获取和处理数组的承诺

I am new to both promises and having trouble understanding what I need to do to code the following logic:

我是两个承诺的新手，并且无法理解我需要做什么来编写以下逻辑：

I am developing a Web service in Node.js and Express to fetch song data from a wiki and return an object, which a client application will consume. The wiki's API does not let me write a batch query; I have to get each page individually. So I will have to get the list of songs, then perform a call for each song.

我正在Node.js和Express中开发一个Web服务来从wiki获取歌曲数据并返回一个客户端应用程序将使用的对象。维基的API不允许我编写批量查询;我必须单独获取每个页面。所以我必须得到歌曲列表，然后为每首歌曲打电话。

I currently intend to use the Q middleware for Node.js as my promises library, though I am open to suggestions on a more appropriate middleware for this task.

我目前打算将Node.js的Q中间件用作我的promises库，尽管我对这个任务的更合适的中间件提出了建议。

Here is my pseudocode:

这是我的伪代码：

app.get('/songs/:criteria', function(request,response) {
    downloadSongList()
    .then(foreach(song) downloadSongData)
    .then(assembleReturnValue)
    .then(response.json(returnValue));
});

What will the actual code look like?

实际代码会是什么样的？

2 个解决方案

#1

The actual code will use function expressions, and around the foreach you will need to use Q.all:

实际的代码将使用函数表达式，并且在foreach周围你需要使用Q.all：

app.get('/songs/:criteria', function(request,response) {
    downloadSongList(request.params)
    .then(function(list) {
        var promises = list.map(function(song) {
            return downloadSongData(song.params) // another promise
        });
        return Q.all(promises);
    }).then(function(allResults) {
        // assemble
        return // Value;
    }).then(response.json);
});

Also have a look at these general rules for promise development.

另请参阅这些承诺开发的一般规则。

#2

Here is an alternative solution with Bluebird, since you said you were interested in different libraries:

以下是Bluebird的替代解决方案，因为您说您对不同的库感兴趣：

  downloadSongList(request.params).
                 map(downloadSongData).
                 call("join",",").
                 then(response.json).catch(sendError)

What we're utilizing here:

我们在这里使用的是：

.map - which takes an array of promises, and calls a method on each one of them, we do that for the list returned from downloadSongList.
.map - 它接受一个promises数组，并在每个promise上调用一个方法，我们为downloadSongList返回的列表执行此操作。
.call which calls an array method, here we're joining the elements as strings, not sure what format you're using here but this will basically do array.join.
调用数组方法的.call，这里我们将元素作为字符串加入，不知道你在这里使用的是什么格式，但这基本上就是array.join。

These are some advantages we gain from Bluebird, other than that this is very similar to Bergi's answer.

这些是我们从Bluebird获得的一些优势，除此之外，它与Bergi的答案非常相似。

#1