{"componentChunkName":"component---src-templates-documentation-tsx","path":"/database/presence-detection","result":{"data":{"mdx":{"body":"var _excluded = [\"components\"];\nfunction _extends() { _extends = Object.assign ? Object.assign.bind() : function (target) { for (var i = 1; i < arguments.length; i++) { var source = arguments[i]; for (var key in source) { if (Object.prototype.hasOwnProperty.call(source, key)) { target[key] = source[key]; } } } return target; }; return _extends.apply(this, arguments); }\nfunction _objectWithoutProperties(source, excluded) { if (source == null) return {}; var target = _objectWithoutPropertiesLoose(source, excluded); var key, i; if (Object.getOwnPropertySymbols) { var sourceSymbolKeys = Object.getOwnPropertySymbols(source); for (i = 0; i < sourceSymbolKeys.length; i++) { key = sourceSymbolKeys[i]; if (excluded.indexOf(key) >= 0) continue; if (!Object.prototype.propertyIsEnumerable.call(source, key)) continue; target[key] = source[key]; } } return target; }\nfunction _objectWithoutPropertiesLoose(source, excluded) { if (source == null) return {}; var target = {}; var sourceKeys = Object.keys(source); var key, i; for (i = 0; i < sourceKeys.length; i++) { key = sourceKeys[i]; if (excluded.indexOf(key) >= 0) continue; target[key] = source[key]; } return target; }\n/* @jsxRuntime classic */\n/* @jsx mdx */\n\nvar _frontmatter = {\n  \"title\": \"Presence Detection\",\n  \"description\": \"Show a realtime list of online users with Realtime Database.\",\n  \"next\": \"/firestore/usage\",\n  \"previous\": \"/database/offline-support\"\n};\nvar layoutProps = {\n  _frontmatter: _frontmatter\n};\nvar MDXLayout = \"wrapper\";\nreturn function MDXContent(_ref) {\n  var components = _ref.components,\n    props = _objectWithoutProperties(_ref, _excluded);\n  return mdx(MDXLayout, _extends({}, layoutProps, props, {\n    components: components,\n    mdxType: \"MDXLayout\"\n  }), mdx(\"p\", null, \"Realtime Database provides the ability to trigger events on the Firebase servers whenever a device is disconnected. This\\ncould be whenever a user has no access to a network or when they quit the app.\"), mdx(\"p\", null, \"One use-case using this functionality is to build a simple presence detection system, whereby we can hold a realtime list\\nof users currently online within our application. This could be useful when building a chat application\\nand you wish to view which of your users are currently online.\"), mdx(\"h1\", {\n    \"id\": \"setup\"\n  }, \"Setup\"), mdx(\"p\", null, \"To get started, we first need a location to store our online users.\"), mdx(\"p\", null, \"To keep things simple we'll assume the user is authenticated (e.g. with \", mdx(\"a\", {\n    parentName: \"p\",\n    \"href\": \"/auth\"\n  }, \"Firebase Authentication\"), \") so we can use their unique user identifier.\"), mdx(\"p\", null, \"Whenever the application opens, \", mdx(\"a\", {\n    parentName: \"p\",\n    \"href\": \"/database/usage#writing-data\"\n  }, \"write a new value\"), \" on a reference node (e.g. \", mdx(\"inlineCode\", {\n    parentName: \"p\"\n  }, \"/online/:userId\"), \"):\"), mdx(\"pre\", {\n    \"className\": \"language-jsx\"\n  }, mdx(\"code\", {\n    parentName: \"pre\",\n    \"className\": \"language-jsx\"\n  }, mdx(\"span\", {\n    parentName: \"code\",\n    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Typically you would execute this functionality from the device\\nitself, however this presents a problem.\"), mdx(\"p\", null, \"If the device suddenly goes offline, or is quit, the app can no longer execute code or connect to Firebase. Luckily, the\\nRealtime Database API provides a way to execute code on the Firebase servers whenever the connection between an app & server\\nis lost.\"), mdx(\"p\", null, \"The \", mdx(\"inlineCode\", {\n    parentName: \"p\"\n  }, \"onDisconnect\"), \" method on a reference returns a new \", mdx(\"a\", {\n    parentName: \"p\",\n    \"href\": \"/reference/database/ondisconnect\"\n  }, mdx(\"inlineCode\", {\n    parentName: \"a\"\n  }, \"OnDisconnect\")), \" instance. This instance\\nprovides functionality to remove or set data whenever a client disconnects. Using the\\n\", mdx(\"a\", {\n    parentName: \"p\",\n    \"href\": \"/reference/database/ondisconnect#remove\"\n  }, mdx(\"inlineCode\", {\n    parentName: \"a\"\n  }, \"remove\")), \" method we can remove the node on the database if the client disconnects:\"), mdx(\"pre\", {\n    \"className\": \"language-jsx\"\n  }, mdx(\"code\", {\n    parentName: \"pre\",\n    \"className\": \"language-jsx\"\n  }, mdx(\"span\", {\n    parentName: \"code\",\n    \"className\": \"token keyword module\"\n  }, \"import\"), \" \", mdx(\"span\", {\n    parentName: \"code\",\n    \"className\": \"token maybe-class-name\"\n  }, \"React\"), mdx(\"span\", {\n    parentName: \"code\",\n    \"className\": \"token punctuation\"\n  }, \",\"), \" \", mdx(\"span\", {\n    parentName: \"code\",\n    \"className\": \"token punctuation\"\n  }, \"{\"), \" useEffect \", mdx(\"span\", {\n    parentName: \"code\",\n    \"className\": \"token punctuation\"\n  }, \"}\"), \" \", mdx(\"span\", {\n    parentName: \"code\",\n 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