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Flutter – What is MapEntry used for?

I am new to dart, and I came across the following code. This apparently updates the document in the firebase. Can someone explain in detail, what it really does?


abstract class FirestoreDocumentUpdater {
  static Future<void> update(

      // ignore: use_function_type_syntax_for_parameters

      DocumentReference<Map<String, dynamic>> documentRef,
      Map<String, dynamic> map) {
    debugPrint('update function called');
    return documentRef.update(
      Map.fromEntries(
        map.entries.map(
          (e) {
            return MapEntry(
                e.key,
                switch (e.value) {
                  DateTime t => toJsonDateTime(t),
                  Duration d => toJsonDuration(d),
                  Enum t => t.name,
                  _ => e.value,
                });
          },
        ),
      ),
    );
  }
}

Timestamp? toJsonDateTime(DateTime? dateTime) {
  if (dateTime == null) return null;
  return Timestamp.fromDate(dateTime);
}


int toJsonDuration(Duration duration) {
  return duration.inMicroseconds;
}
Tags:

2

Answers


  1. 0

    A MapEntry is simply the elements of a Map. Consider this map:

    Map a = {
   'key' : 'value',
   'foo' : 'bar'
}

    This is a map with map entries MapEntry('key','value') and MapEntry('foo','bar');

    Now what that code does is simply change the values of a Map so that certain values are converted to another value. It turns this map:

    enum ExampleEnum {
  test
}

Map example = {
   'date' : DateTime.now(),
   'duration' : Duration(seconds: 6),
   'enum' : ExampleEnum.test,
   'other' : 'other'
}

    into

    Map example = {
   'date' : someTimeStamp,
   'duration' : 6000000,
   'enum' : 'test',
   'other' : 'other'
}
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  2. 0

    The code is slightly more confusing than it needs to be due to a poor name choice. For this reason I am replacing the parameter map with inputMap.

    abstract class FirestoreDocumentUpdater {

  /// Merges the entries of [inputMap] with the entries of the
  /// referenced map.  
  /// Note: The entries of [inputMap] are transformed prior to 
  /// merging according to the rules: 
  /// * value is DateTime => toJsonDateTime(value)
  /// * value is Duration => toJsonDuration(value)
  /// * value is Enum => value.name
  static Future<void> update(
    DocumentReference<Map<String, dynamic>> documentRef,
    Map<String, dynamic> inputMap,
  ) {
    debugPrint('update function called');
    return documentRef.update(
      Map.fromEntries( // Creates a new map from map entries.
        inputMap.entries.map(  //  Iterates through `entries` and calls the 
          (e) {                //  method `map` transforming entries with 
            return MapEntry(   //  value of type `DateTime`, `Duration`, `Enum`.
                e.key,
                switch (e.value) {
                  DateTime t => toJsonDateTime(t),
                  Duration d => toJsonDuration(d),
                  Enum t => t.name,
                  _ => e.value,
                });
          },
        ),
      ),
    );
  }
}
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