Using a stream for PropertyChanges - dart

Question

Trying to understand how streams work so i wrote this
class ViewModelBase{
final List<PropertyChangedRecord> _changeRecords = new List<PropertyChangedRecord>();
Stream<PropertyChangedRecord> _changes;
Stream<PropertyChangedRecord> get changes{
//lazy initialization
if(_changes==null)
_changes = new Stream<PropertyChangedRecord>.fromIterable(_changeRecords);
return _changes;
}
_raisePropertyChanged(oldValue,newValue,propertySymbol){
if(oldValue!=newValue){
_changeRecords.add(new PropertyChangedRecord(this, propertySymbol,oldValue,newValue));
}
return newValue;
}
}
class PropertyChangedRecord{
final ViewModelBase viewModel;
final Symbol propertySymbol;
final Object newValue;
final Object oldValue;
PropertyChangedRecord(this.viewModel,this.propertySymbol,this.oldValue,this.newValue);
}
and used it as
void main() {
var p = new Person('waa',13);
p.age = 33334;
p.name = 'dfa';
p.changes.listen((p)=>print(p));
p.age = 333834;
p.name = 'dfia';
}
class Person extends ViewModelBase{
String _name;
String get name => _name;
set name(String value) => _name = _raisePropertyChanged(_name,value,#name);
int _age;
int get age => _age;
set age(int value) => _age = _raisePropertyChanged(_age,value,#age);
Person(this._name,this._age);
}
and got the following exception
Uncaught Error: Concurrent modification during iteration: Instance(length:4) of '_GrowableList'
which i think is because the stream is removing items from the list while new PropertyChangedRecords are being added, how do i go around that?

Answer 1

The error could be caused by adding an item while the stream iterates the list.
You could use a StreamController to create the stream instead (see How to pass a callback function to a StreamController for an example).
class ViewModelBase{
//final List<PropertyChangedRecord> _changeRecords = new List<PropertyChangedRecord>();
//Stream<PropertyChangedRecord> _changes;
final StreamController _changeRecords = new StreamController<PropertyChangedRecord>();
Stream<PropertyChangedRecord> get changes => _changeRecords.stream;
_raisePropertyChanged(oldValue,newValue,propertySymbol){
if(oldValue!=newValue){
_changeRecords.add(new PropertyChangedRecord(this, propertySymbol,oldValue,newValue));
}
return newValue;
}
}