问题
As I understand, when this following line of code is interpreted/executed by Javascript
ref.on('value',callback)
(similar to document.addEventListener('click', callback)). The callback gets attached to the el/object for that event such that when that event executes then the attached callback (event handler) gets fired.
But I observe that firebase 'value' event will automatically fire when there is some data at this ref, when the above line of code is interpreted/executed by Javascript even though there is NO trigger such as add/delete/modify operation that happens to that ref.
Is this interpretation/assumption correct or the value event works just like any other event that trigger from add/delete/modify operations. In that case what would be that trigger?
Also, if the value event fires automatically does it actually do an async/network call to the firebase database on that ref and then fetches that data (snapshot) or is the ref data cached at the client side i.e. no async/network request.
Can anybuddy, clarify both this confusion? Your help is appreciated.
回答1:
According to the documentation:
You can use the value event to read a static snapshot of the contents at a given path, as they existed at the time of the event. This method is triggered once when the listener is attached and again every time the data, including children, changes. The event callback is passed a snapshot containing all data at that location, including child data. If there is no data, the snapshot will return false when you call exists() and null when you call val() on it.
When you attach a listener, the SDK will use its persistent connection to the database to check if there is new data. If there is not any new data, then the locally cached data is provided.
来源：https://stackoverflow.com/questions/56413242/firebase-realtime-database-does-value-event-fires-automatically