How do I make the map value a constant in function parameter in dart [duplicate]-CodePudding

Why does dart complain "The default value of an optional parameter must be constant". How do I make the map value constant

Map<String, int> toastDuration = {
  "defaut": 4000,
};

void showToast({
  BuildContext context,
  String msg,
  int msDuration = toastDuration["default"], // Error: The default value of an optional parameter must be constant
  bool hidePrev = true,
}) {
  ....
}

CodePudding user response：

toastDuration["default"] can't be constant because it's an expression calculated later (Think about the fact you can put any string in the braces). You can do something similar like that:

const defaultToastDuration = 4000;
Map<String, int> toastDuration = {
  "default": defaultToastDuration,
}

void showToast({
  BuildContext context,
  String msg,
  int msDuration = defaultToastDuration,
  bool hidePrev = true,
}) {
    ...
}

CodePudding user response：

As the error message says The default value of an optional parameter must be constant. Think about what happens if you remove the default key from toastDuration. Instead of using a map here you can simply use the default value directly.

void showToast({
  BuildContext context,
  String msg,
  int msDuration = 4000,
  bool hidePrev = true,
})

Another problem in your original code is that if you change the default key to say 300, the showToast will break because default parameters must be constants.