public interface

# ChildEventListener

com.google.firebase.database.ChildEventListener

## Class Overview

Classes implementing this interface can be used to receive events about changes in the child locations of a given `DatabaseReference` ref. Attach the listener to a location using `addChildEventListener(ChildEventListener)` and the appropriate method will be triggered when changes occur.

## Summary

Public Methods
abstract void | onCancelled(DatabaseError error) This method will be triggered in the event that this listener either failed at the server, or is removed as a result of the security and Firebase rules.
abstract void | onChildAdded(DataSnapshot snapshot, String previousChildName) This method is triggered when a new child is added to the location to which this listener was added.
abstract void | onChildChanged(DataSnapshot snapshot, String previousChildName) This method is triggered when the data at a child location has changed.
abstract void | onChildMoved(DataSnapshot snapshot, String previousChildName) This method is triggered when a child location's priority changes.
abstract void | onChildRemoved(DataSnapshot snapshot) This method is triggered when a child is removed from the location to which this listener was added.

## Public Methods

#### public abstract void onCancelled (DatabaseError error)

This method will be triggered in the event that this listener either failed at the server, or is removed as a result of the security and Firebase rules. For more information on securing your data, see: Security Quickstart

##### Parameters

error | A description of the error that occurred

#### public abstract void onChildAdded (DataSnapshot snapshot, String previousChildName)

This method is triggered when a new child is added to the location to which this listener was added.

##### Parameters

snapshot | An immutable snapshot of the data at the new child location
previousChildName | The key name of sibling location ordered before the new child. This will be null for the first child node of a location.

#### public abstract void onChildChanged (DataSnapshot snapshot, String previousChildName)

This method is triggered when the data at a child location has changed.

##### Parameters

snapshot | An immutable snapshot of the data at the new data at the child location
previousChildName | The key name of sibling location ordered before the child. This will be null for the first child node of a location.

#### public abstract void onChildMoved (DataSnapshot snapshot, String previousChildName)

This method is triggered when a child location's priority changes. See `setPriority(Object)` and Ordered Data for more information on priorities and ordering data.

##### Parameters

snapshot | An immutable snapshot of the data at the location that moved.
previousChildName | The key name of the sibling location ordered before the child location. This will be null if this location is ordered first.

#### public abstract void onChildRemoved (DataSnapshot snapshot)

This method is triggered when a child is removed from the location to which this listener was added.

##### Parameters

snapshot | An immutable snapshot of the data at the child that was removed.