为什么QComboBox忽略其固定的垂直大小策略？

Question

我有一个有两个堆叠层的小部件。第一个看起来是这样的，很好：

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第二个看起来是这样的，这是错误的。

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下面是生成小部件并打印大小策略的完整代码。

#include <QtWidgets>

int main (int argc, char ** argv)
{
    QApplication app (argc, argv);

    QWidget w;
    auto stack = new QStackedWidget ();
    QWidget * labelbox = new QWidget ();
    QWidget * combobox = new QComboBox ();
    QWidget * label = new QLabel ("Label");
    QWidget * button = new QPushButton ("Button");
    QWidget * date = new QDateTimeEdit ();

    auto gl = new QGridLayout (& w);
    auto hl = new QHBoxLayout (labelbox);

    hl -> addWidget (label);
    hl -> addWidget (button);

    stack -> addWidget (labelbox);
    stack -> addWidget (combobox);
    stack -> setCurrentIndex (1); // 0 is the good layer, 1 is the bad

    gl -> addWidget (date);
    gl -> addWidget (stack, 0, 1);

    w .setMinimumSize (300, 200);
    w .show ();

    for (auto x : {(QWidget *) stack, labelbox, combobox, label, button, date})
    {
        qWarning ()
            << x -> metaObject () -> className ()
            << x -> sizePolicy ();
    }
    return app .exec ();
}

标准如下：

QStackedWidget QSizePolicy(horizontalPolicy = 5, verticalPolicy = 5)
QWidget        QSizePolicy(horizontalPolicy = 5, verticalPolicy = 5)
QComboBox      QSizePolicy(horizontalPolicy = 5, verticalPolicy = 0)
QLabel         QSizePolicy(horizontalPolicy = 1, verticalPolicy = 5)
QPushButton    QSizePolicy(horizontalPolicy = 1, verticalPolicy = 0)
QDateTimeEdit  QSizePolicy(horizontalPolicy = 1, verticalPolicy = 0)

这些值的文档如下：

QSizePolicy::Fixed      = 0 The QWidget::sizeHint() is the only acceptable alternative, so the widget can never grow or shrink (e.g. the vertical direction of a push button).
QSizePolicy::GrowFlag   = 1 The widget can grow beyond its size hint if necessary.
QSizePolicy::ExpandFlag = 2 The widget should get as much space as possible.
QSizePolicy::ShrinkFlag = 4 The widget can shrink below its size hint if necessary.
QSizePolicy::IgnoreFlag = 8 The widget's size hint is ignored. The widget will get as much space as possible.

所以看起来垂直大小策略是Fixed，除了QStackedWidget，包含标签和按钮的QWidget，以及QLabel，它们都有GrowFlag|ShrinkFlag。

为什么QComboBox会垂直增长，尽管它有Fixed政策？为什么这种情况不会发生在QPushButton或QDateTimeEdit上？

这发生在Ubuntu15.04上的QT4.8和QT5.1中。

Answer 1

您正在将组合框直接分配给QStackedWidget，但尚未首先将其添加到QLayout中。将其分配给布局，将此布局添加到QWidget，然后将此QWidget添加到QStackedWidget中。

示例：

QWidget * combowidget = new QWidget ();
QWidget * combobox = new QComboBox ();

auto vl = new QVBoxLayout (combowidget);

vl->addWidget(combobox);
stack->addWidget (combowidget);