前言

JS 30 是由加拿大的全端工程師 Wes Bos 免費提供的 JavaScript 簡單應用課程，課程主打 No Frameworks、No Compilers、No Libraries、No Boilerplate 在30天的30部教學影片裡，建立30個JavaScript的有趣小東西。

另外，Wes Bos 也很無私地在 Github 上公開了所有 JS 30 課程的程式碼，有興趣的話可以去 fork 或下載。

本日目標

取得使用者的位置資訊，實作出簡易的羅盤和速度表。

設定本地伺服器

這次的範例會需要取得位置的存取權限，而在取得過程必須是透過https或是localhost這類的secure origin才行，以下我們透過npm install和npm start架起自己的 little server (localhost)。

如果發現下面的指令無效的話，表示沒有安裝Node.js，可以點這邊下載 LTS 的版本，都用預設的安裝就好。

• 第一步 : 先移動到編輯檔案的工作目錄，接著輸入npm install，它會幫你安裝一些套件
  

• 第二步 : 輸入npm start，它會去執行package.json裡的start，開始運行一個 little server。(紅色框是目前的網頁位置)
  

這次不用 VisualStudio 的 Go Live Server，是因為它沒有提供 external server url，所以行動裝置沒辦法直接用來測試。

解析程式碼

HTML 部分

總體而言只有兩個重點，一個是羅盤的 svg(.arrow) 圖，另一個是速度表(.speed)。

<svg class="arrow" xmlns="http://www.w3.org/2000/svg" xmlns:xlink="http://www.w3.org/1999/xlink" version="1.1" x="0px" y="0px" viewBox="0 0 64 64" enable-background="new 0 0 64 64" xml:space="preserve"><g><path fill="#ffffff" d="M32,1.824C15.361,1.824,1.825,15.361,1.825,32c0,16.639,13.537,30.176,30.175,30.176   S62.175,48.639,62.175,32C62.175,15.361,48.639,1.824,32,1.824z M29.715,3.988h1.12l2.333,3.807V3.988h1.069v5.701h-1.155   l-2.298-3.718v3.718h-1.069V3.988z M9.323,33.917H8.102l-1.136-4.262l-1.132,4.262H4.587l-1.361-5.701h1.178l0.859,3.916   l1.042-3.916h1.369l0.999,3.982l0.875-3.982h1.159L9.323,33.917z M33.995,59.828c-0.181,0.285-0.438,0.497-0.77,0.636   c-0.332,0.139-0.745,0.208-1.241,0.208c-0.721,0-1.274-0.167-1.661-0.5c-0.386-0.333-0.617-0.819-0.692-1.456l1.12-0.109   c0.067,0.376,0.204,0.652,0.41,0.828c0.206,0.176,0.484,0.264,0.834,0.264c0.371,0,0.65-0.078,0.838-0.235   c0.188-0.157,0.282-0.34,0.282-0.55c0-0.135-0.04-0.25-0.119-0.344c-0.079-0.095-0.217-0.177-0.414-0.247   c-0.135-0.047-0.442-0.13-0.922-0.249c-0.617-0.153-1.05-0.341-1.299-0.564c-0.35-0.314-0.525-0.696-0.525-1.147   c0-0.29,0.082-0.562,0.247-0.815c0.165-0.253,0.402-0.445,0.712-0.577c0.31-0.132,0.684-0.198,1.122-0.198   c0.716,0,1.254,0.157,1.616,0.471c0.362,0.314,0.552,0.733,0.57,1.256l-1.151,0.051c-0.049-0.293-0.155-0.504-0.317-0.632   c-0.162-0.128-0.405-0.193-0.729-0.193c-0.334,0-0.596,0.069-0.786,0.206c-0.122,0.088-0.183,0.206-0.183,0.354   c0,0.135,0.057,0.25,0.171,0.346c0.145,0.122,0.498,0.249,1.058,0.381c0.56,0.132,0.974,0.269,1.243,0.41   c0.268,0.141,0.478,0.334,0.63,0.58c0.152,0.245,0.227,0.548,0.227,0.908C34.267,59.237,34.176,59.543,33.995,59.828z M32,52.795   c-11.466,0-20.795-9.329-20.795-20.795c0-11.466,9.329-20.795,20.795-20.795S52.795,20.534,52.795,32   C52.795,43.466,43.466,52.795,32,52.795z M55.014,33.917v-5.701h4.227v0.965h-3.076v1.264h2.862v0.96h-2.862v1.552h3.185v0.961   H55.014z"/><g><path fill="#000000" d="M48.904,31.863c-4.074-1.358-8.148-2.717-12.226-4.066c-0.265-0.087-0.399-0.223-0.486-0.486    c-1.349-4.077-2.708-8.151-4.066-12.226c-0.029-0.087-0.074-0.169-0.132-0.3c-0.054,0.152-0.09,0.245-0.122,0.34    c-1.352,4.053-2.707,8.104-4.048,12.161c-0.096,0.292-0.246,0.428-0.532,0.522c-4.056,1.342-8.108,2.696-12.16,4.049    c-0.097,0.032-0.189,0.074-0.344,0.137c0.172,0.06,0.267,0.093,0.362,0.125c4.074,1.358,8.148,2.717,12.224,4.072    c0.204,0.068,0.337,0.158,0.412,0.386c1.243,3.757,2.498,7.511,3.75,11.265c0.144,0.432,0.291,0.862,0.463,1.373    c0.068-0.185,0.108-0.285,0.142-0.386c1.349-4.042,2.701-8.083,4.04-12.128c0.094-0.284,0.231-0.438,0.523-0.534    c4.056-1.341,8.108-2.696,12.161-4.048c0.099-0.033,0.195-0.076,0.347-0.137C49.067,31.925,48.987,31.891,48.904,31.863z     M37.475,32.038c-1.316,0.439-2.631,0.879-3.947,1.314c-0.095,0.031-0.139,0.081-0.17,0.173c-0.434,1.313-0.873,2.625-1.311,3.937    c-0.012,0.033-0.024,0.066-0.046,0.126c-0.056-0.166-0.104-0.306-0.15-0.446c-0.407-1.219-0.814-2.437-1.218-3.657    c-0.025-0.074-0.068-0.104-0.134-0.125c-1.323-0.44-2.646-0.881-3.968-1.322c-0.031-0.01-0.062-0.022-0.118-0.041    c0.05-0.02,0.081-0.034,0.112-0.045c1.315-0.439,2.631-0.879,3.947-1.314c0.093-0.03,0.142-0.075,0.173-0.17    c0.435-1.316,0.875-2.632,1.314-3.947c0.01-0.031,0.022-0.062,0.039-0.11c0.019,0.042,0.033,0.069,0.043,0.097    c0.441,1.323,0.882,2.645,1.321,3.969c0.028,0.085,0.072,0.129,0.158,0.158c1.324,0.438,2.646,0.879,3.969,1.32    c0.027,0.009,0.053,0.02,0.1,0.038C37.538,32.013,37.507,32.027,37.475,32.038z"/><path fill="#000000" d="M24.436,27.633c-1.069-2.137-2.119-4.237-3.216-6.43c2.189,1.094,4.292,2.145,6.433,3.216    c-0.359,0.713-0.706,1.404-1.057,2.091c-0.023,0.045-0.078,0.082-0.127,0.106C25.807,26.949,25.143,27.28,24.436,27.633z"/><path fill="#000000" d="M39.573,27.632c-0.696-0.348-1.351-0.673-2.002-1.005c-0.076-0.038-0.155-0.104-0.193-0.177    c-0.338-0.661-0.666-1.326-1.019-2.033c2.121-1.061,4.228-2.115,6.43-3.217C41.69,23.399,40.635,25.509,39.573,27.632z"/><path fill="#000000" d="M24.436,36.339c0.712,0.357,1.394,0.698,2.074,1.043c0.046,0.024,0.088,0.073,0.113,0.121    c0.339,0.671,0.674,1.345,1.028,2.051c-2.126,1.063-4.232,2.117-6.43,3.216C22.317,40.577,23.37,38.472,24.436,36.339z"/><path fill="#000000" d="M36.358,39.555c0.354-0.707,0.688-1.38,1.028-2.05c0.028-0.056,0.084-0.111,0.14-0.139    c0.67-0.339,1.343-0.674,2.047-1.026c1.066,2.131,2.118,4.235,3.215,6.43C40.601,41.676,38.503,40.628,36.358,39.555z"/></g></g></svg>


