Qt——QScrollArea

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1.QScrollArea是否显示滚动条是由一个主要的子控件决定。
检查滚动条未显示
（1）是否只有一个子控件
（2）是否设置 setWidgetResizable(true);,因为这个的本质是QWidget大于QScrollArea
（3）子控件的大小是否合适。

2.如何在 QScrollArea中显示多个控件？
关键操作：
m_scrollArea->setWidgetResizable(true);
m_scrollArea->setWidget(m_widget);

构造一个 QWidget 作为 QScrollArea的唯一子控件
其他子控件作为 QWidget对象的子控件。
其中有两次布局，
QWidget 对 QScrollArea
others 对 QWidget

3.如何列表方式动态添加子控件到 QScrollArea
（1）动态添加
{
QVBoxLayout *layout = QVBoxLayout(this);
layout->addWidget(new pushbutton);
layout->addStretch(0); // 加弹簧
}
{
layout->insertWidget(0, new pushbutton); // 动态指定位置添加控件
}

（2）增删弹簧添加
{
QVBoxLayout *layout = QVBoxLayout(this);
m_spaceritem = new QSpacerItem(100, 200,
QSizePolicy::Expanding,
QSizePolicy::Expanding);
layout->addSpacerItem(m_spaceritem);
}

{
layout->removeItem(m_spaceritem);
layout->addWidget(new pushbutton);
layout->addSpacerItem(m_spaceritem);
}

4.如何实现网格动态增加？

QScrollArea的唯一子控件 QWidget 使用 QFlowLayout即可,

QFlowLayout实现

#ifndef QFLOWLAYOUT_H
#define QFLOWLAYOUT_H

#include <QLayout>
#include <QRect>
#include <QStyle>

class QFlowLayout : public QLayout
{
public:
    , , );
    , , );
    ~QFlowLayout();

    void addItem(QLayoutItem *item) Q_DECL_OVERRIDE;
    int horizontalSpacing() const;
    int verticalSpacing() const;
    Qt::Orientations expandingDirections() const Q_DECL_OVERRIDE;
    bool hasHeightForWidth() const Q_DECL_OVERRIDE;
    int heightForWidth(int) const Q_DECL_OVERRIDE;
    int count() const Q_DECL_OVERRIDE;
    QLayoutItem *itemAt(int index) const Q_DECL_OVERRIDE;
    QSize minimumSize() const Q_DECL_OVERRIDE;
    void setGeometry(const QRect &rect) Q_DECL_OVERRIDE;
    QSize sizeHint() const Q_DECL_OVERRIDE;
    QLayoutItem *takeAt(int index) Q_DECL_OVERRIDE;

private:
    int doLayout(const QRect &rect, bool testOnly) const;
    int smartSpacing(QStyle::PixelMetric pm) const;

private:
    QList<QLayoutItem *> itemList;
    int m_hSpace;
    int m_vSpace;
};
#endif

#include <QWidget>
#include "QFlowLayout.h"

QFlowLayout::QFlowLayout(QWidget *parent, int margin, int hSpacing, int vSpacing)
    : QLayout(parent),
    m_hSpace(hSpacing),
    m_vSpace(vSpacing)
{
    setContentsMargins(margin, margin, margin, margin);
}

QFlowLayout::QFlowLayout(int margin, int hSpacing, int vSpacing)
    : m_hSpace(hSpacing),
    m_vSpace(vSpacing)
{
    setContentsMargins(margin, margin, margin, margin);
}

QFlowLayout::~QFlowLayout()
{
    QLayoutItem *item;
    )))
        delete item;
}

void QFlowLayout::addItem(QLayoutItem *item)
{
    itemList.append(item);
}

int QFlowLayout::horizontalSpacing() const
{
    ) {
        return m_hSpace;
    }
    else {
        return smartSpacing(QStyle::PM_LayoutHorizontalSpacing);
    }
}

int QFlowLayout::verticalSpacing() const
{
    ) {
        return m_vSpace;
    }
    else {
        return smartSpacing(QStyle::PM_LayoutVerticalSpacing);
    }
}

int QFlowLayout::count() const
{
    return itemList.size();
}

QLayoutItem *QFlowLayout::itemAt(int index) const
{
    return itemList.value(index);
}

QLayoutItem *QFlowLayout::takeAt(int index)
{
     && index < itemList.size())
        return itemList.takeAt(index);
    else
        ;
}

Qt::Orientations QFlowLayout::expandingDirections() const
{
    ;
}

bool QFlowLayout::hasHeightForWidth() const
{
    return true;
}

int QFlowLayout::heightForWidth(int width) const
{
    , , width, ), true);
    return height;
}

void QFlowLayout::setGeometry(const QRect &rect)
{
    QLayout::setGeometry(rect);
    doLayout(rect, false);
}

QSize QFlowLayout::sizeHint() const
{
    return minimumSize();
}

QSize QFlowLayout::minimumSize() const
{
    QSize size;
    QLayoutItem *item;
    foreach(item, itemList)
        size = size.expandedTo(item->minimumSize());

    size += QSize( * margin(),  * margin());
    return size;
}

int QFlowLayout::doLayout(const QRect &rect, bool testOnly) const
{
    int left, top, right, bottom;
    getContentsMargins(&left, &top, &right, &bottom);
    QRect effectiveRect = rect.adjusted(+left, +top, -right, -bottom);
    int x = effectiveRect.x();
    int y = effectiveRect.y();
    ;

    QLayoutItem *item;
    foreach(item, itemList) {
        QWidget *wid = item->widget();
        int spaceX = horizontalSpacing();
        )
            spaceX = wid->style()->layoutSpacing(
                QSizePolicy::PushButton, QSizePolicy::PushButton, Qt::Horizontal);
        int spaceY = verticalSpacing();
        )
            spaceY = wid->style()->layoutSpacing(
                QSizePolicy::PushButton, QSizePolicy::PushButton, Qt::Vertical);

        int nextX = x + item->sizeHint().width() + spaceX;
        ) {
            x = effectiveRect.x();
            y = y + lineHeight + spaceY;
            nextX = x + item->sizeHint().width() + spaceX;
            lineHeight = ;
        }

        if (!testOnly)
            item->setGeometry(QRect(QPoint(x, y), item->sizeHint()));

        x = nextX;
        lineHeight = qMax(lineHeight, item->sizeHint().height());
    }
    return y + lineHeight - rect.y() + bottom;
}

int QFlowLayout::smartSpacing(QStyle::PixelMetric pm) const
{
    QObject *parent = this->parent();
    if (!parent) {
        ;
    }
    else if (parent->isWidgetType()) {
        QWidget *pw = static_cast<QWidget *>(parent);
        , pw);
    }
    else {
        return static_cast<QLayout *>(parent)->spacing();
    }
}