Dart / Flutter Conditional Named Parameters

Question

In Javascript, to conditionally add a value to an Object, I can do something like this:

const obj = {
   ...(myCondition && {someKey: "someValue"})
}

Can I do something similar to pass in a named parameter in Dart / Flutter? For example, if I have the below code, is there a way to conditionally pass the place parameter only if it exists in the json passed into the fromJson factory function.

factory SearchResult.fromJson(Map<String, dynamic> json) {
    return SearchResult(
      id: json['id'],
      displayString: json['displayString'],
      name: json['name'],
      recordType: json['recordType'],
      collection: json['collection'],
      place: GeoJson.fromJson(json['place']),
      properties: json['properties']
    );
  }

Is not passing value to named parameter different from passing null instead? Because I usually do something like place: GeoJson.fromJson(json['place']) ?? null — Federick Jonathan
– Federick Jonathan, Commented Feb 5, 2020 at 3:49
@FederickJonathan that's a good point. The problem that I face is that if I pass null, the GeoJson.fromJson method throws an error because it expects a value. Is there a way to allow a value to be a String or null kind of like in Typescript you can say private someAttribute: string | null. — tlm
– tlm, Commented Feb 5, 2020 at 11:19
In that case, you could do bool isPlaceNotEmpty = json['place'] != null. Then place: isPlaceNotEmpty ? GeoJson.fromJson(json['place']) : null. You won't call fromJson and passing null value this way — Federick Jonathan
– Federick Jonathan, Commented Feb 5, 2020 at 16:35

Richard Ambler · Accepted Answer · 2020-02-05 07:02:44Z

4

You might be looking for Dart's collection operators, specifically collection-if and collection-for capabilities.

You can, for example do something like:

final map = {
  'key1': 'value1',
  'key2': 'value2',
  if (myCondition) 'key3': 'value3'
};

This works in lists, too:

final list = ['value1', 'value2', if (myCondition) 'value3'];

In this case, you might be after something along the lines of:

final keys = [
  'id',
  'displayString',
  'name',
  'recordType',
  'collection',
  'place',
  'properties'
],
obj = {for (final key in keys) if (json.containsKey(key)) key: json[key]};

answered Feb 5, 2020 at 7:02

Richard Ambler

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1 Comment

tlm Over a year ago

That's for sure helpful. Though, I am trying to figure out a way to conditionally (if it exists) pass a parameter to the fromJson method above rather than add it to an object. Seems my only option might be to handle the null when passing values in.

Federick Jonathan · Accepted Answer · 2020-02-05 16:38:23Z

0

factory SearchResult.fromJson(Map<String, dynamic> json) {
    var isNotEmpty = json['place'] != null;

    return SearchResult(
      id: json['id'],
      displayString: json['displayString'],
      name: json['name'],
      recordType: json['recordType'],
      collection: json['collection'],
      /// This way you won't call fromJson and passing null if json['place'] is null in the first place
      place: isNotEmpty ? GeoJson.fromJson(json['place']) : null,
      properties: json['properties']
    );
  }

answered Feb 5, 2020 at 16:38

Federick Jonathan

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Comments

Kohls · Accepted Answer · 2020-02-06 14:44:25Z

In dart if you receive a null object you can use double question mark operator:

??

What it means? : If object is null then take what it is after double question mark

var b = null;
var a = b ?? 'b was null, you got me!';
print(a);

result:

a: b was null, you got me!

For example:

factory SearchResult.fromJson(Map<String, dynamic> json) {
    var isNotEmpty = json['place'] != null;

    return SearchResult(
      id: json['id'] ?? 0,
      displayString: json['displayString'] ?? '',
      name: json['name'] ?? '',
      recordType: json['recordType'] ?? '',
      collection: json['collection'] ? '',
      place: GeoJson.fromJson(json['place'] ?? {}),
      properties: json['properties'] ?? [],
    );
  }

Null safe types are fun to use

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