<h1 class="speed">
    <span class="speed-value">0</span>
    <span class="units">KM/H</span>
</h1>

JS 部分

宣告常數arrow用來放取得的 Svg 羅盤圖片。
宣告常數speed用來放自速度表(.speed)取得的速度(.speed-value)。

const arrow = document.querySelector('.arrow');
const speed = document.querySelector('.speed-value');

Navigator.geolocation會回傳一個唯讀的 Geolocation 物件，這個物件可以被用來存取裝置的位置資訊。

Geolocation物件有一個方法watchPosition()，用來註冊使用者裝置位置更新的監聽器。我們在下面的例子傳入2個參數，分別作為成功取得位置資訊(success)和無法取得位置資訊(error)的回傳函式。

無法取得位置資訊，會拿到PositionError物件。

成功取得位置資訊的話，會拿到Position物件，這個物件的內容如下:

在Position物件的coords屬性裡面，我們可以發現包含經緯度(longitude、latitude)、定位精準度(accuracy)、移動速度(speed)還有偏離北方的度數(heading)等等...。

在成功取得位置資訊的情況下，我們把取得的Position物件(data)丟到第一個回傳函式，然後把速度表上的速度換成自位置資訊得知的速度，再把 svg 圖也旋轉自位置資訊得知的北方偏移量就完成了。

在無法取得位置資訊的情況下，我們把取得的PositionError物件(err)丟到第二個回傳函式，然後把錯誤印到 console 上並且提示使用者。

navigator.geolocation.watchPosition((data)=>{
      console.log(data);
      speed.textContent = data.coords.speed;
      arrow.style.transform = `rotate(${data.coords.heading}deg)`;
    },(err) =>{
      console.err(err);
      alert('Hey! You gotta allow that to happen!!!');
});

今天的內容相對前幾天的 Webcam 要來得和藹可親許多，我覺得學習像這樣用一鬆一緊的方式比較會有持續下去的動力 \(> <)/

補充資料:

Navigator.geolocation
Geolocation
Geolocation.watchPosition()

範例網頁請點此