quicklink解析

簡介

quicklink是一個js庫，能夠預加載出如今視口的網頁連接，提升用戶體驗。它的加載過程以下：
1.檢測網頁中的連接是否出如今視口中，等待連接出如今視口，執行步驟2。
2.等待瀏覽器空閒後執行3。
3.判斷當前的網絡鏈接是不是2G，若是是則中止執行，若是不是2G網絡，執行步驟4。
4.預加載連接指向資源。

使用方式

參考連接https://github.com/GoogleChro...

quicklink源碼解析

quicklink的入口函數接受傳入的配置參數，經過Object.assign函數和默認的配置選項合併。接着執行timeoutFn異步方法，該方法接收一個回調函數，在回調中主要邏輯以下：
若是傳入的options參數中有urls屬性，則直接執行預加載，不然經過document.querySelectorAll方法獲取全部a標籤元素的NodeList，而後便利該元素節點列表，並監視該元素節點

observer.observe(link);

而後判斷該a元素對象的href屬性值所屬的域名是否被容許訪問，若是被容許訪問，繼續判斷該連接是否應該被忽略，判斷邏輯以下：

if (!allowed.length || allowed.includes(link.hostname)) {
   // If there are any filters, the link must not match any of them
   isIgnored(link, ignores) || toPrefetch.add(link.href);
}

若是連接沒有被忽略，則將該節點的href屬性值加入到toPrefetch中

const toPrefetch = new Set();
toPrefetch.add(link.href);

總的代碼邏輯以下

export default function (options) {
  options = Object.assign({
    timeout: 2e3,
    priority: false,
    timeoutFn: requestIdleCallback,
    el: document,
  }, options);

  observer.priority = options.priority;

  const allowed = options.origins || [location.hostname];
  const ignores = options.ignores || [];

  options.timeoutFn(() => {
    // If URLs are given, prefetch them.
    if (options.urls) {
      options.urls.forEach(prefetcher);
    } else {
      // If not, find all links and use IntersectionObserver.
      Array.from(options.el.querySelectorAll('a'), link => {
        observer.observe(link);
        // If the anchor matches a permitted origin
        // ~> A `[]` or `true` means everything is allowed
        if (!allowed.length || allowed.includes(link.hostname)) {
          // If there are any filters, the link must not match any of them
          isIgnored(link, ignores) || toPrefetch.add(link.href);
        }
      });
    }
  }, {timeout: options.timeout});
}

檢測link出如今視口

上面經過observer.observe(link)監視節點元素，其中observer是IntersectionObserver對象的實例，被監聽的節點對象出如今視口時，會執行new操做時傳入的回調函數，並將出如今視口的節點對象經過數組的形式傳給該回調。而後在回調中便利傳入的數組，若是數組中的元素包含在toPrefetch對象中，則取消對該元素的監視，並對該a標籤元素所對應的資源進行預加載。

const observer = new IntersectionObserver(entries => {
  entries.forEach(entry => {
    if (entry.isIntersecting) {
      const link = entry.target;
      if (toPrefetch.has(link.href)) {
        observer.unobserve(link);
        prefetcher(link.href);
      }
    }
  });
});

異步函數實現

若是瀏覽器支持requestIdleCallback，則使用原生的函數，若是不支持，則使用setTimeout函數作ployfill。

const requestIdleCallback = requestIdleCallback ||
  function (cb) {
    const start = Date.now();
    return setTimeout(function () {
      cb({
        didTimeout: false,
        timeRemaining: function () {
          return Math.max(0, 50 - (Date.now() - start));
        },
      });
    }, 1);
  };

export default requestIdleCallback;

資源請求函數實現

預加載策略主要有三種

1.<link> prefetch

function linkPrefetchStrategy(url) {
  return new Promise((resolve, reject) => {
    const link = document.createElement(`link`);
    link.rel = `prefetch`;
    link.href = url;

    link.onload = resolve;
    link.onerror = reject;

    document.head.appendChild(link);
  });
};

2.ajax加載

function xhrPrefetchStrategy(url) {
  return new Promise((resolve, reject) => {
    const req = new XMLHttpRequest();

    req.open(`GET`, url, req.withCredentials=true);

    req.onload = () => {
      (req.status === 200) ? resolve() : reject();
    };

    req.send();
  });
}

3.Fetch請求加載

function highPriFetchStrategy(url) {
  // TODO: Investigate using preload for high-priority
  // fetches. May have to sniff file-extension to provide
  // valid 'as' values. In the future, we may be able to
  // use Priority Hints here.
  //
  // As of 2018, fetch() is high-priority in Chrome
  // and medium-priority in Safari.
  return self.fetch == null
    ? xhrPrefetchStrategy(url)
    : fetch(url, {credentials: `include`});
}

網絡類型判斷

if (conn = navigator.connection) {
    // Don't prefetch if the user is on 2G. or if Save-Data is enabled..
    if ((conn.effectiveType || '').includes('2g') || conn.saveData) return;
  }

將上面三種預加載方法封裝成函數，暴露給外部使用

const supportedPrefetchStrategy = support('prefetch')
  ? linkPrefetchStrategy
  : xhrPrefetchStrategy;

function prefetcher(url, isPriority, conn) {
  if (preFetched[url]) {
    return;
  }

  if (conn = navigator.connection) {
    // Don't prefetch if the user is on 2G. or if Save-Data is enabled..
    if ((conn.effectiveType || '').includes('2g') || conn.saveData) return;
  }

  // Wanna do something on catch()?
  return (isPriority ? highPriFetchStrategy : supportedPrefetchStrategy)(url).then(() => {
    preFetched[url] = true;
  });
};

export default prefetcher;