如何在 Dart 中将 List<Event> 转换为 Map<DateTime, List<Event>>？

Question

我想在 Dart 中将 List 转换为 Map，如下所示。有什么好的方法吗？

class Event {
  const Event(this.date, this.title);
  final DateTime date;
  final String title;
}

List<Event> eventList = [
  Event(DateTime(2021, 1, 1), "buy milk"),
  Event(DateTime(2021, 1, 1), "go to gym"),
  Event(DateTime(2021, 1, 2), "running"),
];

// I want to convert map as follows.

Map<DateTime, List<Event>> eventDayMap = {
  DateTime(2021, 1, 1): [Event(DateTime(2021, 1, 1), "buy milk"), Event(DateTime(2021, 1, 1), "go to gym")],
  DateTime(2021, 1, 2): [Event(DateTime(2021, 1, 2), "running")]
};

Answer 1

假设每个 DateTime 只存储日期而不存储时间，我会使用 ??= 默认为每个日期添加一个空列表，然后我只添加列表元素：

var eventDayMap = <DateTime, List<Event>>{};
for (var event in eventList) {
  (eventDayMap[event.date] ??= []).add(event);
}

如果您正在存储时间信息但想按日期索引，那么您必须先规范化键：

DateTime dateOnly(DateTime dateTime) {
  if (dateTime.isUtc) {
    return DateTime.utc(dateTime.year, dateTime.month, dateTime.day);
  } else {
    return DateTime(dateTime.year, dateTme.month, dateTime.day);
  }
}

var eventDayMap = <DateTime, List<Event>>{};
for (var event in eventList) {
  (eventDayMap[dateOnly(event.date)] ??= []).add(event);
}

如果您希望事件按时间排序：

for (var list in eventDayMap.values) {
  list.sort((event1, event2) => event1.date.compareTo(event2.date));
}

如何在 Dart 中将 List<Event> 转换为 Map<DateTime, List<Event>>？

How to convert List<Event> to Map<DateTime, List<Event>> in Dart?

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