我们程序里面定义了某个结构体(这里简单描述为AStruct)，AStruct包含了一个QFont 类型的成员变量：

struct AStruct {QFont ft;
};

        在具体业务上，AStruct中的QFont会被传递给QPainter去绘制文本。           

        保存工程/加载工程时时，会对AStruct对象进行序列化/反序列化操作：

        
struct AStruct {QFont ft;QString serialize() {QByteArray buf;QDataStream in(&buf, QIODevice::WriteOnly);in << font;return buf.toBase64();}void deserialize(const QString& d) {QByteArray buf = QByteArray::fromBase64(d.toLatin1());QDataStream out(&buf, QIODevice::ReadOnly);out >> font;}
};

        正常情况下，这套序列化/反序列化以及QPainter绘制文本都没有什么问题。但是客户在自己机器上安装了一个新的字体文件后(字体X)，问题来了:

        在AStruct的编辑界面，用户在QFontComboBox里面选择了X字体，QPainter绘制正常。然后保存工程(序列化AStruct)，再重新打开工程(反序列化AStruct)，QPainter绘制异常，没有使用X字体来绘制文本。

        当时的第一反应就是可能序列化或者反序列出问题了，到底哪里出问题了呢？

       先排查序列化吧!  由于在第一次在编辑界面对AStruct设置为X字体后，QPainter绘制是正确的，说明那一次QPainter使用的字体是正确的，所以我把QPainter中的字体获取出来后，使用和AStruct中同样的方法对QFont进行序列化，得到一个序列化后的字符串，再和我工程里面存储的字符串进行比较，发现发现二者在后面一段有差异（标红的部分）:

  工程文件里面的:      AAAAEnm5U2tOZmzViExOZnuAT1MACv9ARAAAAAAAAP8FAAEAMhAAAAEAAAAAAAAAAAAAAAAAA=

 从QPainter的QFont序列化出来的:

AAAAEnm5U2tOZmzViExOZnuAT1MACv9ARAAAAAAAAP8FAAEAMhAAAAEAAAAAAAAAAAAAAAAAAQAAABJ5uVNrTmZs1YhMTmZ7gE9TAAo=

想要知道这部分数据存的到底是什么，只能看QFont的的源码了:

QDataStream &operator<<(QDataStream &s, const QFont &font)
{if (s.version() == 1) {s << font.d->request.family.toLatin1();} else {s << font.d->request.family;if (s.version() >= QDataStream::Qt_5_4)s << font.d->request.styleName;}if (s.version() >= QDataStream::Qt_4_0) {// 4.0double pointSize = font.d->request.pointSize;qint32 pixelSize = font.d->request.pixelSize;s << pointSize;s << pixelSize;} else if (s.version() <= 3) {qint16 pointSize = (qint16) (font.d->request.pointSize * 10);if (pointSize < 0) {pointSize = (qint16)QFontInfo(font).pointSize() * 10;}s << pointSize;} else {s << (qint16) (font.d->request.pointSize * 10);s << (qint16) font.d->request.pixelSize;}s << (quint8) font.d->request.styleHint;if (s.version() >= QDataStream::Qt_3_1) {// Continue writing 8 bits for versions < 5.4 so that we don't write too much,// even though we need 16 to store styleStrategy, so there is some data loss.if (s.version() >= QDataStream::Qt_5_4)s << (quint16) font.d->request.styleStrategy;elses << (quint8) font.d->request.styleStrategy;}s << (quint8) 0<< (quint8) font.d->request.weight<< get_font_bits(s.version(), font.d.data());if (s.version() >= QDataStream::Qt_4_3)s << (quint16)font.d->request.stretch;if (s.version() >= QDataStream::Qt_4_4)s << get_extended_font_bits(font.d.data());if (s.version() >= QDataStream::Qt_4_5) {s << font.d->letterSpacing.value();s << font.d->wordSpacing.value();}if (s.version() >= QDataStream::Qt_5_4)s << (quint8)font.d->request.hintingPreference;if (s.version() >= QDataStream::Qt_5_6)s << (quint8)font.d->capital;if (s.version() >= QDataStream::Qt_5_13)s << font.d->request.families;return s;
}

