Database - Read Data and Change Events

Reading Data and Listening to Value Change events

All data reading is an asynchronous process.

Get a DatabaseReference

To read or write data from the database, you need an instance of DatabaseReference:

var database:DatabaseReference;

database = FirebaseDatabase.service.getReference();

Listen for value events

To read data at a path and listen for changes you add a listener for the VALUE_CHANGED event.

database.addEventListener( DatabaseReferenceEvent.VALUE_CHANGED, valueChangedHandler );

You can also attach these listeners to children to get specific updates when a particular value changes:

database.child("someKey").addEventListener( DatabaseReferenceEvent.VALUE_CHANGED, valueChangedHandler );

In your handler you will have access to a DataSnapshot which represents the data contained in the node at the time of the event:

private function valueChangedHandler( event:DatabaseReferenceEvent ):void 
{
    // event.snapshot will contain the DataSnapshot of the reference this listener was attached
    trace( event.snapshot.toString() );
}

You should also listen for the VALUE_CHANGED_ERROR event at this point. It will be dispatched when there is an issue retrieving the value. For example, a read can fail if the client doesn't have permission to read from a Firebase database location.

When this event is dispatched the event will have the errorCode and errorDescription fields populated with the details of the error.

database.addEventListener( DatabaseReferenceEvent.VALUE_CHANGED_ERROR, valueChangedErrorHandler );

private function valueChangedErrorHandler( event:DatabaseReferenceEvent ):void 
{
    trace( event.errorCode );
    trace( event.errorDescription );
}

Read Data Once

In some cases you may want a callback to be called once and then immediately removed, such as when initializing a UI element that you don't expect to change. This is useful for data that only needs to be loaded once and isn't expected to change frequently or require active listening.

To this end you can use the once( callback:Function ) method and provide a callback function to handle the result.

var ref:DatabaseReference = FirebaseDatabase.service.getReference( "test" );

ref.once( function( value:Object ):void 
{
    if (value != null)
    {
        trace( JSON.stringify( value ) );
    }
    else 
    {
        // There was an error
    }
});

The once function takes a Function as a parameter which will be called when the value has been read. This function should have one parameter of type Object which will be the resulting value of the read operation. This value may be null if an error occurred.