Cv Models

cv models

package:cv maps Dart objects to Map<String, Object?> and back with no code generation. A model is a class whose fields are CvField objects. The field holds the serialized key and a mutable value. The model lists its fields once and gets toMap(), fromMap(), ==, clone(), copyFrom() and JSON for free.

Guidelines

• Extend CvModelBase. Declare each field as final x = CvField<T>('key'); and override CvFields get fields => [...] listing every field. A field not in fields is never serialized.
• Read and write values through the field: model.title.v (alias .value). Use .valueOrThrow when a value is required, .valueOrNull when optional.
• Distinguish unset from null. An unset field is absent from toMap(). A field set to null with .v = null is present with a null value. setValue(null) unsets unless presentIfNull: true. clear() unsets. hasValue is true for present values, including null. isNull is true for both unset and null.
• fromMap() merges: only keys present in the map are written, other fields keep their value. Call clear() first, or build a new object with map.cv<T>(), when you need a full replacement.
• Register every model once with cvAddConstructor(MyModel.new) (or cvAddConstructors([...])) before calling map.cv<T>(), json.cv<T>(), cvNewModel<T>() or clone(). A missing registration throws CvBuilderException with "Missing builder for 'T'". Group registrations in an idempotent initXxxBuilders() function per package.
• Field types must be String, int, double, num, bool, DateTime, List, Map, an enum or another CvModel. Use CvListField<T> for lists of basic types, CvModelField<T> for one nested model, CvModelListField<T> for a list of models and CvModelMapField<T> for a Map<String, T> of models. Nested model types need a registered constructor too.
• Numbers read from a map are coerced to the field type (1.7 into CvField<int> gives 2, 1 into CvField<double> gives 1.0).
• Use CvField.encodedEnum('key', MyEnum.values) to store an enum as its name string, and CvField.encodedDateTime('key') to store a DateTime as an ISO 8601 string. A plain CvField<DateTime> keeps the DateTime object in the map, which is fine for sembast or Firestore but not for JSON.
• Import package:cv/cv_json.dart (re-exports cv.dart) to get toJson(), toJsonPretty(), String.cv<T>() and String.cvList<T>().
• Equality (==) and cvModelsAreEquals compare toMap() content deeply, so two models with the same values are equal.
• Prefer CvFields (a List<CvField>) as the type of fields.
• Put derived helpers in an extension MyModelExt on MyModel rather than in the class, and keep the class limited to field declarations.

Examples

Define a model and convert it

import 'package:cv/cv.dart';

class Note extends CvModelBase {
  final title = CvField<String>('title');
  final content = CvField<String>('content');
  final date = CvField<DateTime>('date');

  @override
  CvFields get fields => [title, content, date];
}

void main() {
  var note = Note()
    ..title.v = 'My note'
    ..content.v = 'My note context'
    ..date.v = DateTime(2021, 08, 16);
  print(note.toMap());
  // {title: My note, content: My note context, date: 2021-08-16 00:00:00.000}

  note = Note()..fromMap({'title': 'Other'});
  print(note.title.v); // Other
  print(note.content.hasValue); // false
}

Register constructors and build from a map

void initNoteBuilders() {
  cvAddConstructor(Note.new);
}

void main() {
  initNoteBuilders();
  var note = {'title': 'My note'}.cv<Note>();
  var notes = [<String, Object?>{'title': 'a'}, {'title': 'b'}].cv<Note>();
  var empty = cvNewModel<Note>();
  var copy = note.clone();
}

Nested models and lists

class Point extends CvModelBase {
  final x = CvField<int>('x');
  final y = CvField<int>('y');

  @override
  CvFields get fields => [x, y];
}

class Shape extends CvModelBase {
  final origin = CvModelField<Point>('origin');
  final points = CvModelListField<Point>('points');
  final tags = CvListField<String>('tags');
  final named = CvModelMapField<Point>('named');

  @override
  CvFields get fields => [origin, points, tags, named];
}

void main() {
  cvAddConstructors([Point.new, Shape.new]);
  var shape = {
    'origin': {'x': 0, 'y': 0},
    'points': [
      {'x': 1, 'y': 2},
      {'x': 3, 'y': 4},
    ],
    'tags': ['a', 'b'],
  }.cv<Shape>();
  print(shape.points.v![1].y.v); // 4

  // Building nested content in code
  shape.origin.v = Point()
    ..x.v = 10
    ..y.v = 20;
  shape.points.v = [Point()..x.v = 1];
  // Or create an empty child through the field:
  var child = shape.points.create({})..x.v = 5;
  shape.points.v!.add(child);
}

Enum and DateTime encoded for JSON

enum Status { draft, published }

class Article extends CvModelBase {
  final status = CvField.encodedEnum('status', Status.values);
  final createdAt = CvField.encodedDateTime('createdAt');

  @override
  CvFields get fields => [status, createdAt];
}

void main() {
  cvAddConstructor(Article.new);
  var article = Article()
    ..status.v = Status.published
    ..createdAt.v = DateTime.utc(2024, 1, 2);
  print(article.toMap());
  // {status: published, createdAt: 2024-01-02T00:00:00.000Z}
  print(article.status.v); // Status.published
}

JSON

import 'package:cv/cv_json.dart';

void main() {
  cvAddConstructor(Note.new);
  var note = '{"title":"My note"}'.cv<Note>();
  var notes = '[{"title":"a"},{"title":"b"}]'.cvList<Note>();
  print(note.toJson()); // {"title":"My note"}
  print(notes.toJson()); // [{"title":"a"},{"title":"b"}]
  print(note.toJsonPretty());
}

Null, unset and partial maps

var note = Note();
note.toMap(); // {}
note.title.v = null;
note.toMap(); // {title: null}
note.title.setValue(null);
note.toMap(); // {}
note.toMap(includeMissingValue: true); // {title: null, content: null, date: null}

// Only export or import some columns
note.toMap(columns: ['title']);
note.fromMap(map, columns: ['title', 'content']);
note.copyFrom(otherNote, columns: ['title']);

Reuse fields through a mixin

mixin TimestampsMixin on CvModelBase {
  final createdAt = CvField.encodedDateTime('createdAt');
  final updatedAt = CvField.encodedDateTime('updatedAt');

  CvFields get timestampFields => [createdAt, updatedAt];
}

class Post extends CvModelBase with TimestampsMixin {
  final title = CvField<String>('title');

  @override
  CvFields get fields => [title, ...timestampFields];
}

Common mistakes

• Forgetting to add a new field to fields. It silently stays out of the map.
• Calling map.cv<T>() before cvAddConstructor(T.new).
• Mixing base classes in one cvAddConstructors call. Register CvModelBase models and CvMapModelBase models in separate calls.
• Expecting fromMap() to reset fields absent from the map.
• Using CvField<DateTime> in a model that is sent as JSON. Use CvField.encodedDateTime instead.
• Declaring a field as CvField<MyModel>. Use CvModelField<MyModel> so the nested map is converted.

More

See references/advanced.md for CvMapModelBase (models that keep unknown keys), raw Model map helpers, tree paths (cvPath, cvTreeValueAtPath), withParent flattened keys, multi-type fields, and the test helpers in package:cv/cv_matcher.dart (cvEquals, cvEqualsDiffReport, fillModel).