获取pushmanager订阅时出错-javascript

我有下面的代码来注册一个服务工作者并请求用户允许通知。当用户允许推送通知返回承诺后,我会收到一个错误。 serviceWorkerRegistration.pushManager.getSubscription() 是 null . 当我关闭浏览器并再次强制此函数调用时,它不会出错。我不明白为什么。这是我的代码:

window.vapidPublicKey = new Uint8Array([4, 45, ...]); 
if (navigator.serviceWorker) {
  navigator.serviceWorker.register('/serviceworker.js')
  .then(function(reg) {
     console.log('Service worker change, registered the service worker');
  });
}
// Otherwise, no push notifications :(
else {
  console.error('Service worker is not supported in this browser');
}

navigator.serviceWorker.ready.then((serviceWorkerRegistration) => {
  console.log("ready");
  serviceWorkerRegistration.pushManager
  .subscribe({
    userVisibleOnly: true,
    applicationServerKey: window.vapidPublicKey
  });
});



// application.js

// Let's check if the browser supports notifications
if (!("Notification" in window)) {
  console.error("This browser does not support notifications.");
}

// Let's check whether notification permissions have already been granted
else if (Notification.permission === "granted") {
  console.log("Permission to receive notifications has been granted");
}

// Otherwise, we need to ask the user for permission
else if (Notification.permission !== 'denied') {
  Notification.requestPermission(function (permission) {
    // If the user accepts, let's create a notification
    if (permission === "granted") {
      console.log("Permission to receive notifications has been granted");
      saveSubscriptionToDatabase();
    }
  });
}

function saveSubscriptionToDatabase(){

      navigator.serviceWorker.ready
      .then((serviceWorkerRegistration) => {
        console.log(serviceWorkerRegistration.pushManager.getSubscription());
        serviceWorkerRegistration.pushManager.getSubscription()
        .then((subscription) => {
          $.post("/users/save_subscription", { subscription: subscription.toJSON() });
        });
      });

}

错误:

Uncaught (in promise) TypeError: Cannot read property 'toJSON' of null
at serviceWorkerRegistration.pushManager.getSubscription.then

更新:

所以我添加了以下按钮:

 <input type="button" onclick="saveSubscriptionToDatabase();" value="test">

只要我通过按钮click调用函数,所有的JS和Post请求就可以正常工作,但是当我从检查权限状态的条件调用函数时,它仍然像以前一样失败。

更新2:

我试过再次编写代码,但从另一个角度。

function registerServiceWorker() {
  return navigator.serviceWorker.register('serviceworker.js')
  .then(function(registration) {
    console.log('Service worker successfully registered.');
    return registration;
  })
  .catch(function(err) {
    console.error('Unable to register service worker.', err);
  });
}


function askPermission() {
  return new Promise(function(resolve, reject) {
    const permissionResult = Notification.requestPermission(function(result) {
      resolve(result);
    });

    if (permissionResult) {
      permissionResult.then(resolve, reject);
    }
  })
  .then(function(permissionResult) {
    if (permissionResult !== 'granted') {
      throw new Error('We weren\'t granted permission.');
    }
  });
}


function subscribeUserToPush() {
  return navigator.serviceWorker.register('serviceworker.js')
  .then(function(registration) {
    const subscribeOptions = {
      userVisibleOnly: true,
      applicationServerKey: new Uint8Array("key goes here")

    };

    return registration.pushManager.subscribe(subscribeOptions);
  })
  .then(function(pushSubscription) {
    console.log("never called");
    console.log('Received PushSubscription: ', JSON.stringify(pushSubscription));
    return pushSubscription;
  });
}

registerServiceWorker();
askPermission();
subscribeUserToPush();

命令链的最后一部分 subscribeUserToPush() 我在哪里 console.log("never called"); 从不执行并且没有错误。我用Chrome67测试这个。我肯定 registration.pushManager.subscribe(subscribeOptions); 被调用。