apollo-client/src/main/java/com/ctrip/framework/apollo/spring/config/CachedCompositePropertySource.java/CachedCompositePropertySource/addPropertySource
class CachedCompositePropertySource:  @Override
  public void addPropertySource(PropertySource<?> propertySource) {
    super.addPropertySource(propertySource);
    if (propertySource instanceof ConfigPropertySource) {
      ((ConfigPropertySource) propertySource).addChangeListener(this);
    }
  }