jQuery Deferred 和 Promise 用于同步和异步函数的顺序执行 [英] jQuery Deferred and Promise for sequential execution of synchronous and asynchronous functions

问题描述

如果我想以特定顺序执行同步和异步函数，我可以使用 jQuery promise，但它似乎并没有像我期望的那样工作.

If I want to have synchronous and asynchronous functions execute in a particular order I could use jQuery promise but it doesn't seem to work the way I'd expect it to work.

函数 a,b 和 c 应该在调用 deferred.resolve() 时按该顺序执行我希望函数 b 被执行，但所有函数都会立即执行，无论是否解析被调用.

Functions a,b and c should execute in that order when in a the deferred.resolve() is called I'd expect function b to be executed but all functions are executed immediately no matter if the resolve is called.

代码如下:

function a(){
  var deferred = $.Deferred();
  setTimeout(function(){
    console.log("status in a:",deferred.state());
    //this should trigger calling a or not?
    deferred.resolve("from a");
  },200);
  console.log("a");
  return deferred.promise();
};
function b(){
  var deferred = $.Deferred();
  setTimeout(function(){
    console.log("status in b:",deferred.state());
    deferred.resolve("from b");
  },200);
  console.log("b");
  return deferred.promise();
}
//synchronous function
function c(){
  var deferred = $.Deferred();
  console.log("c");
  console.log("status in c:",deferred.state());
  deferred.resolve("from c");
  return deferred.promise();
}
function test(){
  fn=[a,b,c],i=-1,
  len = fn.length,d,
  d = jQuery.Deferred(),
  p=d.promise();
  while(++i<len){
    p=p.then(fn[i]);
  }
  p.then(function(){
    console.log("done");
  },
  function(){
    console.log("Failed");
  });
  d.resolve();
  //instead of the loop doing the following has the same output
  //p.then(a).then(b).then(c);
  //d.resolve();
}
test();

输出为:

a
b
status in c: pending
c
done
status in a: pending
status in b: pending

预期输出:

a
status in a: pending
b
status in b: pending
c
status in c: pending
done

尝试了以下修改的一些组合:

Tried a some combinations of the following modifications:

  d = jQuery.Deferred();
  setTimeout(function(){d.resolve();},100);
  var p=d.promise();
  while(++i<len){
    p.then(fn[i]);
  }

但是所有的结果都是一样的，在解析 a 的延迟之前 b 被调用，在解析 b 的延迟之前调用 c.

But all with same unexpected results, b gets called before deferred of a is resolved, c is called before deferred of b is resolved.

推荐答案

对于 1.8 之前的 jQuery，这是一个问题，但对于新版本的 jQuery，这不再是问题:

For jQuery prior to 1.8, this is a problem, but for new versions of jQuery, this is not a problem anymore:

function test(){
  var d = jQuery.Deferred(), 
  p=d.promise();
  //You can chain jQuery promises using .then
  p.then(a).then(b).then(c);
  d.resolve();
}
test();

演示

以下是jQuery 1.7.2的demo

Below is the demo of jQuery 1.7.2

演示

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