Dart Set.add用法及代码示例

将value 添加到集合中。

如果集合中还没有value(或相等的值)，则返回true。否则返回 false 并且集合不会改变。

例子：

final dateTimes = <DateTime>{};
final time1 = DateTime.fromMillisecondsSinceEpoch(0);
final time2 = DateTime.fromMillisecondsSinceEpoch(0);
// time1 and time2 are equal, but not identical.
assert(time1 == time2);
assert(!identical(time1, time2));
final time1Added = dateTimes.add(time1);
print(time1Added); // true
// A value equal to time2 exists already in the set, and the call to
// add doesn't change the set.
final time2Added = dateTimes.add(time2);
print(time2Added); // false

print(dateTimes); // {1970-01-01 02:00:00.000}
assert(dateTimes.length == 1);
assert(identical(time1, dateTimes.first));
print(dateTimes.length);