sequencing function calls in javascript - are callbacks the only way?

问题

I read through various threads like this one for example.

But it really escapes me how to accomplish the following:

I have 4 functions, and want them happen one after another in sequence. Notice they are in incorrect order, to get my point across. I want the result that will output "1, 2, 3, 4'

function firstFunction(){
  // some very time consuming asynchronous code...
  console.log('1');
}
function thirdFunction(){
  // definitely dont wanna do this until secondFunction is finished
  console.log('3');
}
function secondFunction(){
  // waits for firstFunction to be completed
  console.log('2');
}
function fourthFunction(){
  // last function, not executed until the other 3 are done.
  console.log('4');
}

I tried to figure out callbacks but am getting lost :(

Isn't there some simple way to do this? Like looping through an array...

回答1:

It's a great chance to start using jQuery Deferred.

Apart from the callbacks-based solution the code is readable, flexible and highly maintainable

http://jsfiddle.net/zerkms/zJhph/

function firstFunction(){
  var d = $.Deferred();
  // some very time consuming asynchronous code...
  setTimeout(function() {
    console.log('1');
    d.resolve();
  }, 1000);
  return d.promise();
}
function thirdFunction(){
  var d = $.Deferred();
  // definitely dont wanna do this until secondFunction is finished
  setTimeout(function() {
    console.log('3');
    d.resolve();
  }, 1000);
  return d.promise();
}
function secondFunction(){
  var d = $.Deferred();
  setTimeout(function() {
    console.log('2');
    d.resolve();
  }, 1000);
  return d.promise();
}
function fourthFunction(){
  var d = $.Deferred();
  // last function, not executed until the other 3 are done.
  setTimeout(function() {
    console.log('4');
    d.resolve();
  }, 1000);
  return d.promise();
}

firstFunction().pipe(secondFunction).pipe(thirdFunction).pipe(fourthFunction);​

PS: as an example of asynchronous code I've used setTimeout. The main thing is that in the end of the asynchronous part you need to call d.resolve() to continue chaining methods.

Further reading: http://joseoncode.com/2011/09/26/a-walkthrough-jquery-deferred-and-promise/

回答2:

The idea is you'd do something like the following so that once the first function was done running, it'd know what to run as opposed to you having to figure it out on your own outside the function:

function firstFunction(callback){
  // some very time consuming asynchronous code...
  console.log('1');

  return callback(function(){
    alert("Second function finished.");
    return true;
  });
}
function secondFunction(callback){
  // waits for firstFunction to be completed
  console.log('2');

  return callback();
}

firstFunction(secondFunction);

Also look up .apply() and .call().

回答3:

If I'm using callbacks, my working solution now looks like this:

    one(two);
    function one(callb){
        console.log('1');
        callb(three);
    }
    function four(){
        console.log('4');
    }
    function two(callb){
        console.log('2');
        callb(four);
    }
    function three(callb){
        console.log('3');
        callb();
    }

I find that hideous. How am I supposed to keep track of this stuff if there is more than 2-3 sequences? Shudder...

回答4:

It's been a while and I noticed something about deferreds in jquery documentation, specifically the when core API function.

$.when( $.ajax("test.aspx") ).then(function(ajaxArgs){ 
     alert(ajaxArgs[1]); /* ajaxArgs is [ "success", statusText, jqXHR ] */
});

Code sample taken from http://jqapi.com/#p=jQuery.when

回答5:

I have played with the Promise, Sequence, Exception, Callback to understand how it works and finally made this code.

Call functions with callback and send result as parameter to another function in sequence and have a catch errors.

function firstFunction(par) {
    return new Promise(function (resolve, reject) {
        console.log("start " + par);
        setTimeout(function (par) {
            console.log(par);
            resolve(par + 1);
        }, 1000, par);
    });
}
function secondFunction(par) {
    return new Promise(function (resolve, reject) {
        console.log("start " + par);
        setTimeout(function (par) {
            console.log(par);
            try{
                throw "Let's make an error...";
            }
            catch(err)
            {
                reject(err);
            }
            resolve(par + 1);
        }, 1000, par);
    })
}
function thirdFunction(par) {
    return new Promise(function (resolve, reject) {
        console.log("start " + par);
        setTimeout(function (par) {
            console.log(par);
            resolve(par + 1);
        }, 1000, par);
    });
}

function CatchError(error) {
    console.log("Exception: " + error);
}

//Break all chain in second function
function ChainBrake() {
    firstFunction(1)
    .then(secondFunction)
    .then(thirdFunction)
    .catch(CatchError);    
}

//Log error and continue executing chain
function ChainContinue() {
    firstFunction(1)
    .catch(CatchError)
    .then(secondFunction)
    .catch(CatchError)
    .then(thirdFunction)
    .catch(CatchError);
}

来源：https://stackoverflow.com/questions/12274114/sequencing-function-calls-in-javascript-are-callbacks-the-only-way