How to dynamically call constructor, based on class name?

1

Hi!
Working on a website, using Haxe, with a JavaScript target.
I have a question:

Suppose I have a class “Base”, and a number of descendent classes:

class Base {
  //
}

class Desc1 extends Base {
  //
}

class Desc2 extends Base {
  //
}

class Desc3 extends Base {
  //
}

Now suppose I have a string, loaded at runtime, that contains the name of any one of those descendent classes:

var s: String;
… // Loading happens...
trace(s); // E.g., 'Desc2'

What I want to do is create an object, of declared type Base, at runtime, with the name of its runtime type corresponding to that string:

var o: Base = new …; // o has now type Desc2

How do I dynamically call that particular descendent constructor (in this case, new Desc2()), going ONLY by the name in that string?

Many thanks!

2

See Type.createInstance() and Type.createEmptyInstance(), which you should be able to feed Type.resolveClass(s) to.

Edit: note that those will need to be included in the compilation. Make sure you include those types (you might need to add a reference to them in your hxml or reference them somewhere. You also need to make sure dce (dead code elimination) doesn’t get rid of them; that can be done by adding @:keep to those types.

1 Like
3

If you don’t want to rely on reflection, a simple Map can work:

var constructors = [
  "Desc1" => Desc1.new,
  "Desc2" => Desc2.new,
  // etc
];
var s:String = ...;
// maybe check that constructors contains s ?
var o:Base = constructors[s]();

You can use macros to generate the constructors expression automatically, e.g. for all classes in a given package. Since all the child classes are mentioned in the code, there should be no need to annotate them with @:keep.

3 Likes
4

Awesome, awesome, awesome!
This solved it, many many thanks.
Keep safe,
Matt