Customizing context menus

5. Precondition: External state

Use of the preconditionFn is not limited to checking the type of the Blockly component that the context menu was invoked on. You can use it to check for conditions entirely outside of Blockly. For instance, let's disable helloWorldItem for the second half of every minute:

    preconditionFn: function (scope) {
      if (
        scope.focusedNode instanceof Blockly.WorkspaceSvg ||
        scope.focusedNode instanceof Blockly.BlockSvg
      ) {
        const now = new Date(Date.now());
        if (now.getSeconds() < 30) {
          return 'enabled';
        }
        return 'disabled';
      }
      return 'hidden';
    },

Test it

Reload your workspace, check your watch, and open a context menu on the workspace to confirm the timing. The option will always be in the menu, but will sometimes be greyed out.