concept G22Variable extends ConceptWithChangeListeners is IPersistenceInfo {
  settableWithVar S name;
  volatile O value;
  settableWithVar bool persistent;
  
  // big = value is stored in separate file
  settableWithVar bool big;
  
  // field "value" is persisted iff field "persistent" is set
  public Map _persistenceInfo() { ret litmap(value := persistent && !big); }
  
  public simplyCached FieldVar<O> varValue() {
    ret new FieldVar<O>(this, "value", () -> value(), value -> value(value));
  }
  
  void setValueIfNull(O defaultValue) {
    if (defaultValue != null && value == null) value(defaultValue);
  }
  
  O get aka value() {
    if (value != null) ret value;
    if (big) ret getBigValue();
    ret value;
  }
  
  G22Utils g22utils() {
    ret main g22utils(_concepts());
  }
  
  File bigFile() {
    ret newFile(assertNotNull(g22utils().projectDir()), id + "-" + sanitizeFileName(name) + ".structgz");
  }
  
  O getBigValue() {
    File f = bigFile();
    ret value = unstructureGZFile(f, g22utils().classFinder());
  }
  
  void setBigValue(O value) {
    if (value == null) {
      if (hasBigValue()) {
        deleteBigValue();
        change();
      }
    } else {
      File f = bigFile();
      saveGZStructureToFile(f, value);
      change();
    }
  }
  
  void deleteBigValue() {
    deleteFile(bigFile());
  }
  
  bool hasBigValue() {
    ret fileExists(bigFile());
  }
  
  void makeBig {
    if (big) ret;
    setBigValue(value);
    big(true);
    persistent(true);
  }
  
  void makeSmall {
    if (!big) ret;
    get(); // will cache the value
    big(false);
    deleteBigValue();
  }
  
  selfType value aka setValue(O o) {
    if (big)
      setBigValue(o);
    else if (!eq(value, o)) {
      value = o;
      change();
    }
    this;
  }
  
  bool notNull() {
    ret big ? hasBigValue() : value != null;
  }
  
  void unload() {
    if (big) value = null;
  }
}