navigator.serviceWorker.controller 在 serviceWorker.js 的 Create React App 中的作用

Question

以下是 CRA 生成的 serviceWorker.js 的摘录。

里面有if (navigator.serviceWorker.controller)有什么意义 if (installingWorker.state === 'installed')？

新的service worker已经安装成功。 'else' 部分是否处理尚未安装以前的 service worker 的场景？

navigator.serviceWorker
    .register(swUrl)
    .then(registration => {
      registration.onupdatefound = () => {
        const installingWorker = registration.installing;
        if (installingWorker == null) {
          return;
        }
        installingWorker.onstatechange = () => {
          if (installingWorker.state === 'installed') {
            if (navigator.serviceWorker.controller) {
              // At this point, the updated precached content has been fetched,
              // but the previous service worker will still serve the older
              // content until all client tabs are closed.
              console.log(
                'New content is available and will be used when all ' +
                  'tabs for this page are closed.'
              );

              // Execute callback
              if (config && config.onUpdate) {
                config.onUpdate(registration);
              }
            } else {
              // At this point, everything has been precached.
              // It's the perfect time to display a
              // "Content is cached for offline use." message.
              console.log('Content is cached for offline use.');

              // Execute callback
              if (config && config.onSuccess) {
                config.onSuccess(registration);
              }
            }
          }
        };
      };
    })
    .catch(error => {
      console.error('Error during service worker registration:', error);
    });

Answer 1

navigator.serviceWorker.controller will be set to a truthy value (a ServiceWorker object) 如果当前页面在 service worker 的控制之下。

如果这是第一次注册 scope 包含当前页面的 URL 的服务工作者，则不会设置 navigator.serviceWorker.controller（除非 service worker 自己调用 clients.claim()).

如果这是对已注册 service worker scope 下的页面的后续访问，则将设置 navigator.serviceWorker.controller，并且该页面将在 service worker 的控制下。