How to access or iterate parameters in Dart

Question

In a function with optional parameters, I want to print all the parameters passed in, not just value but also identifier of each parameter. For example, below is a function with optional named parameters. I print out both arg name and its value using a map instance, that's what I need.

Map temp = new Map();

argumentTest({String a, String b, String c: "DDD"}){
  if (a != null) {
    temp.addAll({"a":a});
  }

  if (b != null) {
    temp.addAll({"b":b});
  }

  if (c != null) {
    temp.addAll({"c":c});
  }
}

void main() {
  argumentTest(a:"AAA");
  print('$temp'); //{a: AAA, c: DDD}

}

Though ugly, it works. But, is there anything which looks like;

Map temp = new Map();

argumentTest({String a, String b, String c: "DDD"}){
  arguments.forEach((arg) => temp.addAll({"arg": arg}));
}

void main() {
  argumentTest(a:"AAA");
  print('$temp'); //of course, not working.
}

Thank you all and always.

Actually dart:mirrors but I have never seen a method to access local variables using mirrors only fields and methods/functions. There is no way to know which parameters where passed. You can only check if a parameter value is null or the default value you specified but this value could also have been passed by the caller. — Günter Zöchbauer
– Günter Zöchbauer, Commented Sep 14, 2014 at 13:17

Günter Zöchbauer · Accepted Answer · 2014-09-14 14:07:14Z

4

You could simplify your workaround

Map temp = new Map();

argumentTest({String a, String b, String c: "DDD"}) {
  ({'a': a, 'b': b, 'c': c}).forEach((k, v) {
    if(v != null) temp[k] = v;
  });
}

to support default values different from null you need to extend a little - for example like

argumentTest({String a, String b, String c: "DDD"}) {
  ({
      'a': [a, null], 'b': [b, null], 'c': [c, "DDD"]
  }).forEach((k, v) {
    if(v[0] != v[1]) temp[k] = v[0];
  });
}

edited Sep 14, 2014 at 14:07

answered Sep 14, 2014 at 13:38

Günter Zöchbauer

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5 Comments

Günter Zöchbauer Over a year ago

I have shortened it further. You should be aware that it doesn't check whether c was passed because c gets the value DDD if it is omitted by the caller but you had the same code in your workaround.

Letsgo Over a year ago

much more elegant. Thank you Mr.Zöchbauer. But, could you explain the second version a bit more?

Günter Zöchbauer Over a year ago

If a parameter is omitted by the caller the default value is assigned to the parameter (if no default value is specified the default value is null). My second version creates a map with the parameter name as key and as value a list containing the actual parameter value and the default value. The forEach doesn't simply compare to null but checks if the parameter value differs from the default value (v[0] is the actual value, v[1] is the default value). It is bit cumbersome because the default value has to be maintained twice - in the argument list and in the list assigned to the map.

Letsgo Over a year ago

I see, the second one removes default value from the map being created. :)

Günter Zöchbauer Over a year ago

The second one only adds these arguments to temp where the actual value differs from the default value (I thought this was the intention of the example in your question). This is the closest you can get when you try to figure out which parameters were passed - at least as far as I know.

luizmineo · Accepted Answer · 2014-09-14 13:37:49Z

2

There is no such thing in Dart. However, if you want to build mock objects for automated tests, you can take a look at this article, which shows how to use the mock package

answered Sep 14, 2014 at 13:37

luizmineo

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Comments

Danny Tuppeny · Accepted Answer · 2014-09-14 13:28:01Z

1

I don't believe this is possible (even using mirrors). The names of your variables will be changed during minification. The "messy" coder you have is really the only way of preserving them in a way you can access like this.

answered Sep 14, 2014 at 13:28

Danny Tuppeny

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1 Comment

Günter Zöchbauer Over a year ago

Minification doesn't prevent reflection but the mirrors library doesn't provide any functionality for this use case.

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