QT源码拾贝0-5（qimage和qpainter）

目录

0   qt源码查看方法

1.  qimage.cpp中线程池使用方法

2.  qpainter_p.h中SmallStack模板元结构体存放智能指针

3.  qpainter.cpp的保存函数，状态对象赋值使用std::exchange函数

4.  qpainter.cpp中获得类对象的方法

5.  qpainter.cpp中QChar字节操作，使用u在字节前面

---

0   qt源码查看方法

使用vscode工具，加载qt源码路径，比如：C:\Qt\6.5.0\Src

再安装一个C/C++插件：

按住ctrl+鼠标，可以方便查看引用函数的定义。

返回上次光标位置，使用快捷键Alt和方向左/右键

跳过单个变量，使用快捷键ctrl加方向左/右键

1.  qimage.cpp中线程池使用方法

 
/*!
    \since 5.14
    Applies the color transformation \a transform to all pixels in the image.
*/
void QImage::applyColorTransform(const QColorTransform &transform)
{
    if (transform.isIdentity())
        return;
    detach();
    if (!d)
        return;
    if (pixelFormat().colorModel() == QPixelFormat::Indexed) {
        for (int i = 0; i < d->colortable.size(); ++i)
            d->colortable[i] = transform.map(d->colortable[i]);
        return;
    }
    QImage::Format oldFormat = format();
    if (qt_fpColorPrecision(oldFormat)) {
        if (oldFormat != QImage::Format_RGBX32FPx4 && oldFormat != QImage::Format_RGBA32FPx4
                && oldFormat != QImage::Format_RGBA32FPx4_Premultiplied)
            convertTo(QImage::Format_RGBA32FPx4);
    } else if (depth() > 32) {
        if (oldFormat != QImage::Format_RGBX64 && oldFormat != QImage::Format_RGBA64
                && oldFormat != QImage::Format_RGBA64_Premultiplied)
            convertTo(QImage::Format_RGBA64);
    } else if (oldFormat != QImage::Format_ARGB32 && oldFormat != QImage::Format_RGB32
                && oldFormat != QImage::Format_ARGB32_Premultiplied) {
        if (hasAlphaChannel())
            convertTo(QImage::Format_ARGB32);
        else
            convertTo(QImage::Format_RGB32);
    }
 
    QColorTransformPrivate::TransformFlags flags = QColorTransformPrivate::Unpremultiplied;
    switch (format()) {
    case Format_ARGB32_Premultiplied:
    case Format_RGBA64_Premultiplied:
    case Format_RGBA32FPx4_Premultiplied:
        flags = QColorTransformPrivate::Premultiplied;
        break;
    case Format_RGB32:
    case Format_RGBX64:
    case Format_RGBX32FPx4:
        flags = QColorTransformPrivate::InputOpaque;
        break;
    case Format_ARGB32:
    case Format_RGBA64:
    case Format_RGBA32FPx4:
        break;
    default:
        Q_UNREACHABLE();
    }
 
    std::function<void(int,int)> transformSegment;
 
    if (qt_fpColorPrecision(format())) {
        transformSegment = [&](int yStart, int yEnd) {
            for (int y = yStart; y < yEnd; ++y) {
                QRgbaFloat32 *scanline = reinterpret_cast(d->data + y * d->bytes_per_line);
                QColorTransformPrivate::get(transform)->apply(scanline, scanline, width(), flags);
            }
        };
    } else  if (depth() > 32) {
        transformSegment = [&](int yStart, int yEnd) {
            for (int y = yStart; y < yEnd; ++y) {
                QRgba64 *scanline = reinterpret_cast(d->data + y * d->bytes_per_line);
                QColorTransformPrivate::get(transform)->apply(scanline, scanline, width(), flags);
            }
        };
    } else {
        transformSegment = [&](int yStart, int yEnd) {
            for (int y = yStart; y < yEnd; ++y) {
                QRgb *scanline = reinterpret_cast(d->data + y * d->bytes_per_line);
                QColorTransformPrivate::get(transform)->apply(scanline, scanline, width(), flags);
            }
        };
    }
 
