渐进式Web应用程序未捕获(已承诺)TypeError:无法获取

问题描述

我开始学习PWA(渐进式Web应用程序)，但遇到问题，控制台抛出"错误未捕获(答应)TypeError:无法获取.

I started learning PWA (Progressive Web App) and I have problem, console "throws" error Uncaught (in promise) TypeError: Failed to fetch.

有人知道是什么原因吗?

Anyone know what could be the cause?

let CACHE = 'cache';

self.addEventListener('install', function(evt) {
    console.log('The service worker is being installed.');
    evt.waitUntil(precache());
});

self.addEventListener('fetch', function(evt) {
    console.log('The service worker is serving the asset.');
    evt.respondWith(fromCache(evt.request));
});
function precache() {
    return caches.open(CACHE).then(function (cache) {
        return cache.addAll([
            '/media/wysiwyg/homepage/desktop.jpg',
            '/media/wysiwyg/homepage/bottom2_desktop.jpg'
        ]);
    });
}
function fromCache(request) {
    return caches.open(CACHE).then(function (cache) {
        return cache.match(request).then(function (matching) {
            return matching || Promise.reject('no-match');
        });
    });
}

推荐答案

我认为这是由于您没有后备策略而引起的. event.respondWith 带有一个承诺，如果出现错误，您必须抓住这个承诺.

I think this is due to the fact that you don't have a fallback strategy. event.respondWith comes with a promise which you have to catch if there's some error.

因此，我建议您从此更改代码:

So, I'd suggest that you change your code from this:

self.addEventListener('fetch', function(evt) {        
    console.log('The service worker is serving the asset.');
    evt.respondWith(fromCache(evt.request));
});                   

对于这样的事情:

addEventListener('fetch', function(event) {
  event.respondWith(
    caches.match(event.request)
      .then(function(response) {
        if (response) {
          return response;     // if valid response is found in cache return it
        } else {
          return fetch(event.request)     //fetch from internet
            .then(function(res) {
              return caches.open(CACHE_DYNAMIC_NAME)
                .then(function(cache) {
                  cache.put(event.request.url, res.clone());    //save the response for future
                  return res;   // return the fetched data
                })
            })
            .catch(function(err) {       // fallback mechanism
              return caches.open(CACHE_CONTAINING_ERROR_MESSAGES)
                .then(function(cache) {
                  return cache.match('/offline.html');
                });
            });
        }
      })
  );
});          

注意:有很多缓存策略，我在这里展示的是离线优先方法.有关更多信息，请此&这是必读内容.

NOTE: There are many strategies for caching, what I've shown here is offline first approach. For more info this & this is a must read.

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