Ref

If you want to access direclty DOM nodes created by asm-dom, for example to managing focus, text selection, or integrating with third-party DOM libraries, you can use refs callbacks. ref is a special callback that takes the DOM node as param and can return true or false unconditionally, this is just for semplicity, to mantain the same signatures of other events. ref is called after that the DOM node is mounted, if the ref callback changes or after that the DOM node is removed from the DOM tree, in this case the param is evaluated emscripten::val::null. Here is an example of the first and the last case:

bool refCallback(emscripten::val node) {
  // check if node === null
  if (node.strictlyEquals(emscripten::val::null())) {
    // node unmounted
    // do nothing
  } else {
    // node mounted
    // focus input
    node.call<void>("focus");
  }

  return true;
};

int main() {
  Config config = Config();
  init(config);

  VNode* vnode1 = h("div",
    h("input",
      Data(
        Callbacks {
          {"ref", refCallback}
        }
      )
    )
  );

  patch(
    emscripten::val::global("document").call<emscripten::val>(
      "getElementById",
      std::string("root")
    ),
    vnode1
  );

  VNode* vnode2 = h("div");
  patch(vnode1, vnode2);

  deleteVNode(vnode2);

  return 0;
}

As we said before ref callback is also invoked if it changes, in the following example asm-dom will call refCallback after that the DOM node is mounted and then anotherRefCallback after the update:

VNode* vnode1 = h("div",
  h("input",
    Data(
      Callbacks {
        {"ref", refCallback}
      }
    )
  )
);

patch(
  emscripten::val::global("document").call<emscripten::val>(
    "getElementById",
    std::string("root")
  ),
  vnode1
);

VNode* vnode2 = h("div",
  h("input",
    Data(
      Callbacks {
        {"ref", anotherRefCallback}
      }
    )
  )
);

patch(vnode1, vnode2);

Please note that if you want to use a lambda as a ref asm-dom will call it on every update, so, you probably want to avoid something like this:

VNode* vnode1 = h("div",
  h("input",
    Data(
      Callbacks {
        {"ref", [&](emscripten::val node) -> bool {
          if (!node.strictlyEquals(emscripten::val::null())) {
            // node mounted
            // focus input
            node.call<void>("focus");
          }

          return true;
        }}
      }
    )
  )
);