LegacyRenderSubtreeIntoContainer

legacyRenderSubtreeIntoContainer.js

function legacyRenderSubtreeIntoContainer(parentComponent, children, container, forceHydrate, callback) {
  ...
  let root = container._reactRootContainer;
  if (!root) {
    // Initial mount
    root = container._reactRootContainer = legacyCreateRootFromDOMContainer(container, forceHydrate);
    if (typeof callback === 'function') {
      const originalCallback = callback;
      callback = function () {
        const instance = DOMRenderer.getPublicRootInstance(root._internalRoot);
        originalCallback.call(instance);
      };
    }
    // Initial mount should not be batched.
    DOMRenderer.unbatchedUpdates(() => {
      if (parentComponent != null) {
        root.legacy_renderSubtreeIntoContainer(
          parentComponent,
          children,
          callback,
        );
      } else {
        root.render(children, callback);
      }
    });
  } else {
    ...
  }
  return DOMRenderer.getPublicRootInstance(root._internalRoot);
}