如何在 QLayout 中查找给定类型的小部件?

问题描述

我可以通过这种方式找到 QRadioButton:

I am able to find the QRadioButton this way:

for(int i = 0; i < ui->verticalLayout->count(); i++)
{
    QRadioButton* r = qobject_cast<QRadioButton*>(ui->verticalLayout->itemAt(i)->widget());
    if(r->isChecked())
        //found it!
}

但我不喜欢这种迭代元素的方式，并希望使用 foreach 构造.我的第一次尝试失败了:

But I don't like this way of iterating over elements and would like to use the foreach construct. My first attempt fails:

foreach(QRadioButton* child, ui->verticalLayout->findChildren<QRadioButton*>())
{
    if(child->isChecked())
        //found it!
}

问题在于 ui->verticalLayout->findChildren() 返回零元素.它也不返回带有 findChildren() 的元素.有人可以解释一下这种行为吗?

Problem is that the ui->verticalLayout->findChildren<QRadioButton*>() returns zero elements. It also returns no elements with findChildren<QObject*>(). Can someone please explain this behaviour?

注意:这个标题 问题与我的几乎相同，但它与python Qt有关，并且不包含任何对我有用的信息.

Note: the title of this question is almost identical to mine, but it is related to python Qt, and does not contain any helpful information for me.

实验上我认为 ui->verticalLayout->children().count() 返回零，其中 ui->verticalLayout->count() 返回我在 verticalLayout 中的元素数量.这意味着 itemAt(i) 和 findChild() 不会访问同一个列表.查看关于 children() 的 Qt 文档对我没有帮助.

Experimentally I figured that ui->verticalLayout->children().count() returns zero where as ui->verticalLayout->count() returns the number of elements I have in the verticalLayout. This implies that itemAt(i) and findChild<QRadioButton*>() do not access the same list. Looking at the Qt documentation on children() did not help me.

有人能给我指点关于 Qt 子父概念的好材料吗?我假设这与访问嵌套对象无关，而这正是我想要完成的.

Can someone point me to a good material on Qt child parent concepts? I am assuming that this has nothing to do with accesing nested objects which is what I am trying to accomplish.

正如 Kuba Ober 所建议的，这个问题的答案包含关于另一个主题的宝贵信息，而他的回答澄清了我关于布局子项的问题.因此这不是一个重复的问题.

As suggested by Kuba Ober, the answers to this question contains valuable information on another topic, whereas his answer makes clarification on my question about children of layout. Thus this is not a duplicate question.

推荐答案

就 QObject 子项而言，小部件不是布局的子项 - 它们是父小部件的子项.QWidget 只能是另一个 QWidget 的子项 - 因此您不能期望小部件是布局的子项.虽然 new QWidget(new QWidget()) 有效，但 new QWidget(new QHBoxLayout()) 不会编译.

The widgets are not children of the layout in the sense of being QObject children - they are children of the parent widget. A QWidget can only be a child of another QWidget - thus you can't ever expect widgets to be layout's children. While new QWidget(new QWidget()) works, new QWidget(new QHBoxLayout()) won't compile.

您可以按如下方式迭代给定类型的小部件的子部件:

You could iterate a widget's children of a given type as follows:

// C++11
for (auto button : findChildren<QRadioButton*>()) if (button->isChecked()) {
  ...
}

// C++98
Q_FOREACH (QWidget * button, findChildren<QRadioButton*>())
  if (button->isChecked()) {
    ...
  }

如果你使用 C++11，你应该使用 range-基于 for 循环，而不是现在过时的 foreach 或 Q_FOREACH.

If you're using C++11, you should use the range-based for loop, not the now obsolete foreach or Q_FOREACH.

