How to use the @jupyterlab/apputils.CommandToolbarButton function in @jupyterlab/apputils

// Make a refresh button for the toolbar.
    toolbar.addItem(
      'refresh',
      new ToolbarButton({
        iconClassName: 'jp-RefreshIcon jp-Icon jp-Icon-16',
        onClick: () => {
          this._updateClusterList();
        },
        tooltip: 'Refresh Cluster List'
      })
    );

    // Make a new cluster button for the toolbar.
    toolbar.addItem(
      this._launchClusterId,
      new CommandToolbarButton({
        commands: this._registry,
        id: this._launchClusterId
      })
    );

    layout.addWidget(toolbar);
    layout.addWidget(this._clusterListing);

    // Do an initial refresh of the cluster list.
    this._updateClusterList();
    // Also refresh periodically.
    this._poll = new Poll({
      factory: async () => {
        await this._updateClusterList();
      },
      frequency: { interval: REFRESH_INTERVAL, backoff: true, max: 60 * 1000 },

it('should add main class', async () => {
        const button = new CommandToolbarButton({
          commands,
          id: testLogCommandId
        });
        await render(button);
        const buttonNode = button.node.firstChild as HTMLButtonElement;
        expect(buttonNode.classList.contains('test-log-class')).to.equal(true);
        button.dispose();
      });

it('should add an icon with icon class and label', async () => {
        const button = new CommandToolbarButton({
          commands,
          id: testLogCommandId
        });
        await render(button);
        const buttonNode = button.node.firstChild as HTMLButtonElement;
        expect(buttonNode.title).to.equal('Test log command caption');
        const wrapperNode = buttonNode.firstChild as HTMLElement;
        const iconNode = wrapperNode.firstChild as HTMLElement;
        expect(iconNode.classList.contains('test-icon-class')).to.equal(true);
        button.dispose();
      });

it('should use the command label if no icon class/label', async () => {
        const id = 'to-be-removed';
        const cmd = commands.addCommand(id, {
          execute: () => {
            return;
          },
          label: 'Label-only button'
        });
        const button = new CommandToolbarButton({
          commands,
          id
        });
        await render(button);
        const buttonNode = button.node.firstChild as HTMLButtonElement;
        expect(buttonNode.textContent).to.equal('Label-only button');
        cmd.dispose();
      });

it('should create a button', () => {
        const button = new CommandToolbarButton({
          commands,
          id: testLogCommandId
        });
        expect(button).to.be.an.instanceof(CommandToolbarButton);
        button.dispose();
      });

[revert, save].forEach(name => {
      const item = new CommandToolbarButton({ commands: registry, id: name });
      toolbar.addItem(name, item);
    });
  }

: nbtracker.currentWidget
        ? nbtracker.currentWidget.context.path
        : null;

    logConsoleWidget = new MainAreaWidget({ content: logConsolePanel });
    logConsoleWidget.addClass('jp-LogConsole');
    logConsoleWidget.title.closable = true;
    logConsoleWidget.title.label = 'Log Console';
    logConsoleWidget.title.iconClass = 'jp-ListIcon';

    const addCheckpointButton = new CommandToolbarButton({
      commands: app.commands,
      id: CommandIDs.addCheckpoint
    });

    const clearButton = new CommandToolbarButton({
      commands: app.commands,
      id: CommandIDs.clear
    });

    logConsoleWidget.toolbar.addItem(
      'lab-log-console-add-checkpoint',
      addCheckpointButton
    );
    logConsoleWidget.toolbar.addItem('lab-log-console-clear', clearButton);

    logConsoleWidget.toolbar.addItem(
      'level',
      new LogLevelSwitcher(logConsoleWidget.content)
    );

    logConsolePanel.sourceChanged.connect(() => {

: nbtracker.currentWidget
        ? nbtracker.currentWidget.context.path
        : null;

    logConsoleWidget = new MainAreaWidget({ content: logConsolePanel });
    logConsoleWidget.addClass('jp-LogConsole');
    logConsoleWidget.title.closable = true;
    logConsoleWidget.title.label = 'Log Console';
    logConsoleWidget.title.iconClass = 'jp-LogConsoleIcon';

    const addCheckpointButton = new CommandToolbarButton({
      commands: app.commands,
      id: CommandIDs.addCheckpoint
    });

    const clearButton = new CommandToolbarButton({
      commands: app.commands,
      id: CommandIDs.clear
    });

    logConsoleWidget.toolbar.addItem(
      'lab-log-console-add-checkpoint',
      addCheckpointButton
    );
    logConsoleWidget.toolbar.addItem('lab-log-console-clear', clearButton);

    logConsoleWidget.toolbar.addItem(
      'level',
      new LogLevelSwitcher(logConsoleWidget.content)
    );

    logConsolePanel.sourceChanged.connect(() => {

logConsolePanel = new LogConsolePanel(loggerRegistry);

    logConsolePanel.source =
      options.source !== undefined
        ? options.source
        : nbtracker.currentWidget
        ? nbtracker.currentWidget.context.path
        : null;

    logConsoleWidget = new MainAreaWidget({ content: logConsolePanel });
    logConsoleWidget.addClass('jp-LogConsole');
    logConsoleWidget.title.closable = true;
    logConsoleWidget.title.label = 'Log Console';
    logConsoleWidget.title.iconClass = 'jp-LogConsoleIcon';

    const addCheckpointButton = new CommandToolbarButton({
      commands: app.commands,
      id: CommandIDs.addCheckpoint
    });

    const clearButton = new CommandToolbarButton({
      commands: app.commands,
      id: CommandIDs.clear
    });

    logConsoleWidget.toolbar.addItem(
      'lab-log-console-add-checkpoint',
      addCheckpointButton
    );
    logConsoleWidget.toolbar.addItem('lab-log-console-clear', clearButton);

    logConsoleWidget.toolbar.addItem(

public createNew(
    nb: NotebookPanel,
    context: DocumentRegistry.IContext
  ): IDisposable {
    const btn = new CommandToolbarButton({
      commands: this.app.commands,
      id: Constants.FORMAT_ALL_COMMAND
    });
    nb.toolbar.insertAfter(
      'cellType',
      this.app.commands.label(Constants.FORMAT_ALL_COMMAND),
      btn
    );
    return new DisposableDelegate(() => {
      btn.dispose();
    });
  }