For passing upwards I typically fire a custom event, with data in the detail field. For passing downwards, I have the parent delegate attribute changes on the c

For passing upwards I typically fire a custom event, with data in the detail field. For passing downwards, I have the parent delegate attribute changes on the children as much as possible. This limits you to only passing string values downwards, but it keeps the html mirrored to the state, which has a simplicity to it that I prefer. An example of this would be a radio-group and radio-element. When an element is clicked it fires a "clicked" event to the parent radio-group. The radio-group then toggles on "selected" for the clicked element, and removes the previously selected element's "selected" attribute.