通过调试发现，序列化AStruct中的QFont和序列化QPainter中的QFont，差异就在上面这个函数的最后一个if：

 if (s.version() >= QDataStream::Qt_5_13)s << font.d->request.families;

AStruct序列化时，request.families为空，QPainter的QFont序列化时request.families不为空。

但是QPainter的字体明明是通过setFont()方法把AStruct的QFont设置进去的，怎么序列化就不一样了呢？ 莫非是QPainter里面有偷偷摸摸干了啥？ 一查代码，还真是:

/** QPainter::setFont() ***************************************/
void QPainter::setFont(const QFont &font)
{Q_D(QPainter);#ifdef QT_DEBUG_DRAWif (qt_show_painter_debug_output)printf("QPainter::setFont(), family=%s, pointSize=%d\n", font.family().toLatin1().constData(), font.pointSize());
#endifif (!d->engine) {qWarning("QPainter::setFont: Painter not active");return;}d->state->font = QFont(font.resolve(d->state->deviceFont), device());if (!d->extended)d->state->dirtyFlags |= QPaintEngine::DirtyFont;
}/** QFont::resolve() *****************************************/
QFont QFont::resolve(const QFont &other) const
{if (resolve_mask == 0 || (resolve_mask == other.resolve_mask && *this == other)) {QFont o(other);o.resolve_mask = resolve_mask;return o;}QFont font(*this);font.detach();font.d->resolve(resolve_mask, other.d.data());return font;
}/** QFontPrivate::resolve() *************************/
void QFontPrivate::resolve(uint mask, const QFontPrivate *other)
{Q_ASSERT(other != nullptr);dpi = other->dpi;if ((mask & QFont::AllPropertiesResolved) == QFont::AllPropertiesResolved) return;// assign the unset-bits with the set-bits of the other font defif (! (mask & QFont::FamilyResolved))request.family = other->request.family;if (!(mask & QFont::FamiliesResolved)) {request.families = other->request.families;// Prepend the family explicitly set so it will be given// preference in this caseif (mask & QFont::FamilyResolved)request.families.prepend(request.family);}if (! (mask & QFont::StyleNameResolved))request.styleName = other->request.styleName;if (! (mask & QFont::SizeResolved)) {request.pointSize = other->request.pointSize;request.pixelSize = other->request.pixelSize;}..........................................
}

上面贴出了调用QPainter::setFont()时和字体相关的几个关键函数，调用时序为:

QPainter::setFont() -> QFont::resovle() ->QFontPrivate::resolve()

看QFontPrivate::resolve()，里面有这么一段:

if ((mask & QFont::AllPropertiesResolved) == QFont::AllPropertiesResolved) return;// assign the unset-bits with the set-bits of the other font defif (! (mask & QFont::FamilyResolved))request.family = other->request.family;if (!(mask & QFont::FamiliesResolved)) {request.families = other->request.families;// Prepend the family explicitly set so it will be given// preference in this caseif (mask & QFont::FamilyResolved)request.families.prepend(request.family);}

说人话就是:

如果字体的属性掩码不是QFont::AllPropertiesResolved，那么就需要根据属性掩码对没有复制的属性进行复制。于是马上有了对FamiliesResolved属性的处理: 如果FamiliesResolved没有赋值，那么就用Family属性填充families。   

既然QPainter每次设置字体都会调用一遍QFont::resolve()来填充families字段，按理说重新打开工程之后，使用反序列化得到的QFont设置给QPainter时，也会自动填充families才对啊，为什么绘制就不对了呢？

注意上面对QFont的属性填充有个判断条件:

