为什么当引用不存在时，firebase“on”“value”不会运行监听器功能，但“一次”“值”是什么？

Question

Say I'm listening to a widget:

const ref = firebase.database().ref(`widgets/${widgetId}`);
ref.on('value', function(snapshot){
  console.log(snapshot.val()); // <-- I never see this if ref doesn't exist!
}

If that widget doesn't exist I want to know about it. I'd expect snapshot.val() to return null . But the problem is the function isn't run at all. So I can't event check for null . It just stays silent.

The weird thing is, this fixes it:

const ref = firebase.database().ref(`widgets/${widgetId}`);
ref.once('value', function(){});
ref.on('value', function(snapshot){
  console.log(snapshot.val()); // <-- Now is null!
}

Now the function provided to on is run with snapshot.val() === null . Is this a bug or am I doing something wrong?

Answer 1

我需要将Firebase升级到最新版本（3.0.5 - > 3.2.0）

npm install --save firebase