Viewer模型加载本地缓存实战

由于Autodesk Forge是完全基于RESTful API框架的云平台，且暂时没有本地部署方案，特别是Viewer.js暂不支持本地搭建，必须外部引用位于服务器端的脚本，如何满足离线应用的需求一直是广大开发者关注的问题。文本将介绍来自Forge顾问团队的国际同事Petr原创的Viewer缓存范例，利用HTML5广泛用于PWA（Progessive Web App）开发的典型接口实现。

时至今日，要把来自网络应用或服务的数据缓存至本地设备，我们有几种不同的技术与方式可选，本文将示范使用Service Worker，Cache和Channel Messaging等API实现，它们都是开发Progrssive Web App的常客。虽然这些API相对较为新锐，但已获得新版浏览器的广发支持，详细支持情况可以参考：

当我们在JavaScript中注册了Service Worker之后，该Worker会拦截浏览器所有页面对于指定网络源或域的请求，返回缓存中的内容，Service Worker亦可以调用 IndexDB 、Channel Messaging、Push等API。Service Worker在Worker上下文中执行，无法直接对DOM进行操作，可以独立于页面的形式控制页面的加载。一个Service Worker可以控制多个页面，每当指定范围内的页面加载时，Service Worker便于会对其进行安装并操作，所以请小心使用全局变量，每个页面并没有自身独立的Worker。

Service Worker的生命周期如下：

• 在JavaScript中注册Service Worker
• 浏览器下载并执行Worker脚本
• Worker收到“install”（安装）事件，一次性配置所需资源
• 等待其他正在执行的Service Worker结束
• Worker收到“activate”(激活)事件，清除Worker的旧cache
• Worker开始接受“fetch”（拦截网络请求并返回缓存中的资源）和“message”（与前端代码通讯）事件

Cache是一个存储API，与LocalStorage类似IndexDB，每个网络源或域都有自己对应的存储空间，其中包括不重名的cache对象，用于存储HTTP请求与应答内容。

Channel Messaging是脚本之间通讯API，支持包括主页面、iframe、Web Worker、Service Worker之间的双向通讯。

缓存策略

缓存诸如静态资源和API端口返回的数据并不复杂，可在Service Worker安装时缓存即可。然后，当页面向API端口发送请求时，Service Worker会当即返回缓存的内容，且可按需在后台拉取资源并更新缓存内容。

缓存模型就稍许繁琐，一个模型通常会转换生成数个资源，生成的资源也时常引用其他素材，所以需要找出所有这些依赖并按需将其缓存。在本文的代码示例中，我们在后台写有接口，可根据模型的URN查询并返回所需资源的URL列表。因而在缓存模型时，Service Worker可以调用该接口缓存所有相关的URL，无需用到Viewer。

代码示例

我们制作了让用户选择模型作离线缓存的例子，查看代码请访问： https://github.com/petrbroz/f... ，在线演示请访问： https://forge-offline.herokua... 。接下来我们讲解一些具体的代码片段。

例子的后台基于Express， public 目录的内容作静态托管，其它服务端口位于以下三个路径：

GET /api/token
GET /api/models
GET /api/models/:urn/files

客户端包括两个核心脚本： public/javascript/main.js 和 public/service-worker.js ，其中 public/javascript/main.js 主要用于配置Viewer和UI逻辑，有两个重要的函数在脚本底部： initServiceWorker 和 submitWorkerTask ，前者触发Service Worker的注册，后者向其发送消息：

async function initServiceWorker() {
    try {
        const registration = await navigator.serviceWorker.register('/service-worker.js');
        console.log('Service worker registered', registration.scope);
    } catch (err) {
        console.error('Could not register service worker', err);
    }
}

On the activate event, we claim control of all instances of our web application potentially running in different browser tabs.

async function activateAsync() {
    const clients = await self.clients.matchAll({ includeUncontrolled: true });
    console.log('Claiming clients', clients.map(client => client.url).join(','));
    await self.clients.claim();
}

