如何在 dart 中将默认参数分配给除 map 参数之外的函数

Question

Any idea how to assign a default parameter to a function aside of Map parameter in dart?

static Future functionName(
  bool isDone,
  [
    Map<String, String> params,
  ]
) async {
    ....
}

Note that if I put functionName([...],{bool isDone = false}) it doesn't work until I remove [...],

Answer 1

If you want isDone to have a default value, that means it will need to become an optional parameter. Your function has an existing optional positional parameter ( params ), and Dart currently does not allow mixing optional positional parameters with optional named parameters . Therefore for isDone to be optional, it also must be positional. To avoid breaking existing callers, it should remain the first parameter:

static Future functionName([
  bool isDone = false,
  Map<String, String>? params, // Remove the `?` if you don't enable null-safety.
]) async {
    ....
}

and then you would call it the same way that you already do:

functionName(
  true,
  {
    'key1': value1,
    'key2': value2,
    ...
  },
);

The only difference between this and your existing code is that you now will be able to call your function as just functionName() . If you want callers to be able to call your function with params but not with isDone , then that would be a breaking change (you would need to update all callers) and would require reordering the parameters or changing the parameters to optional named parameters.

Answer 2

A friend helped me with this and figured out that function args can take default values have to be given inside [] or {}, so this is how we pass it:

static Future functionName(
  [
    Map<String, String> params,
    bool isDone = false,
  ]
) async {
    ....
}

And call it like this:

functionName(
  {
    'key1': value1,
    'key2': value2,
    ...
  },
  true,
);