我仍在进行实验，以掌握服务工作者，并面临一个问题，可能是由于我在JavaScript和服务工作者方面缺乏专业知识。
问题发生在我想要使用postMessage()让新的服务工作者skipWaiting()时。如果我展示一个带有按钮的弹窗，并在那里绑定对postMessage()的调用，一切正常。如果我直接调用postMessage()，它就不能工作。这是由于竞态条件，因为有时它能够工作，但我无法确定竞争条件。
顺便提一下，postMessage()调用可以工作，服务工作者记录了当获取消息时应该记录的内容：
// Listen to messages from clients.
self.addEventListener('message', event => {
switch(event.data) {
case 'skipWaiting': self.skipWaiting(); console.log('I skipped waiting... EXTRA');
break;
}
});
这里是代码。关键部分在if (registration.waiting)条件语句中。未注释的代码可以工作，而注释掉的则不行：
// Register service worker.
if ('serviceWorker' in navigator) {
// Helpers to show and hide the update toast.
let hideUpdateToast = () => {
document.getElementById('update_available').style.visibility = 'hidden';
};
let showUpdateToast = (serviceworker) => {
document.getElementById('update_available').style.visibility = 'visible';
document.getElementById('force_install').onclick = () => {
serviceworker.postMessage('skipWaiting');
hideUpdateToast();
};
document.getElementById('close').onclick = () => hideUpdateToast();
};
window.addEventListener('load', () => {
let refreshing = false;
navigator.serviceWorker.addEventListener('controllerchange', () => {
if (refreshing) return;
refreshing = true;
window.location.reload();
});
navigator.serviceWorker.register('/sw.js').then(registration => {
// A new service worker has been fetched, watch for state changes.
//
// This event is fired EVERY TIME a service worker is fetched and
// succesfully parsed and goes into 'installing' state. This
// happens, too, the very first time the page is visited, the very
// first time a service worker is fetched for this page, when the
// page doesn't have a controller, but in that case there's no new
// version available and the notification must not appear.
//
// So, if the page doesn't have a controller, no notification shown.
registration.addEventListener('updatefound', () => {
// return; // FIXME
registration.installing.onstatechange = function () { // No arrow function because 'this' is needed.
if (this.state == 'installed') {
if (!navigator.serviceWorker.controller) {
console.log('First install for this service worker.');
} else {
console.log('New service worker is ready to activate.');
showUpdateToast(this);
}
}
};
});
// If a service worker is in 'waiting' state, then maybe the user
// dismissed the notification when the service worker was in the
// 'installing' state or maybe the 'updatefound' event was fired
// before it could be listened, or something like that. Anyway, in
// that case the notification has to be shown again.
//
if (registration.waiting) {
console.log('New service worker is waiting.');
// showUpdateToast(registration.waiting);
// The above works, but this DOESN'T WORK.
registration.waiting.postMessage('skipWaiting');
}
}).catch(error => {
console.log('Service worker registration failed!');
console.log(error);
});
});
}
为什么使用按钮的 onclick 事件进行间接调用可以正常工作，但是调用 postMessage() 却不行呢？
我真的很困惑，我敢打赌答案很简单，只是我太糊涂了看不出来。
提前非常感谢。