When intercepting requests via the fetch event, we reply with a cached response if there is one. One exception is the GET /api/token endpoint. Since our access token has an expiration time, we try to get a fresh token first, and only fall back to the cached one if we don't succeed.

async function fetchAsync(event) {
    // When requesting an access token, try getting a fresh one first
    if (event.request.url.endsWith('/api/token')) {
        try {
            const response = await fetch(event.request);
            return response;
        } catch(err) {
            console.log('Could not fetch new token, falling back to cache.', err);
        }
    }

    // If there's a cache match, return it
    const match = await caches.match(event.request.url, { ignoreSearch: true });
    if (match) {
        // If this is a static asset or known API, try updating the cache as well
        if (STATIC_URLS.includes(event.request.url) || API_URLS.includes(event.request.url)) {
            caches.open(CACHE_NAME)
                .then((cache) => cache.add(event.request))
                .catch((err) => console.log('Cache not updated, but that\'s ok...', err));
        }
        return match;
    }

    return fetch(event.request);
}

Finally, using the message event we execute "tasks" from the client.

async function messageAsync(event) {
    switch (event.data.operation) {
        case 'CACHE_URN':
            try {
                const urls = await cacheUrn(event.data.urn, event.data.access_token);
                event.ports[0].postMessage({ status: 'ok', urls });
            } catch(err) {
                event.ports[0].postMessage({ error: err.toString() });
            }
            break;
        case 'CLEAR_URN':
            try {
                const urls = await clearUrn(event.data.urn);
                event.ports[0].postMessage({ status: 'ok', urls });
            } catch(err) {
                event.ports[0].postMessage({ error: err.toString() });
            }
            break;
        case 'LIST_CACHES':
            try {
                const urls = await listCached();
                event.ports[0].postMessage({ status: 'ok', urls });
            } catch(err) {
                event.ports[0].postMessage({ error: err.toString() });
            }
            break;
    }
}

async function cacheUrn(urn, access_token) {
    console.log('Caching', urn);
    // First, ask our server for all derivatives in this URN, and their file URLs
    const baseUrl = 'https://developer.api.autodesk.com/derivativeservice/v2';
    const res = await fetch(`/api/models/${urn}/files`);
    const derivatives = await res.json();
    // Prepare fetch requests to cache all the URLs
    const cache = await caches.open(CACHE_NAME);
    const options = { headers: { 'Authorization': 'Bearer ' + access_token } };
    const fetches = [];
    const manifestUrl = `${baseUrl}/manifest/${urn}`;
    fetches.push(fetch(manifestUrl, options).then(resp => cache.put(manifestUrl, resp)).then(() => manifestUrl));
    for (const derivative of derivatives) {
        const derivUrl = baseUrl + '/derivatives/' + encodeURIComponent(derivative.urn);
        fetches.push(fetch(derivUrl, options).then(resp => cache.put(derivUrl, resp)).then(() => derivUrl));
        for (const file of derivative.files) {
            const fileUrl = baseUrl + '/derivatives/' + encodeURIComponent(derivative.basePath + file);
            fetches.push(fetch(fileUrl, options).then(resp => cache.put(fileUrl, resp)).then(() => fileUrl));
        }
    }
    // Fetch and cache all URLs in parallel
    const urls = await Promise.all(fetches);
    return urls;
}

async function clearUrn(urn) {
    console.log('Clearing cache', urn);
    const cache = await caches.open(CACHE_NAME);
    const requests = (await cache.keys()).filter(req => req.url.includes(urn));
    await Promise.all(requests.map(req => cache.delete(req)));
    return requests.map(req => req.url);
}

async function listCached() {
    console.log('Listing caches');
    const cache = await caches.open(CACHE_NAME);
    const requests = await cache.keys();
    return requests.map(req => req.url);
}

And that's pretty much it. If you want to see this code in action, head over to https://forge-offline.herokua... with your favorite (modern) browser, open the dev. tools, and try caching on of the listed models using the ☆ symbol next to their title.