C++ TextTrackCue::getDisplayTree方法代码示例

void MediaControlTextTrackContainerElement::updateDisplay()
{
    HTMLMediaElement* mediaElement = toParentMediaElement(this);

    // 1. If the media element is an audio element, or is another playback
    // mechanism with no rendering area, abort these steps. There is nothing to
    // render.
    if (!mediaElement->isVideo())
        return;

    // 2. Let video be the media element or other playback mechanism.
    HTMLVideoElement* video = static_cast<HTMLVideoElement*>(mediaElement);

    // 3. Let output be an empty list of absolutely positioned CSS block boxes.
    Vector<RefPtr<HTMLDivElement> > output;

    // 4. If the user agent is exposing a user interface for video, add to
    // output one or more completely transparent positioned CSS block boxes that
    // cover the same region as the user interface.

    // 5. If the last time these rules were run, the user agent was not exposing
    // a user interface for video, but now it is, let reset be true. Otherwise,
    // let reset be false.

    // There is nothing to be done explicitly for 4th and 5th steps, as
    // everything is handled through CSS. The caption box is on top of the
    // controls box, in a container set with the -webkit-box display property.

    // 6. Let tracks be the subset of video's list of text tracks that have as
    // their rules for updating the text track rendering these rules for
    // updating the display of WebVTT text tracks, and whose text track mode is
    // showing or showing by default.
    // 7. Let cues be an empty list of text track cues.
    // 8. For each track track in tracks, append to cues all the cues from
    // track's list of cues that have their text track cue active flag set.
    CueList activeCues = video->currentlyActiveCues();

    // 9. If reset is false, then, for each text track cue cue in cues: if cue's
    // text track cue display state has a set of CSS boxes, then add those boxes
    // to output, and remove cue from cues.

    // There is nothing explicitly to be done here, as all the caching occurs
    // within the TextTrackCue instance itself. If parameters of the cue change,
    // the display tree is cleared.

    // 10. For each text track cue cue in cues that has not yet had
    // corresponding CSS boxes added to output, in text track cue order, run the
    // following substeps:
    for (size_t i = 0; i < activeCues.size(); ++i) {
        TextTrackCue* cue = activeCues[i].data();

        ASSERT(cue->isActive());
        if (!cue->track() || !cue->track()->isRendered())
            continue;

        RefPtr<TextTrackCueBox> displayBox = cue->getDisplayTree();

        if (displayBox->hasChildNodes() && !contains(static_cast<Node*>(displayBox.get())))
            // Note: the display tree of a cue is removed when the active flag of the cue is unset.
            m_cueContainer->appendChild(displayBox, ASSERT_NO_EXCEPTION, false);
    }

    // 11. Return output.
    if (hasChildNodes()) {
        show();
        if (mediaElement->requiresTextTrackRepresentation()) {
            if (!m_textTrackRepresentation)
                m_textTrackRepresentation = TextTrackRepresentation::create(this);
            mediaElement->setTextTrackRepresentation(m_textTrackRepresentation.get());

            if (Page* page = document()->page())
                m_textTrackRepresentation->setContentScale(page->deviceScaleFactor());

            m_textTrackRepresentation->update();
            setInlineStyleProperty(CSSPropertyWidth, String::number(m_videoDisplaySize.size().width()) + "px");
            setInlineStyleProperty(CSSPropertyHeight, String::number(m_videoDisplaySize.size().height()) + "px");
        }
    } else {
        hide();
        m_textTrackRepresentation = nullptr;
        mediaElement->setTextTrackRepresentation(0);
        removeInlineStyleProperty(CSSPropertyWidth);
        removeInlineStyleProperty(CSSPropertyHeight);
    }
}