新在JavaScript异步编程。任何建议？ [英] New to asynchronous programming in JavaScript. Any advice?

问题描述

异步回调是伟大的，但，当一个回调依赖于他人的结果，我有一个包含了回调，等API调用回调。

Asynchronous callbacks are great but when one callback depends on the result of another I have callbacks with api calls that have callbacks, and so on.

apiCall(function () { apiCall(function () { apiCall(function () ...

我能说出其中包括内嵌的回调函数。这看起来prettier和少筑巢，但我没有找到任何更容易阅读。

I can name the callback functions instead of including them inline. That looks prettier and has less nesting but I do not find it any easier to read.

下面是一个例子。我需要查询本地SQLite数据库，用结果来查询服务器，然后使用响应更新本地数据库。

Here is an example. I need to query the local sqlite database, use the result to query a server, then use the response to update the local database.

function sync() {
  db.transaction(
function (transaction) {
  execute(transaction, 'SELECT max(server_time) AS server_time FROM syncs;', [],
      function (transaction, results) { // Query results callback
        var t = results.rows.item(0).server_time;
        $.post('sync.json', { last_sync_time: (t || '1980-01-01') },
           function (data) { // Ajax callback
             db.transaction(
               function(transaction) {
                 $(data.thing).each(function () {
                              var thing = new Thing(this.thing);
                              thing.insert(transaction);
                            });
               });
           });
      });
});
}

有没有办法来untagle本（除命名回调）？

Is there a way to untagle this (other than naming the callbacks)?

推荐答案

命名回调是一件事你可以做，但你需要做其他的事情是不重叠的SQL事务。 （ 1 ）

Naming the callbacks is one thing you can do, but the other thing that you need to do is to have non-overlapping SQL transactions. (1)

您第一笔交易应该是这样的：

Your first transaction should be like this:

// The whole thing starts here
db.transaction(selectTimeCB, null, ajaxPost);

事务与回调开始选择的时间，并且当所述的交易的是完整的，使向ajaxPost动作的呼叫

The transaction starts with the callback to select the time, and when the transaction is complete, make a call to the ajaxPost operation.

// Initial transaction to get server_time
var selectTimeCB = function(t) {
  var query = 'SELECT max(server_time) AS server_time FROM syncs';
  t.executeSql(query, [], postLastSyncCB);
};

// This saves the results from the above select, and nothing else.
var server_time;
var postLastSyncCB = function(t, results) {
  server_time = results.rows.item(0).server_time;
};

var ajaxPost = function() {
  $.post('sync.json', { last_sync_time: (server_time || '1980-01-01') }, nextDbTransaction);
};

如果你有重叠的SQL事务，真正能够杀死对数据库的性能。我最近做了一个有点超过500行的数据库200混合交易测试，发现保持交易建立单独的减少超过90秒的运行时间为3-5秒。

If you have overlapped SQL transactions, that can really kill the performance of the database. I recently did a test of 200 mixed transactions on a database with a bit over 500 rows, and found that keeping the transactions separate reduced a run time of over 90 seconds to 3-5 seconds.

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