How to use the @lumino/messaging.MessageLoop.sendMessage function in @lumino/messaging

protected attachWidget(widget: Widget): void {
    // Send a `'before-attach'` message if the parent is attached.
    if (this.parent!.isAttached) {
      MessageLoop.sendMessage(widget, Widget.Msg.BeforeAttach);
    }

    // Add the widget's node to the parent.
    this.parent!.node.appendChild(widget.node);

    // Send an `'after-attach'` message if the parent is attached.
    if (this.parent!.isAttached) {
      MessageLoop.sendMessage(widget, Widget.Msg.AfterAttach);
    }

    // Post a fit request for the parent widget.
    this.parent!.fit();
  }

protected attachWidget(widget: Widget): void {
    // Send a `'before-attach'` message if the parent is attached.
    if (this.parent!.isAttached) {
      MessageLoop.sendMessage(widget, Widget.Msg.BeforeAttach);
    }

    // Add the widget's node to the parent.
    this.parent!.node.appendChild(widget.node);

    // Send an `'after-attach'` message if the parent is attached.
    if (this.parent!.isAttached) {
      MessageLoop.sendMessage(widget, Widget.Msg.AfterAttach);
    }
  }

createBody(value: Body): Widget {
      let body: Widget;
      if (typeof value === 'string') {
        body = new Widget({ node: document.createElement('span') });
        body.node.textContent = value;
      } else if (value instanceof Widget) {
        body = value;
      } else {
        body = ReactWidget.create(value);
        // Immediately update the body even though it has not yet attached in
        // order to trigger a render of the DOM nodes from the React element.
        MessageLoop.sendMessage(body, Widget.Msg.UpdateRequest);
      }
      body.addClass('jp-Dialog-body');
      Styling.styleNode(body.node);
      return body;
    }

protected moveWidget(fromIndex: number, toIndex: number, widget: Widget): void {
    // Send a `'before-detach'` message if the parent is attached.
    if (this.parent!.isAttached) {
      MessageLoop.sendMessage(widget, Widget.Msg.BeforeDetach);
    }

    // Remove the widget's node from the parent.
    this.parent!.node.removeChild(widget.node);

    // Send an `'after-detach'` and  message if the parent is attached.
    if (this.parent!.isAttached) {
      MessageLoop.sendMessage(widget, Widget.Msg.AfterDetach);
    }

    // Look up the next sibling reference node.
    let ref = this.parent!.node.children[toIndex];

    // Send a `'before-attach'` message if the parent is attached.
    if (this.parent!.isAttached) {
      MessageLoop.sendMessage(widget, Widget.Msg.BeforeAttach);
    }

    // Insert the widget's node before the sibling.
    this.parent!.node.insertBefore(widget.node, ref);

    // Send an `'after-attach'` message if the parent is attached.
    if (this.parent!.isAttached) {
      MessageLoop.sendMessage(widget, Widget.Msg.AfterAttach);

function openRootMenu(menu: Menu, x: number, y: number, forceX: boolean, forceY: boolean): void {
    // Ensure the menu is updated before attaching and measuring.
    MessageLoop.sendMessage(menu, Widget.Msg.UpdateRequest);

    // Get the current position and size of the main viewport.
    let px = window.pageXOffset;
    let py = window.pageYOffset;
    let cw = document.documentElement.clientWidth;
    let ch = document.documentElement.clientHeight;

    // Compute the maximum allowed height for the menu.
    let maxHeight = ch - (forceY ? y : 0);

    // Fetch common variables.
    let node = menu.node;
    let style = node.style;

    // Clear the menu geometry and prepare it for measuring.
    style.top = '';

protected detachWidget(index: number, widget: Widget): void {
    // Send a `'before-detach'` message if the parent is attached.
    if (this.parent!.isAttached) {
      MessageLoop.sendMessage(widget, Widget.Msg.BeforeDetach);
    }

    // Remove the widget's node from the parent.
    this.parent!.node.removeChild(widget.node);

    // Send an `'after-detach'` message if the parent is attached.
    if (this.parent!.isAttached) {
      MessageLoop.sendMessage(widget, Widget.Msg.AfterDetach);
    }
  }

it('should trigger an update request', async () => {
        const msg = Widget.ResizeMessage.UnknownSize;
        MessageLoop.sendMessage(widget, msg);
        expect(widget.methods).to.contain('onResize');
        await framePromise();
        expect(widget.methods).to.contain('onUpdateRequest');
      });
    });

let options: Completer.IOptions = {
          editor: anchor.editor,
          model: new CompleterModel()
        };
        let value = '';
        let listener = (sender: any, selected: string) => {
          value = selected;
        };
        options.model!.setOptions(['foo', 'bar']);
        Widget.attach(anchor, document.body);

        let widget = new Completer(options);

        widget.selected.connect(listener);
        Widget.attach(widget, document.body);
        MessageLoop.sendMessage(widget, Widget.Msg.UpdateRequest);
        expect(value).to.equal('');
        widget.selectActive();
        expect(value).to.equal('foo');
        widget.dispose();
        anchor.dispose();
      });
    });

hide(): void {
    if (this.testFlag(Widget.Flag.IsHidden)) {
      return;
    }
    if (this.isAttached && (!this.parent || this.parent.isVisible)) {
      MessageLoop.sendMessage(this, Widget.Msg.BeforeHide);
    }
    this.setFlag(Widget.Flag.IsHidden);
    this.addClass('p-mod-hidden');
    if (this.isAttached && (!this.parent || this.parent.isVisible)) {
      MessageLoop.sendMessage(this, Widget.Msg.AfterHide);
    }
    if (this.parent) {
      let msg = new Widget.ChildMessage('child-hidden', this);
      MessageLoop.sendMessage(this.parent, msg);
    }
  }

show(): void {
    if (!this.testFlag(Widget.Flag.IsHidden)) {
      return;
    }
    if (this.isAttached && (!this.parent || this.parent.isVisible)) {
      MessageLoop.sendMessage(this, Widget.Msg.BeforeShow);
    }
    this.clearFlag(Widget.Flag.IsHidden);
    this.removeClass('p-mod-hidden');
    if (this.isAttached && (!this.parent || this.parent.isVisible)) {
      MessageLoop.sendMessage(this, Widget.Msg.AfterShow);
    }
    if (this.parent) {
      let msg = new Widget.ChildMessage('child-shown', this);
      MessageLoop.sendMessage(this.parent, msg);
    }
  }