WebKit的历史项管理

标准定义

关于历史的管理，和HTML页面加载一样，都有其相应的标准。地址如下:
WhatWG: https://html.spec.whatwg.org/multipage/browsers.html#history

其中关于历史项的要点如下:

1. 在onload之前，非用户操作引起的导航操作不建立历史项。

非用户操作比如页面中指定的Timer修改location或iframe的src引发的导航操作。而用户点击发起的Timer,则在timer中记录下在发起timer时标记手势来源。但有一个例外是由另一个Timer发起的Timer或是重复执行的Timer, 则仅针对第一次执行有效(以nesting level标识)。

2. 子Frame加载完成仍有未加载的上层Frame, 则不创建历史项。

3. 当前Frame只有一个历史项，且为about:blank, 则不创建历史项。

4. 如果页面跳转的间隔小于1s，则不创建历史项。

关于Timer的nesting level可以参考这里:
http://www.w3.org/html/wg/drafts/html/CR/webappapis.html#timer-nesting-level

以上规则对应于以下三个WebKit的函数:

a. LockBackForwardList NavigationScheduler::mustLockBackForwardList(Frame& targetFrame)
LockBackForwardList NavigationScheduler::mustLockBackForwardList(Frame& targetFrame)
{
    // Non-user navigation before the page has finished firing onload should not create a new back/forward item.
    // See https://webkit.org/b/42861 for the original motivation for this.   
    if (!ScriptController::processingUserGesture() && targetFrame.loader().documentLoader() && !targetFrame.loader().documentLoader()->wasOnloadHandled())
        return LockBackForwardList::Yes;
   
    // Navigation of a subframe during loading of an ancestor frame does not create a new back/forward item.
    // The definition of "during load" is any time before all handlers for the load event have been run.
    // See https://bugs.webkit.org/show_bug.cgi?id=14957 for the original motivation for this.
    for (Frame* ancestor = targetFrame.tree().parent(); ancestor; ancestor = ancestor->tree().parent()) {
        Document* document = ancestor->document();
        if (!ancestor->loader().isComplete() || (document && document->processingLoadEvent()))
            return LockBackForwardList::Yes;
    }
    return LockBackForwardList::No;
}

b. HistoryController::currentItemShouldBeReplaced() const
{
    // From the HTML5 spec for location.assign():
    //  "If the browsing context‘s session history contains only one Document,
    //   and that was the about:blank Document created when the browsing context
    //   was created, then the navigation must be done with replacement enabled."
    return m_currentItem && !m_previousItem && equalIgnoringCase(m_currentItem->urlString(), blankURL());
}

c. void NavigationScheduler::scheduleRedirect(double delay, const URL& url)
{
    if (!shouldScheduleNavigation(url))
        return;
    if (delay < 0 || delay > INT_MAX / 1000)
        return;
    if (url.isEmpty())
        return;
    // We want a new back/forward list item if the refresh timeout is > 1 second.
    if (!m_redirect || delay <= m_redirect->delay()) {
        LockBackForwardList lockBackForwardList = delay <= 1 ? LockBackForwardList::Yes : LockBackForwardList::No;
        schedule(std::make_unique<ScheduledRedirect>(delay, m_frame.document()->securityOrigin(), url, LockHistory::Yes, lockBackForwardList));
    }
}

页面问题分析

1. Timer问题

2. 加载规则导致多余历史项问题