Button structure in Flex Message

flex-component-button.json

{
  "type": "bubble",
  "body": {
    "type": "box",
    "layout": "vertical",
    "spacing": "md",
    "contents": [
      {
        "type": "button",
        "style": "primary",
        "action": {
          "type": "uri",
          "label": "Primary style button",
          "uri": "https://developers.line.me"
        }
      },
      {
        "type": "button",
        "style": "secondary",
        "action": {
          "type": "uri",
          "label": "Secondary style button",
          "uri": "https://developers.line.me"
        }
      },
      {
        "type": "button",
        "style": "link",
        "action": {
          "type": "uri",
          "label": "Link style button",
          "uri": "https://developers.line.me"
        }
      }
    ]
  }
}