Don't execute assignment if value is null-CodePudding

I am still coming up to speed with dart and wanted to know if there was an easier way to not execute a statement if the value is null. See example below:

I can always do the if statements below for setting field3 and field4, but felt like something like field5 should work. But when I try to do that, it complains that a null check operator is used on a null value.

Also I don't want to change the Map to have a dynamic value.

Is there a single one liner to do what I am trying to do, or do I just need to check for null before setting the value.

  Map<String, Object> myMap = {};

  print('running now');
  try {
    myMap['field1'] = DummyClass.getString('hello');
    myMap['field2'] = DummyClass.getString('good');

    //Is there a more concise way to do this than the 2 options below?

    if (DummyClass.getOptionalString('goodbye') != null) {
      myMap['field3'] = DummyClass.getOptionalString('goodbye')!;
    }

    String? temp = DummyClass.getOptionalString('go');
    if (temp != null) {
      myMap['field4'] = temp;
    }

    // This gives an error 'null check operator used on a null value'
    // myMap['field5'] ??= DummyClass.getOptionalString('to')!;

  } catch (e) {
    print('error condition, $e');
  }

  print(myMap);
}

class DummyClass {

  static String getString(String? strParam) {
    String? retString = getOptionalString(strParam);
    if (retString == null) {
      throw ('nulls are not allowed');
    }
    return retString;
  }

  static String? getOptionalString(String? strParam) {
    if (strParam == null || strParam.length < 3) {
      return null;
    }
    return strParam;
  }
}

CodePudding user response：

The issue is that the ??= operator assigns to the left if it is null. Expanded, it would look something like this:

a ??= b;

// Equivalent to:

if (a == null) {
  a = b;
}

Which is not something that you're trying to achieve. AFAIK, there is no such operator yet in Dart. However, you can try this:

final possiblyNullValue = '';
final myMap = <String, String>{};

myMap['key'] = possiblyNullValue ?? myMap['key'];

// Equivalent to:

if (possiblyNullValue != null) {
  myMap['key'] = possiblyNullValue;
}

// or:

myMap['key'] = possiblyNullValue != null? possiblyNullValue : myMap['key'];

Which would work in your case as a one-liner.

CodePudding user response：

You could create your map with all entries, even null, and then filter the null values out:

void main() {
  try {
    final myMap = <String, dynamic>{
      'field1': DummyClass.getString('hello'),
      'field2': DummyClass.getString('good'),
      'field3': DummyClass.getOptionalString('goodbye'),
      'field4': DummyClass.getOptionalString('go'),
    }..removeWhere((k, v) => v == null);
    print(myMap);
  } catch (e) {
    print('error condition, $e');
  }
}

CodePudding user response：

There's no built-in way to do what you want, but you could write a function (or extension method) to do it. For example:

extension MapTrySet<K, V> on Map<K, V> {
  void trySet(K key, V? value) {
    if (value != null) {
      this[key] = value;
    }
  }
}

and then you could do:

myMap.trySet('field3', DummyClass.getOptionalString('goodbye'));
myMap.trySet('field4', DummyClass.getOptionalString('go'));

Alternatively, if you really want to use normal Map syntax, you could create your own Map class that has a void operator []=(K key, V? value) override and does nothing when the value is null, but that probably would not be worth the effort.