How to write an enum factory in dart?

Question

I have this enum:

/// Firmware update  BLE OTA transfert speed
enum FirmwareUpdateBleOtaTransfertSpeed { fast, medium, moderate }

With the following extension:

extension FirmwareUpdateBleOtaTransfertSpeedExtension
    on FirmwareUpdateBleOtaTransfertSpeed {

  static FirmwareUpdateBleOtaTransfertSpeed fromSliderIndex(double value) {
    if (value > 66)
      return FirmwareUpdateBleOtaTransfertSpeed.fast;
    else if (value > 33)
      return FirmwareUpdateBleOtaTransfertSpeed.medium;
    else
      return FirmwareUpdateBleOtaTransfertSpeed.moderate;
  }
}

And I'd like to use my factory as follows:

speed = FirmwareUpdateBleOtaTransfertSpeed
                        .fromSliderIndex(value);

But it does not compile. I must use the name of the extension as follows:

speed = FirmwareUpdateBleOtaTransfertSpeedExtension
                        .fromSliderIndex(value);

How can I use the enum class directly to generate the right enum?

Answer 1

If you want the static method to be called on your enum, you need to define the static method directly inside the enum itself rather than in an extension.

enum FirmwareUpdateBleOtaTransfertSpeed {
  fast,
  medium,
  moderate;

  static FirmwareUpdateBleOtaTransfertSpeed fromSliderIndex(double value) {
    if (value > 66)
      return FirmwareUpdateBleOtaTransfertSpeed.fast;
    else if (value > 33)
      return FirmwareUpdateBleOtaTransfertSpeed.medium;
    else
      return FirmwareUpdateBleOtaTransfertSpeed.moderate;
  }
}

You can also define as an actual factory constructor:

enum FirmwareUpdateBleOtaTransfertSpeed {
  fast,
  medium,
  moderate;

  factory FirmwareUpdateBleOtaTransfertSpeed.fromSliderIndex(double value) {
    if (value > 66)
      return FirmwareUpdateBleOtaTransfertSpeed.fast;
    else if (value > 33)
      return FirmwareUpdateBleOtaTransfertSpeed.medium;
    else
      return FirmwareUpdateBleOtaTransfertSpeed.moderate;
  }
}

Note that you have to use dart 2.17 or higher in order to use the enhanced enums feature.