 也就是说当mask没有覆盖了所有字体属性时，才会进入到下面的逻辑。那么这个mask是如何赋值的呢？

mask的赋值有几种方式，一种是调用QFont的resolve()函数，一种是在QFont的几个构造函数中自动赋值，另一种则是QDataStream反序列化中赋值。我们要关心的正是QDataStream的反序列化:

QDataStream &operator>>(QDataStream &s, QFont &font)
{font.d = new QFontPrivate;font.resolve_mask = QFont::AllPropertiesResolved;quint8 styleHint, charSet, weight, bits;quint16 styleStrategy = QFont::PreferDefault;if (s.version() == 1) {QByteArray fam;s >> fam;font.d->request.family = QString::fromLatin1(fam);} else {s >> font.d->request.family;if (s.version() >= QDataStream::Qt_5_4)s >> font.d->request.styleName;}if (s.version() >= QDataStream::Qt_4_0) {// 4.0double pointSize;qint32 pixelSize;s >> pointSize;s >> pixelSize;font.d->request.pointSize = qreal(pointSize);font.d->request.pixelSize = pixelSize;} else {qint16 pointSize, pixelSize = -1;s >> pointSize;if (s.version() >= 4)s >> pixelSize;font.d->request.pointSize = qreal(pointSize / 10.);font.d->request.pixelSize = pixelSize;}s >> styleHint;if (s.version() >= QDataStream::Qt_3_1) {if (s.version() >= QDataStream::Qt_5_4) {s >> styleStrategy;} else {quint8 tempStyleStrategy;s >> tempStyleStrategy;styleStrategy = tempStyleStrategy;}}s >> charSet;s >> weight;s >> bits;font.d->request.styleHint = styleHint;font.d->request.styleStrategy = styleStrategy;font.d->request.weight = weight;set_font_bits(s.version(), bits, font.d.data());if (s.version() >= QDataStream::Qt_4_3) {quint16 stretch;s >> stretch;font.d->request.stretch = stretch;}if (s.version() >= QDataStream::Qt_4_4) {quint8 extendedBits;s >> extendedBits;set_extended_font_bits(extendedBits, font.d.data());}if (s.version() >= QDataStream::Qt_4_5) {int value;s >> value;font.d->letterSpacing.setValue(value);s >> value;font.d->wordSpacing.setValue(value);}if (s.version() >= QDataStream::Qt_5_4) {quint8 value;s >> value;font.d->request.hintingPreference = QFont::HintingPreference(value);}if (s.version() >= QDataStream::Qt_5_6) {quint8 value;s >> value;font.d->capital = QFont::Capitalization(value);}if (s.version() >= QDataStream::Qt_5_13) {QStringList value;s >> value;font.d->request.families = value;}return s;
}

看，这个函数第二行就把mask赋值成了font.resolve_mask = QFont::AllPropertiesResolved，但是又因为AStruct的QFont序列化时，request.families为空，所以在反序列化时没有任何数据解析出来。

至此，问题就比较明确了:

AStruct的QFont序列化时，families属性为空；反序列化时，families属性也为空，同时，QFont的mask被设置成了font.resolve_mask = QFont::AllPropertiesResolved;   于是当我将反序列化得到的AStruct的QFont设置给QPainter后，QPainter调用QFont::resolve()方法无法填充families。最后QPainter使用字体失败。

如何解决这个问题？

这个问题的根本原因在于AStruct中的QFont属性不完整，我们得想办法让他完整。其实QPainter已经给出了解决方案：调用QFont::resolve()来填充属性。 当然，调用resolve还有一些坑，这里我直接给出我的代码。很简单:

AStruct astru;
connect(ui.fontComboBox, &QFontComboBox::currentFontChanged, this, [this, astru](const QFont& ft) {/*有问题的写法: astru.font中的families属性为空 astru.font = ft;*//*修正后的写法: 通过调用resolve给families属性赋值*/astru.font = QFont(ft.family()).resolve(ft);
});