Firebase.ServerValue.TIMESTAMP在侦听器和实际添加数据的客户端之间未同步 [英] Firebase.ServerValue.TIMESTAMP not synched between listeners and the client that actually adds data

问题描述

  var fb = new Firebase（'https://xxxxxxxxxxx.firebaseio.com /测试'）; 
 
 fb.limitToLast（1）.on（'child_added'，function（snap）{
 console.log（'key'，snap.key（））; 
 console .log（'val'，snap.val（））; 
}）; 
 
 fb.push（{
 date_now：Firebase.ServerValue.TIMESTAMP 
}）; 
  

如果我用这个脚本打开两个选项卡，那么实际推送数据的选项是 local 在child_added回调中的时间戳，而刚刚侦听的另一个选项卡获取正确的服务器生成的一个。据我所知，这样做是为了排除往返并节省带宽。

但是对于我的任务来说，这种行为是不可接受的。

这是推送者的console.log：

  key -K59mrvEUhTaoNIQQoA4 
 val Object {date_now：1449732570832} 
  

相当于仪表板中显示的服务器数据）：

$ p $ 键-K59mrvEUhTaoNIQQoA4
对象{date_now：1449732571759}


解决方案

Firebase会为该写入操作触发两个本地事件：

1. 立即使用本地时间戳激发一个 child_added 事件）
   
2. 它随后在服务器指定的时间触发带有实际时间戳的 child_changed 事件。

所以你可以通过监听这两个事件来解决问题：

  var fb = new Firebase（'https://xxxxxxxxxxx.firebaseio.com/test'）; 
 
 var query = fb.limitToLast（1）; 
 query.on（'child_added'，function（snap）{
 console.log（'key'，snap.key（））; 
 console.log（'val'，snap。 val（））; 
}）; 
 query.on（'child_changed'，function（snap）{
 console.log（'key'，snap.key（））; 
 console.log（'val'，snap。 val（））; 
}）; 
 
 fb.push（{
 date_now：Firebase.ServerValue.TIMESTAMP 
}）; 
  

一般而言，建议您处理所有子元素_ * 事件，而不仅仅是 child_added 。可能有更多的原因，为什么服务器必须更新或删除值，以纠正本地事件。

如果你喜欢有一个单一的回调/事件处理程序，你还可以监听值事件：

  var query = fb。 limitToLast（1）; 
 query.on（'value'，function（snap）{
 snap.forEach（function（child）{
 console.log（'key'，child.key（））; 
 console.log（'val'，child.val（））; 
}）; 
}）; 
  

您将注意到 forEach（）在回调中。

Here is the simplest example possible:

var fb = new Firebase('https://xxxxxxxxxxx.firebaseio.com/test');

fb.limitToLast(1).on('child_added', function(snap) {
    console.log('key', snap.key());
    console.log('val', snap.val());
});

fb.push({
    date_now: Firebase.ServerValue.TIMESTAMP
});

If I open two tabs with this script, the one that actually pushes data gets local timestamp in child_added callback and the other tab that just listens gets proper server-generated one. As far as I understand it's done to exclude round-trip and save bandwidth.

But for my task this behaviour is unacceptable. How can I overcome it?

This is the console.log from pusher:

key -K59mrvEUhTaoNIQQoA4
val Object {date_now: 1449732570832}

and listeners (equals to server data seen in dashboard):

key -K59mrvEUhTaoNIQQoA4
val Object {date_now: 1449732571759}

解决方案

Firebase fires two local events for that write operation:

1. it immediately fires a child_added event with the local timestamp (corrected for your expected offset to the server)
2. it later fires a child_changed event with the actual timestamp as the server specified it.

So you can solve the problem by listening for both events:

var fb = new Firebase('https://xxxxxxxxxxx.firebaseio.com/test');

var query = fb.limitToLast(1);
query.on('child_added', function(snap) {
    console.log('key', snap.key());
    console.log('val', snap.val());
});
query.on('child_changed', function(snap) {
    console.log('key', snap.key());
    console.log('val', snap.val());
});

fb.push({
    date_now: Firebase.ServerValue.TIMESTAMP
});

In general it is recommended to handle all child_* events and not just child_added. There may be more reasons why the server has to update or remove the value, to correct for the local event.

If you prefer having a single callback/event handler, you can also listen for the value event:

var query = fb.limitToLast(1);
query.on('value', function(snap) {
    snap.forEach(function(child) {
        console.log('key', child.key());
        console.log('val', child.val());
    });
});

You'll note the use of forEach() in the callback.

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