Flutter Web中如何将serviceWorkerVersion替换为{{flutter_service_worker_version}}模板令牌？

按照Flutter的提示，需要把本地变量serviceWorkerVersion替换成模板令牌{{flutter_service_worker_version}}，具体修改步骤如下：

• 删除弃用的本地变量：直接移除代码里的var serviceWorkerVersion = null;声明，这个变量已经不需要了。
• 替换Service Worker URL中的变量：将构建serviceWorkerUrl的代码从

  var serviceWorkerUrl = 'flutter_service_worker.js?v=' + serviceWorkerVersion;
  

  修改为

  var serviceWorkerUrl = 'flutter_service_worker.js?v={{flutter_service_worker_version}}';
  

• 替换脚本版本校验逻辑：把判断Service Worker版本的条件从

  } else if (!reg.active.scriptURL.endsWith(serviceWorkerVersion)) {
  

  修改为

  } else if (!reg.active.scriptURL.endsWith('{{flutter_service_worker_version}}')) {
  

修改后的完整代码 #

<body>
  <img src="img/logo_symbol.png" class="center"/>

  <script>
    var scriptLoaded = false;
    function loadMainDartJs() {
      if (scriptLoaded) {
        return;
      }
      scriptLoaded = true;
      var scriptTag = document.createElement('script');
      scriptTag.src = 'main.dart.js';
      scriptTag.type = 'application/javascript';
      document.body.append(scriptTag);
    }

    if ('serviceWorker' in navigator) {
      // Service workers are supported. Use them.
      window.addEventListener('load', function () {
        // Wait for registration to finish before dropping the <script> tag.
        // Otherwise, the browser will load the script multiple times,
        // potentially different versions.
        var serviceWorkerUrl = 'flutter_service_worker.js?v={{flutter_service_worker_version}}';
        navigator.serviceWorker.register(serviceWorkerUrl)
          .then((reg) => {
            function waitForActivation(serviceWorker) {
              serviceWorker.addEventListener('statechange', () => {
                if (serviceWorker.state == 'activated') {
                  console.log('Installed new service worker.');
                  loadMainDartJs();
                }
              });
            }
            if (!reg.active && (reg.installing || reg.waiting)) {
              // No active web worker and we have installed or are installing
              // one for the first time. Simply wait for it to activate.
              waitForActivation(reg.installing || reg.waiting);
            } else if (!reg.active.scriptURL.endsWith('{{flutter_service_worker_version}}')) {
              // When the app updates the serviceWorkerVersion changes, so we
              // need to ask the service worker to update.
              console.log('New service worker available.');
              reg.update();
              waitForActivation(reg.installing);
            } else {
              // Existing service worker is still good.
              console.log('Loading app from service worker.');
              loadMainDartJs();
            }
          });

        // If service worker doesn't succeed in a reasonable amount of time,
        // fallback to plaint <script> tag.
        setTimeout(() => {
          if (!scriptLoaded) {
            console.warn(
              'Failed to load app from service worker. Falling back to plain <script> tag.',
            );
            loadMainDartJs();
          }
        }, 4000);
      });
    } else {
      // Service workers not supported. Just drop the <script> tag.
      loadMainDartJs();
    }
  </script>
</body>

内容的提问来源于stack exchange，提问作者Roar Grønmo