self.addEventListener("install", function(event) {
event.waitUntil(initialCache());
});

var initialCache = function() {
return caches.open("sw-cache").then(function(cache) {
return cache.addAll(["inizio.php", "sw_offline.html"]);
});
};

self.addEventListener("fetch", function(event) {
event.respondWith(checkOffline(event.request).catch(function() {
return recoverFromCache(event.request);
}));
event.waitUntil(updateCache(event.request));
});

var checkOffline = function(request) {
return new Promise(function(fulfill, reject) {
fetch(request).then(function(response) {
if(response.status !== 404) {
fulfill(response);
} else {
reject();
}
}, reject);
});
};

var updateCache = function(request) {
return caches.open("sw-cache").then(function (cache) {
return fetch(request).then(function (response) {
return cache.put(request, response);
});
});
};

var recoverFromCache = function(request) {
return caches.open("sw-cache").then(function (cache) {
return cache.match(request).then(function (matching) {
if(!matching || matching.status == 404) {
return cache.match("sw_offline.html");
} else {
return matching;
}
});
});
};