Wrappers

Wrappers allow Eval to use instances created outside of the Eval environment. This is necessary when calling from Eval into a native Dart function that returns a value, and can be used when passing an argument into an Eval function.

For example, a Flutter Text bridge class may look something like this (abbreviated for clarity):

class $Text$bridge extends Text with $Bridge {
  @override
  EvalValue? $bridgeGet(String identifier) {
    switch (identifier) {
      case 'build':
        return EvalFunctionImpl((rt, target, args) {
            return $Widget.wrap(super.build(args[0].$value));
        });
    }
    throw UnimplementedError();
  }

  Widget? build(BuildContext context) => 
      $_invoke('build', [$BuildContext.wrap(context)]);
} 

Here we are using wrappers in two places:

1. When calling build in bridgeGet, a wrapper ($Widget) is used to give the Eval environment access to the resulting Widget object that Text will natively produce if we don't override its build method in an Eval subclass.
2. When overriding build for native Dart use, we wrap the BuildContext argument with $BuildContext, so that the Eval environment understands the arguments if we do override its build method in an Eval subclass.