#if QT_CONFIG(thread) && !defined(Q_OS_WASM)
    int segments = (qsizetype(width()) * height()) >> 16;
    segments = std::min(segments, height());
    QThreadPool *threadPool = QThreadPool::globalInstance();
    if (segments > 1 && threadPool && !threadPool->contains(QThread::currentThread())) {
        QSemaphore semaphore;
        int y = 0;
        for (int i = 0; i < segments; ++i) {
            int yn = (height() - y) / (segments - i);
            threadPool->start([&, y, yn]() {
                transformSegment(y, y + yn);
                semaphore.release(1);
            });
            y += yn;
        }
        semaphore.acquire(segments);
    } else
#endif
        transformSegment(0, height());
 
    if (oldFormat != format())
        *this = std::move(*this).convertToFormat(oldFormat);
}

2.  qpainter_p.h中SmallStack模板元结构体存放智能指针

 
    std::unique_ptr state;
    template <typename T, std::size_t N = 8>
    struct SmallStack : std::stack> {
        void clear() { this->c.clear(); }
    };
    SmallStack> savedStates;
 
    mutable std::unique_ptr dummyState;

3.  qpainter.cpp的保存函数，状态对象赋值使用std::exchange函数

void QPainter::save()
{
#ifdef QT_DEBUG_DRAW
    if (qt_show_painter_debug_output)
        printf("QPainter::save()\n");
#endif
    Q_D(QPainter);
    if (!d->engine) {
        qWarning("QPainter::save: Painter not active");
        return;
    }
 
    std::unique_ptr prev;
    if (d->extended) {
        // separate the creation of a new state from the update of d->state, since some
        // engines access d->state directly (not via createState()'s argument)
        std::unique_ptr next(d->extended->createState(d->state.get()));
        prev = std::exchange(d->state, std::move(next));
        d->extended->setState(d->state.get());
    } else {
        d->updateState(d->state);
        prev = std::exchange(d->state, std::make_unique(d->state.get()));
        d->engine->state = d->state.get();
    }
    d->savedStates.push(std::move(prev));
}

4.  qpainter.cpp中获得类对象的方法

 
QBrush QPaintEngineState::brush() const
{
    return static_cast<const QPainterState *>(this)->brush;
}

5.  qpainter.cpp中QChar字节操作，使用u在字节前面

int old_offset = offset;
    for (; offset < text.size(); offset++) {
        QChar chr = text.at(offset);
        if (chr == u'\r' || (singleline && chr == u'\n')) {
            text[offset] = u' ';
        } else if (chr == u'\n') {
            text[offset] = QChar::LineSeparator;
        } else if (chr == u'&') {
            ++maxUnderlines;
        } else if (chr == u'\t') {
            if (!expandtabs) {
                text[offset] = u' ';
            } else if (!tabarraylen && !tabstops) {
                tabstops = qRound(fm.horizontalAdvance(u'x')*8);
            }
        } else if (chr == u'\x9c') {
            // string with multiple length variants
            hasMoreLengthVariants = true;
            break;
        }
    }
 
    QList underlineFormats;
    int length = offset - old_offset;
    if ((hidemnmemonic || showmnemonic) && maxUnderlines > 0) {
        QChar *cout = text.data() + old_offset;
        QChar *cout0 = cout;
        QChar *cin = cout;
        int l = length;
        while (l) {
            if (*cin == u'&') {
                ++cin;
                --length;
                --l;
                if (!l)
                    break;
                if (*cin != u'&' && !hidemnmemonic && !(tf & Qt::TextDontPrint)) {
                    QTextLayout::FormatRange range;
                    range.start = cout - cout0;
                    range.length = 1;
                    range.format.setFontUnderline(true);
                    underlineFormats.append(range);
                }
#ifdef Q_OS_MAC
            } else if (hidemnmemonic && *cin == u'(' && l >= 4 &&
                       cin[1] == u'&' && cin[2] != u'&' &&
                       cin[3] == u')') {
                int n = 0;
                while ((cout - n) > cout0 && (cout - n - 1)->isSpace())
                    ++n;
                cout -= n;
                cin += 4;
                length -= n + 4;
                l -= 4;
                continue;
#endif //Q_OS_MAC
            }
            *cout = *cin;
            ++cout;
            ++cin;
            --l;
        }
    }