dart How to get an enum with index?

问题

I defined an enum:

enum TestEnum {
  test1,
  test2,
}

and I want make an enum with index:

E buildEnum<E extends ?????????????????>(int index) {
  try {
    return E.values[index];
  }
  catch(e) {
    return null;
  }
}

I don't know the type of enum.

回答1:

enum A { a1, a2, a3}
A.values[index]

回答2:

You cannot make static accesses on a type parameter, so this cannot work.

There is no way except reflection (dart:mirrors) to go from a value to a static member of its type.

回答3:

I know what you mean, but it doesn't seem to be supported. At least you can override it:

enum YourEnum { a1, a2, a3 }
enum AnotherEnum { b1, b2, b3 }

abstract class ToolsEnum<T> {
  T getEnumFromString(String value);
}

class YourClass with ToolsEnum<YourEnum> {
  @override
  YourEnum getEnumFromString(String value) {
    return YourEnum.values
        .firstWhere((e) => e.toString() == "YourEnum." + value);
  }
}

class AnotherClass with ToolsEnum<AnotherEnum> {
  @override
  AnotherEnum getEnumFromString(String value) {
    return AnotherEnum.values
        .firstWhere((e) => e.toString() == "AnotherEnum." + value);
  }
}

回答4:

I know this is an old question and, for now at least, it seems not possible to return E where E is a generic enum , but i'm using this code:

static dynamic asEnumValue(List<dynamic> enumValues, int index) {
    try {
      return enumValues[index];
    } catch (err) {
      return null;
    }
  }

This just allows you not to check every time if index is a correct value for your Enum type. If you pass the correct values you can still use enum properties like its index after.

Of course you will get a TypeError if you don't pass the correct enum values.

Example of use (DartPad):

enum Test {
  a,
  b,
  c
}

dynamic asEnumValue(List<dynamic> enumValues, int index) {
    try {
      return enumValues[index];
    } catch (err) {
      return null;
    }
  }

Test aTest;
Test nullTest1;
Test nullTest2;


void main() {
  
  aTest = asEnumValue(Test.values, 0);
  nullTest1 = asEnumValue(Test.values, -1);
  nullTest2 = asEnumValue(Test.values, null);
  
  print(aTest);
  print(nullTest1);
  print(nullTest2);
  
  print(aTest?.index);
  print(nullTest1?.index);
  print(nullTest2?.index);
}

Output:

Test.a
null
null
0
null
null

来源：https://stackoverflow.com/questions/51452433/dart-how-to-get-an-enum-with-index

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