Feature: added Map to CSV converter

lib/csv.dart

@@ -8,6 +8,7 @@ import 'csv_settings_autodetection.dart';
 
 part 'csv_to_list_converter.dart';
 part 'list_to_csv_converter.dart';
+part 'map_to_csv_converter.dart';
 
 /// The RFC conform default value for field delimiter.
 const String defaultFieldDelimiter = ',';

lib/map_to_csv_converter.dart

@@ -0,0 +1,88 @@
+part of csv;
+
+enum ExtrasActions { raise, ignore }
+
+/// Converts rows -- a [List] of [Map]s into a csv String.
+///
+/// Usage:
+///   var sb = new StringBuffer();
+///
+///   var writer = MapToCsvConverter(sb, field_names);
+///   writer.writeheader();
+///   writer.writerows(data_list);
+///
+///   print(f.toString());
+///
+class MapToCsvConverter {
+  StringBuffer sb;
+  List<String> fieldNames;
+  ExtrasActions extrasAction;
+
+  ListToCsvConverter _listConverter;
+
+  /// This relies on [ListToCsvConverter] to do the actual converting.
+  ///
+  /// Required arguments:
+  ///
+  /// [sb] is [StringBuffer] to write results to
+  /// [fieldNames] is a list of [Map] keys in the desired output order
+  ///
+  /// Optional arguments:
+  ///
+  /// [extrasAction] determines whether or not it is ok for the data
+  /// to have fields not present in [fieldNames]. Will raise an exception
+  /// by default.
+  ///
+  /// Can also receive any arguments that [ListToCsvConverter] takes.
+  ///
+  MapToCsvConverter(
+    this.sb,
+    this.fieldNames, {
+    this.extrasAction: ExtrasActions.raise,
+    String fieldDelimiter: defaultFieldDelimiter,
+    String textDelimiter: defaultTextDelimiter,
+    String textEndDelimiter,
+    String eol: defaultEol,
+    bool delimitAllFields: defaultDelimitAllFields,
+  }) {
+    _listConverter = new ListToCsvConverter(
+      fieldDelimiter: fieldDelimiter,
+      textDelimiter: textDelimiter,
+      textEndDelimiter: textEndDelimiter,
+      eol: eol,
+      delimitAllFields: delimitAllFields,
+    );
+  }
+
+  void writerow(Map rowdict) {
+    _listConverter.convertSingleRow(sb, _mapToList(rowdict));
+    sb.write(_listConverter.eol);
+  }
+
+  void writerows(List<Map> rowdicts) {
+    rowdicts.forEach((row_dict) {
+      writerow(row_dict);
+    });
+  }
+
+  void writeheader() {
+    var header_map = new Map.fromIterable(fieldNames,
+        key: (field) => field, value: (field) => field);
+    writerow(header_map);
+  }
+
+  List<String> _mapToList(Map rowdict) {
+    if (extrasAction == ExtrasActions.raise) {
+      var extraFields = _findExtraFields(rowdict);
+      if (extraFields.length > 0) {
+        throw new Exception(
+            'Map contains fields not in fieldNames: "${extraFields.join(",")}"');
+      }
+    }
+
+    return fieldNames.map((field) => rowdict[field]).toList();
+  }
+
+  List<String> _findExtraFields(Map data) =>
+      data.keys.toList()..removeWhere((e) => fieldNames.contains(e));
+}

test/map_to_csv_test.dart

@@ -0,0 +1,134 @@
+import 'package:test/test.dart';
+import "package:csv/csv.dart";
+
+void main() {
+  var sb = new StringBuffer();
+
+  tearDown(() {
+    sb.clear();
+  });
+
+  group("MapToCsvConverter Tests", () {
+    test("writes header row", () {
+      var fieldNames = ["one", "two", "three"];
+      var writer = new MapToCsvConverter(sb, fieldNames);
+      writer.writeheader();
+
+      expect(sb.toString(), equals("one,two,three${defaultEol}"));
+    });
+
+    test("writes regular row", () {
+      var fieldNames = ["one", "two", "three"];
+      var writer = new MapToCsvConverter(sb, fieldNames);
+
+      var data = {"one": "this", "two": "thing", "three": "works"};
+      writer.writerow(data);
+
+      expect(sb.toString(), equals("this,thing,works${defaultEol}"));
+    });
+
+    test("writes multiple rows", () {
+      var fieldNames = ["one", "two", "three"];
+      var writer = new MapToCsvConverter(sb, fieldNames);
+
+      var data = [
+        {"one": "this", "two": "thing", "three": "works"},
+        {"one": "second", "two": "works", "three": "too"}
+      ];
+      writer.writerows(data);
+
+      expect(
+          sb.toString(),
+          equals("this,thing,works${defaultEol}"
+              "second,works,too${defaultEol}"));
+    });
+
+    test("ignores when extra fields present in data", () {
+      var fieldNames = ["one", "two", "three"];
+      var writer = new MapToCsvConverter(sb, fieldNames,
+          extrasAction: ExtrasActions.ignore);
+
+      var data = {
+        "one": "this",
+        "two": "thing",
+        "three": "works",
+        "extra": "ignore"
+      };
+      writer.writerow(data);
+
+      expect(sb.toString(), equals("this,thing,works${defaultEol}"));
+    });
+
+    test("raises exception when extra fields present in data", () {
+      var fieldNames = ["one", "two", "three"];
+      var writer = new MapToCsvConverter(sb, fieldNames,
+          extrasAction: ExtrasActions.raise);
+
+      var data = {
+        "one": "this",
+        "two": "thing",
+        "three": "works",
+        "extra": "raise error",
+        "extra_two": "other",
+      };
+
+      var expectedMessage =
+          'Map contains fields not in fieldNames: "extra,extra_two"';
+      expect(() => writer.writerow(data),
+          throwsA(predicate((e) => e.message == expectedMessage)));
+    });
+
+    test("can customize field delimiter", () {
+      var fieldNames = ["one", "two", "three"];
+      var writer = new MapToCsvConverter(sb, fieldNames, fieldDelimiter: "|");
+
+      var data = [
+        {"one": "this", "two": "thing", "three": "works"},
+        {"one": "second", "two": "works", "three": "too"}
+      ];
+      writer.writerows(data);
+
+      expect(
+          sb.toString(),
+          equals("this|thing|works${defaultEol}"
+              "second|works|too${defaultEol}"));
+    });
+
+    test("can customize text delimiter", () {
+      var fieldNames = ["one", "two", "three"];
+      var writer = new MapToCsvConverter(sb, fieldNames, textDelimiter: "'");
+
+      var data = [
+        {"one": "that's", "two": "thing", "three": "works"},
+      ];
+      writer.writerows(data);
+
+      expect(sb.toString(), equals("'that''s',thing,works${defaultEol}"));
+    });
+
+    test("can customize eol delimiter", () {
+      var fieldNames = ["one", "two", "three"];
+      var writer = new MapToCsvConverter(sb, fieldNames, eol: "***");
+
+      var data = [
+        {"one": "this", "two": "thing", "three": "works"},
+      ];
+      writer.writerows(data);
+
+      expect(sb.toString(), equals("this,thing,works***"));
+    });
+
+    test("can customize text end delimiter", () {
+      var fieldNames = ["one", "two", "three"];
+      var writer =
+          new MapToCsvConverter(sb, fieldNames, textEndDelimiter: "&");
+
+      var data = [
+        {"one": '"air quotes"', "two": "thing", "three": "works"},
+      ];
+      writer.writerows(data);
+
+      expect(sb.toString(), equals('""air quotes"&,thing,works$defaultEol'));
+    });
+  });
+}