Question

我使用for for each来处理列表，我希望只有在元素n-1结束时才执行元素n：

var elements = ["stuff1", "stuff2", "stuff3"];

elements.forEach(function(element){
    someAsyncFunction(element, function(){
       console.log('do more stuff');
   });
});

我需要他们彼此等待并以正确的顺序执行。

编辑：我碰到了process.nextTick（），但我不确定它是否能保证等待和订购。

Answer 1

只需将元素存储在数组中，就像在问题中一样。然后使用elements.shift()检索并删除第一个元素。

var elements = ["stuff1", "stuff2", "stuff3"];
(function next () {
    if (!elements.length) {
        return;
    }

    var element = elements.shift();
    someAsyncFunction(element, function (err, result) {
        …
        next();
    });
})();

Answer 2

一种不错的方法是使用诸如Q或Bluebird之类的承诺库，然后实施promiseWhile like this。

var Promise = require('bluebird');

var promiseWhile = function(condition, action) {
    var resolver = Promise.defer();

    var loop = function() {
        if (!condition()) return resolver.resolve();
        return Promise.cast(action())
            .then(loop)
            .catch(resolver.reject);
    };

    process.nextTick(loop);

    return resolver.promise;
};

用法：

var sum = 0,
    stop = 10;

promiseWhile(function() {
    // Condition for stopping
    return sum < stop;
}, function() {
    // Action to run, should return a promise
    return new Promise(function(resolve, reject) {
        // Arbitrary 250ms async method to simulate async process
        // In real usage it could just be a normal async event that 
        // returns a Promise.
        setTimeout(function() {
            sum++;
            // Print out the sum thus far to show progress
            console.log(sum);
            resolve();
        }, 250);
    });
}).then(function() {
    // Notice we can chain it because it's a Promise, 
    // this will run after completion of the promiseWhile Promise!
    console.log("Done");
});

用于使用异步函数进行迭代

2 个答案: