pass initial data into a singleton factory in dart - dart

Question

I would like to pass some initial information into a singleton in dart.
Unfortunately, the information I like to access is null (see dartpad output below)
It seems like I get a new instance of my object and not the singleton but I can not wrap my head around it. Any idea?
ElmCommandProvider.fromMetaData
ElmCommandProvider._internal()
ElmCommandProvider._init
ElmCommandProvider()
null
This is the code which can be pasted in DartPad
class Command {
Command(this.i);
final int i;
}
class MetaData {
MetaData(this.i);
final int i;
}
class ElmCommandProvider {
List<Command> commandsList;
bool _lock = false;
static Map<String, MetaData> _metaDataPool;
factory ElmCommandProvider.fromMetaData(Map<String, MetaData> metaDataPool) {
print('ElmCommandProvider.fromMetaData');
assert(!_singleton._lock, "it's a singleton that can't re-defined");
ElmCommandProvider._metaDataPool = metaDataPool;
_singleton._lock = true;
ElmCommandProvider._init();
return _singleton;
}
factory ElmCommandProvider() {
print('ElmCommandProvider()');
return _singleton;
}
static final ElmCommandProvider _singleton =
new ElmCommandProvider._internal();
ElmCommandProvider._internal() {
print('ElmCommandProvider._internal()');
}
ElmCommandProvider._init() {
print('ElmCommandProvider._init');
commandsList =
_metaDataPool.values.map((bloc) => Command(bloc.i)).toList();
}
}
void main() {
ElmCommandProvider.fromMetaData({'1': MetaData(1), '2': MetaData(2)});
print( ElmCommandProvider().commandsList);
}

Answer 1

_init() should not be a constructor. Or at least there is no need for it to be one and it's confusing you. It should be changed to a static method or a private instance method.
When you do commandsList= in ElmCommandProvider._init(), commandsList is referring to the commandsList instance variable in the new ElmCommandProvider object you're creating with the constructor. You likely actually mean to modify the singleton's commandsList so you should have been doing singleton.commandsList = instead of just commandsList =.
Example working code with static method:
class Command {
Command(this.i);
final int i;
}
class MetaData {
MetaData(this.i);
final int i;
}
class ElmCommandProvider {
List<Command> commandsList;
bool _lock = false;
static Map<String, MetaData> _metaDataPool;
factory ElmCommandProvider.fromMetaData(Map<String, MetaData> metaDataPool) {
print('ElmCommandProvider.fromMetaData');
assert(!_singleton._lock, "it's a singleton that can't re-defined");
ElmCommandProvider._metaDataPool = metaDataPool;
_singleton._lock = true;
_init();
return _singleton;
}
factory ElmCommandProvider() {
print('ElmCommandProvider()');
return _singleton;
}
static final ElmCommandProvider _singleton =
new ElmCommandProvider._internal();
ElmCommandProvider._internal() {
print('ElmCommandProvider._internal()');
}
static _init() {
print('ElmCommandProvider._init');
_singleton.commandsList =
_metaDataPool.values.map((bloc) => Command(bloc.i)).toList();
}
}
void main() {
ElmCommandProvider.fromMetaData({'1': MetaData(1), '2': MetaData(2)});
print( ElmCommandProvider().commandsList);
}
Example working code with private instance method:
class Command {
Command(this.i);
final int i;
}
class MetaData {
MetaData(this.i);
final int i;
}
class ElmCommandProvider {
List<Command> commandsList;
bool _lock = false;
static Map<String, MetaData> _metaDataPool;
factory ElmCommandProvider.fromMetaData(Map<String, MetaData> metaDataPool) {
print('ElmCommandProvider.fromMetaData');
assert(!_singleton._lock, "it's a singleton that can't re-defined");
ElmCommandProvider._metaDataPool = metaDataPool;
_singleton._lock = true;
_singleton._init();
return _singleton;
}
factory ElmCommandProvider() {
print('ElmCommandProvider()');
return _singleton;
}
static final ElmCommandProvider _singleton =
new ElmCommandProvider._internal();
ElmCommandProvider._internal() {
print('ElmCommandProvider._internal()');
}
void _init() {
print('ElmCommandProvider._init');
commandsList =
_metaDataPool.values.map((bloc) => Command(bloc.i)).toList();
}
}
void main() {
ElmCommandProvider.fromMetaData({'1': MetaData(1), '2': MetaData(2)});
print( ElmCommandProvider().commandsList);
}