console.log(text) } );

回答3:

Q = require('q');
FS = require('fs');

function qread() {
  var deferred = Q.defer();
  FS.readFile("foo.txt", "utf-8", function (error, text) {
    if (error) {
  deferred.reject(new Error(error));
    } else {
  deferred.resolve(text);
    }
  });
  return deferred.promise;
};   

var foo = qread();

setTimeout(function() {
  console.log(""+foo);
},1000);

It's strange you cannot see the output for console.log(foo). Dont' know why.

Check more examples here https://github.com/kriskowal/q/wiki/Examples-Gallery

回答4:

deferred.promise.then(function (text) {
  console.log(text); // Bingo!
});

回答5:

Q = require('q');
FS = require('fs');

var deferred = Q.defer();
FS.readFile("client-02.html", "utf-8", function (error, text) {
  if (error) {
    deferred.reject(new Error(error));
    } else {
    deferred.resolve(text);
    }
return deferred.promise.done( setTimeout(console.log(text),1000 ));
});

来源：https://stackoverflow.com/questions/12505850/q-js-using-deferred

标签

node.js

deferred