apollo-client/src/main/java/com/ctrip/framework/apollo/internals/DefaultConfig.java/DefaultConfig/updateAndCalcConfigChanges
class DefaultConfig:  private Map<String, ConfigChange> updateAndCalcConfigChanges(Properties newConfigProperties,
      ConfigSourceType sourceType) {
    List<ConfigChange> configChanges =
        calcPropertyChanges(m_namespace, m_configProperties.get(), newConfigProperties);

    ImmutableMap.Builder<String, ConfigChange> actualChanges =
        new ImmutableMap.Builder<>();

    /** === Double check since DefaultConfig has multiple config sources ==== **/

    //1. use getProperty to update configChanges's old value
    for (ConfigChange change : configChanges) {
      change.setOldValue(this.getProperty(change.getPropertyName(), change.getOldValue()));
    }

    //2. update m_configProperties
    updateConfig(newConfigProperties, sourceType);
    clearConfigCache();

    //3. use getProperty to update configChange's new value and calc the final changes
    for (ConfigChange change : configChanges) {
      change.setNewValue(this.getProperty(change.getPropertyName(), change.getNewValue()));
      switch (change.getChangeType()) {
        case ADDED:
          if (Objects.equals(change.getOldValue(), change.getNewValue())) {
            break;
          }
          if (change.getOldValue() != null) {
            change.setChangeType(PropertyChangeType.MODIFIED);
          }
          actualChanges.put(change.getPropertyName(), change);
          break;
        case MODIFIED:
          if (!Objects.equals(change.getOldValue(), change.getNewValue())) {
            actualChanges.put(change.getPropertyName(), change);
          }
          break;
        case DELETED:
          if (Objects.equals(change.getOldValue(), change.getNewValue())) {
            break;
          }
          if (change.getNewValue() != null) {
            change.setChangeType(PropertyChangeType.MODIFIED);
          }
          actualChanges.put(change.getPropertyName(), change);
          break;
        default:
          //do nothing
          break;
      }
    }
    return actualChanges.build();
  }