Make JsonKey's defaultValue a Function? type

[erf]

I would like to supply a DateTime.now() as a defaultValue to JsonKey, but as it's not a const it's not possible to add it, however if it was of type Function? i could pass it a function which would return a value in the same way as toJson and fromJson, e.g. static DateTime newDate() => DateTime.now(); This would be a breaking change so not sure if it's feasible, but it would make the API more flexible perhaps?

Dart SDK version: 2.17.6 (stable) (Tue Jul 12 12:54:37 2022 +0200) on "macos_arm64"

[kevmoo]

You know what I COULD do. We could allow defaultValue (which is already typed as Object?) to accept a function of type T Function() where T is the field type. Not a bad idea!

[kevmoo]

Not a priority for me, but I'd accept a reasonable PR.

[erf]

I'm looking into the code and perhaps point me in the right direction if you have an idea already?

If i try to pass a function expression to the current defaultValue type of Object? it complains with the following:

Arguments of a constant creation must be constant expressions.
Try making the argument a valid constant, or use 'new' to call the constructor.dart[const_with_non_constant_argument](https://dart.dev/diagnostics/const_with_non_constant_argument)

The JsonKey is an annotation type so it's constructor needs to be const it seem and a function expression is apparently not that.

Were you thinking of changing the type of defaultValue from Object? to something else?

[kevmoo]

@erf – you will ONLY be able to pass in top-level or static functions. No instance functions will be allowed.

[erf]

@kevmoo that seem to work now - so not sure how to improve this per now. I'm closing this. Feel free to open this if you have any suggestions for further improvement.

[kevmoo]

@erf – like...the feature actually works? I don't remember adding that feature. (goes to look at code...)

[kevmoo]

This is NOT implemented. Build will break if you use a function. I actually have a todo here for this!

[kevmoo]

@erf – see #1216

[erf]

Sorry, i did not test it well, i just did not get any errors, i guess i misunderstood. It seems like you've fixed it now? Great stuff! I tried to pull down a local version now and link to it using dependency_overrides and path given a top function which returned a date as a defaultValue but it seem to be null. Must be doing something wrong? (i did pub upgrade and checked out the correct branch). Btw, maybe you could add an example on how to use it in json_annotation

[eximius313]

@kevmoo - when do you plan the release?

[kevmoo]

Hopefully today!

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[kevmoo]

Done! v6.5.0

[erf]

Are you able to pass a function which returns a DateTime now in the defaultValue? I can't seem to make that work.

[kevmoo]

You just need to use a top-level or static function.

[kevmoo]

Always good to share source code...

[erf]

I tried the following:

@JsonSerializable()
class Task {
  @JsonKey(defaultValue: '')
  String task;

  @JsonKey(defaultValue: DateTime.now)
  DateTime? date0;
}

I also tried:

DateTime getDateTime() => DateTime.now();


@JsonSerializable()
class Task {
  @JsonKey(defaultValue: '')
  String task;

  @JsonKey(defaultValue: getDateTime)
  DateTime? date0;
}

The last one produces:

Task _$TaskFromJson(Map<String, dynamic> json) => Task(
      json['task'] as String? ?? '',
      date0: json['date0'] == null
          ? null
          : DateTime.parse(json['date0'] as String) ?? getDateTime(),
      date1: json['date1'] == null
          ? null
          : DateTime.parse(json['date1'] as String),
    );

Here the getDateTime() has a squiggly yellow line indicating that:

"The left operand can't be null, so the right operand is never executed.
Try removing the operator and the right operand."

It also seem strange that getDateTime() is not called if json['date0'] == null ..?

[kevmoo]

Remove the ? from date0.

[kevmoo]

Hrm...I'm digging now and seeing the problem. Will have to investigate.

[kevmoo]

Will need to follow-up here: #1219

[erf]

Ok. I was thinking if i kept the date values optional / nullable, i did not have to set them at construction time and then if i write them to json or parse them and the value is null, then the default value is used, not sure if this is possible. When i think about it i could just make them non-nullable, and just make a custom constructor for these cases and set the date values there.

[kevmoo]

@erf you actually FOUND A BUG!! It affects BigInt, DateTime and Uri. Completely unrelated to this feature.

Well done!

[kevmoo]

@erf – well, it's weird. Turns out the type helpers for certain types NEED to explicitly support default values. I'd never done "the work" for three types, because they weren't const-able. That's now fixed! See #1220

[kevmoo]

Fixed now and published @erf