QT的动态翻译功能，可能依赖于消息（事件）机制

QTranslator translator;
bool b = translator.load(QString(":/qm/lang_en"));
以后，无论使用QObject的tr，还是使用QApplication的translate，都依赖于消息（事件）循环，有代码为证：

QString QCoreApplication::translate(const char *context, const char *sourceText,
                                    const char *disambiguation, int n)
{
    QString result;

    if (!sourceText)
        return result;

    if (self && !self->d_func()->translators.isEmpty()) {
        QList<QTranslator*>::ConstIterator it;
        QTranslator *translationFile;
        for (it = self->d_func()->translators.constBegin(); it != self->d_func()->translators.constEnd(); ++it) {
            translationFile = *it;
            result = translationFile->translate(context, sourceText, disambiguation, n);
            if (!result.isNull())
                break;
        }
    }

    if (result.isNull())
        result = QString::fromUtf8(sourceText);

    replacePercentN(&result, n);
    return result;
}

static QString getMessage(const uchar *m, const uchar *end, const char *context,
                          const char *sourceText, const char *comment, uint numerus)
{
    const uchar *tn = 0;
    uint tn_length = 0;

    for (;;) {
        uchar tag = 0;
        if (m < end)
            tag = read8(m++);
        switch((Tag)tag) {
        case Tag_End:
            goto end;
        case Tag_Translation: {
            int len = read32(m);
            if (len % 1)
                return QString();
            m += 4;
            if (!numerus--) {
                tn_length = len;
                tn = m;
            }
            m += len;
            break;
        }
        case Tag_Obsolete1:
            m += 4;
            break;
        case Tag_SourceText: {
            quint32 len = read32(m);
            m += 4;
            if (!match(m, sourceText, len))
                return QString();
            m += len;
        }
            break;
        case Tag_Context: {
            quint32 len = read32(m);
            m += 4;
            if (!match(m, context, len))
                return QString();
            m += len;
        }
            break;
        case Tag_Comment: {
            quint32 len = read32(m);
            m += 4;
            if (*m && !match(m, comment, len))
                return QString();
            m += len;
        }
            break;
        default:
            return QString();
        }
    }
end:
    if (!tn)
        return QString();
    QString str = QString((const QChar *)tn, tn_length/2);
    if (QSysInfo::ByteOrder == QSysInfo::LittleEndian) {
        for (int i = 0; i < str.length(); ++i)
            str[i] = QChar((str.at(i).unicode() >> 8) + ((str.at(i).unicode() << 8) & 0xff00));
    }
    return str;
}

QString QTranslatorPrivate::do_translate(const char *context, const char *sourceText,
                                         const char *comment, int n) const
{
    if (context == 0)
        context = "";
    if (sourceText == 0)
        sourceText = "";
    if (comment == 0)
        comment = "";

    uint numerus = 0;
    size_t numItems = 0;

    if (!offsetLength)
        goto searchDependencies;

    /*
        Check if the context belongs to this QTranslator. If many
        translators are installed, this step is necessary.
    */
    if (contextLength) {
        quint16 hTableSize = read16(contextArray);
        uint g = elfHash(context) % hTableSize;
        const uchar *c = contextArray + 2 + (g << 1);
        quint16 off = read16(c);
        c += 2;
        if (off == 0)
            return QString();
        c = contextArray + (2 + (hTableSize << 1) + (off << 1));

        for (;;) {
            quint8 len = read8(c++);
            if (len == 0)
                return QString();
            if (match(c, context, len))
                break;
            c += len;
        }
    }

    numItems = offsetLength / (2 * sizeof(quint32));
    if (!numItems)
        goto searchDependencies;

    if (n >= 0)
        numerus = numerusHelper(n, numerusRulesArray, numerusRulesLength);

    for (;;) {
        quint32 h = 0;
        elfHash_continue(sourceText, h);
        elfHash_continue(comment, h);
        elfHash_finish(h);

        const uchar *start = offsetArray;
        const uchar *end = start + ((numItems-1) << 3);
        while (start <= end) {
            const uchar *middle = start + (((end - start) >> 4) << 3);
            uint hash = read32(middle);
            if (h == hash) {
                start = middle;
                break;
            } else if (hash < h) {
                start = middle + 8;
            } else {
                end = middle - 8;
            }
        }

        if (start <= end) {
            // go back on equal key
            while (start != offsetArray && read32(start) == read32(start-8))
                start -= 8;

            while (start < offsetArray + offsetLength) {
                quint32 rh = read32(start);
                start += 4;
                if (rh != h)
                    break;
                quint32 ro = read32(start);
                start += 4;
                QString tn = getMessage(messageArray + ro, messageArray + messageLength, context,
                                        sourceText, comment, numerus);
                if (!tn.isNull())
                    return tn;
            }
        }
        if (!comment[0])
            break;
        comment = "";
    }

searchDependencies:
    foreach (QTranslator *translator, subTranslators) {
        QString tn = translator->translate(context, sourceText, comment, n);
        if (!tn.isNull())
            return tn;
    }
    return QString();
}

本来在GUI下使用好好的tr和translate函数，在QtService中再开线程就出了问题（在QtService中使用本身没有问题）

解决方法：
1. 自己读取翻译文件xml以后，用它做一个Map，供我自己随时动态把它的内容都查询出来
2. 在每一个线程里申请一个Service，重新安装QTranslator，然后再使用tr或者translate函数（没试，不知道行不行）
3. 把tr或者translate转发到执行exec()的那个全局service里去，相当于转到主事件循环里去，翻译完了再跳回来（没试，不知道行不行）

官方参考文档：
http://doc.qt.io/qt-5/i18n-source-translation.html