What's the setter of a built_value's object

Question

I'm trying using built_value in flutter, and found if I declared a Type use built_value, I can commonly use dot syntax to assign value to it's properties: my declaration is:

abstract class Post implements Built<Post, PostBuilder> {
    Post._();
    int get userId;
    int get id;
    String get title;
    String get body;
    factory Post([updates(PostBuilder b)]) = _$Post;
    static Serializer<Post> get serializer => _$postSerializer;
}

and use it like this:

Post p = Post();
p.titie = "hello world";

got error:

[dart] No setter named 'title' in class 'Post'.

I'm not familar the builder thing, even I found out that the PostBuilder have the setter of all properties: PostBuilder().title = 'hello world'; but how can I use it?

Answer 1

BuiltValue classes are immutable. That's one of its main features.
Immutability means you can't modify an instance. Each modification has to result in a new instance.

One of several ways is

p = (p.toBuilder().titie = 'hello world').build();

to get an updated instance.

Alternatively

p = p.rebuild((b) => b..title = 'hello world');

Answer 2

In fact, there's a full example alone with source code.

// Values must be created with all required fields.
final value = new SimpleValue((b) => b..anInt = 3);

// Nullable fields will default to null if not set.
final value2 = new SimpleValue((b) => b
..anInt = 3
..aString = 'three');

// All values implement operator==, hashCode and toString.
assert(value != value2);

// Values based on existing values are created via "rebuild".
final value3 = value.rebuild((b) => b..aString = 'three');
assert(value2 == value3);

// Or, you can convert to a builder and hold the builder for a while.
final builder = value3.toBuilder();
builder.anInt = 4;
final value4 = builder.build();
assert(value3 != value4);