Example of native interop (NativeFunction, etc)?

[alastaircoote]

I'm curious to play around with Hermes outside of the React Native context, with my own C++ code. From looking through the code it looks as though things like NativeFunction are what I want to be using, but I'm curious to know if there is a good example anywhere of declaring a native function or class, adding it to a context and running it?

[dulinriley]

There are some examples in the React Native codebase, and the important file to look at is jsi.h.
There’s a function called jsi::Function::createFromHostFunction.

It’s used like this:

  /// Create a function which, when invoked, calls C++ code. If the
  /// function throws an exception, a JS Error will be created and
  /// thrown.
  /// \param name the name property for the function.
  /// \param paramCount the length property for the function, which
  /// may not be the number of arguments the function is passed.
  static Function createFromHostFunction(
      Runtime& runtime,
      const jsi::PropNameID& name,
      unsigned int paramCount,
      jsi::HostFunctionType func);

…

void foo(Runtime &rt) {
  const auto fooImpl = [](Runtime &rt, const Value& thisVal, const Value* args, size_t count) -> Value {
    // Do whatever you want in here.
    return args[0];
  };
  auto fooPropName = PropNameID::forAscii(rt, “foo”);
  auto func = Function::createFromHostFunction(rt, fooPropName, 1, fooImpl);
  rt.global().setProperty(rt, fooPropName, std::move(func));
  // You can also call existing host functions.
  rt.global().getPropertyAsFunction(rt, “foo”).call(rt, {Value(2)});
}

That should be the basics of setting up your own NativeFunctions.

[alastaircoote]

Awesome. Thank you so much!