Create class instance from string in dartlang

Question

I have created this class

class Opacity {
  String a,b,c;

  Opacity({this.a, this.b,this.c});
}

And I'm trying to dynamically create an instance of this class only using strings and an hashmap for arguments.

String type = "Opacity";
List<String> args = {'a': 'a', 'b': 'b','c': 'c'}

And I have one constraint, I can't modify the Opacity class.

For creating the instance I thought about using reflection to dynamically create the class from string but I can't figure out how to pass the arguments dynamically.

You could do that using reflection, but that's only available for server/command-line apps, but not for Web, or Flutter. For such Web and Flutter apps it's probably better to create a factory constructor/method that knows how to deal with such strings and to create instances and pass parameters for a predefined set of classes. — Günter Zöchbauer
– Günter Zöchbauer, Commented Mar 14, 2019 at 20:30

Johnykutty · Accepted Answer · 2019-03-15 15:23:07Z

4

For passing arguments dynamically to the constructor you can use newInstance method of ClassMirror.

For example

  MirrorSystem mirrors = currentMirrorSystem();
  ClassMirror classMirror = mirrors.findLibrary(Symbol.empty).declarations[new Symbol('Opacity')];
  print(classMirror);
  var arguments = {'a': 'a', 'b': 'b', 'c': 'c'}.map((key, value) {
    return MapEntry(Symbol(key), value);
  });
  var op = classMirror.newInstance(Symbol.empty, [], arguments);
  Opacity opacity = op.reflectee;
  print("opacity.a: ${opacity.a}");
  print("opacity.b: ${opacity.b}");
  print("opacity.c: ${opacity.c}");

answered Mar 15, 2019 at 15:23

Johnykutty

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lrn · Accepted Answer · 2019-03-15 10:34:53Z

Going from a string to a source name, and further to the thing denoted by that source name, is reflection. It's only available through the dart:mirrors library, or if you generate code ahead-of-time, perhaps using package:reflectable.

This is a point where Dart differs from a language like JavaScript, where you can inspect all values at run-time.

Without reflection, the only way you can call a constructor is if you have actual code performing that call in your code. That would mean that you have to have code like Opacity(a: ..., b: ..., c: ...) at least in one place in your code. You could define a function like:

Opacity createOpacity(Map<String, String> args) => 
    Opacity(a: args["a"], b: args["b"], c: args["c"]);

Then you could perhaps register it by name as:

Map<String, Function> factories = {"Opacity": createOpacity};

and finally use it as:

var type = "Opacity";
var args = {'a': 'a', 'b': 'b', 'c': 'c'};
Opacity myOpacity = factories[type](args);

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