要迭代由布局管理的子小部件，您需要一个用于布局的迭代器适配器.例如:

To iterate the child widgets managed by a layout, you need an iterator adapter for the layout. For example:

#include <QLayout>
#include <QDebug>
#include <QPointer>
#include <utility>

template<class WT> class IterableLayoutAdapter;

template<typename WT>
class LayoutIterator {
   QPointer<QLayout> m_layout;
   int m_index;
   friend class IterableLayoutAdapter<WT>;
   LayoutIterator(QLayout * layout, int dir) :
      m_layout(layout), m_index(dir>0 ? -1 : m_layout->count()) {
      if (dir > 0) ++*this;
   }
   friend QDebug operator<<(QDebug dbg, const LayoutIterator & it) {
      return dbg << it.m_layout << it.m_index;
   }
   friend void swap(LayoutIterator& a, LayoutIterator& b) {
      using std::swap;
      swap(a.m_layout, b.m_layout);
      swap(a.m_index, b.m_index);
   }
public:
   LayoutIterator() : m_index(0) {}
   LayoutIterator(const LayoutIterator & o) :
      m_layout(o.m_layout), m_index(o.m_index) {}
   LayoutIterator(LayoutIterator && o) { swap(*this, o); }
   LayoutIterator & operator=(LayoutIterator o) {
      swap(*this, o);
      return *this;
   }
   WT * operator*() const { return static_cast<WT*>(m_layout->itemAt(m_index)->widget()); }
   const LayoutIterator & operator++() {
      while (++m_index < m_layout->count() && !qobject_cast<WT*>(m_layout->itemAt(m_index)->widget()));
      return *this;
   }
   LayoutIterator operator++(int) {
      LayoutIterator temp(*this);
      ++*this;
      return temp;
   }
   const LayoutIterator & operator--() {
      while (!qobject_cast<WT*>(m_layout->itemAt(--m_index)->widget()) && m_index > 0);
      return *this;
   }
   LayoutIterator operator--(int) {
      LayoutIterator temp(*this);
      --*this;
      return temp;
   }
   bool operator==(const LayoutIterator & o) const { return m_index == o.m_index; }
   bool operator!=(const LayoutIterator & o) const { return m_index != o.m_index; }
};

template <class WT = QWidget>
class IterableLayoutAdapter {
   QPointer<QLayout> m_layout;
public:
   typedef LayoutIterator<WT> const_iterator;
   IterableLayoutAdapter(QLayout * layout) : m_layout(layout) {}
   const_iterator begin() const { return const_iterator(m_layout, 1); }
   const_iterator end() const { return const_iterator(m_layout, -1); }
   const_iterator cbegin() const { return const_iterator(m_layout, 1); }
   const_iterator cend() const { return const_iterator(m_layout, -1); }
};

template <class WT = QWidget>
class ConstIterableLayoutAdapter : public IterableLayoutAdapter<const WT> {
public:
   ConstIterableLayoutAdapter(QLayout * layout) : IterableLayoutAdapter<const WT>(layout) {}
};

用法如下:

#include <QApplication>
#include <QLabel>
#include <QHBoxLayout>

int main(int argc, char ** argv) {
   QApplication app(argc, argv);
   tests();
   QWidget a, b1, b3;
   QLabel b2;
   QHBoxLayout l(&a);
   l.addWidget(&b1);
   l.addWidget(&b2);
   l.addWidget(&b3);

   // Iterate all widget types as constants
   qDebug() << "all, range-for";
   for (auto widget : ConstIterableLayoutAdapter<>(&l)) qDebug() << widget;
   qDebug() << "all, Q_FOREACH";
   Q_FOREACH (const QWidget * widget, ConstIterableLayoutAdapter<>(&l)) qDebug() << widget;

   // Iterate labels only
   qDebug() << "labels, range-for";
   for (auto label : IterableLayoutAdapter<QLabel>(&l)) qDebug() << label;
   qDebug() << "labels, Q_FOREACH";
   Q_FOREACH (QLabel * label, IterableLayoutAdapter<QLabel>(&l)) qDebug() << label;
}

一些基本测试如下:

void tests() {
   QWidget a, b1, b3;
   QLabel b2;
   QHBoxLayout l(&a);

   IterableLayoutAdapter<> l0(&l);
   auto i0 = l0.begin();
   qDebug() << i0; Q_ASSERT(i0 == l0.begin() && i0 == l0.end());

   l.addWidget(&b1);
   l.addWidget(&b2);
   l.addWidget(&b3);

   IterableLayoutAdapter<> l1(&l);
   auto i1 = l1.begin();
         qDebug() << i1; Q_ASSERT(i1 == l1.begin() && i1 != l1.end());
   ++i1; qDebug() << i1; Q_ASSERT(i1 != l1.begin() && i1 != l1.end());
   ++i1; qDebug() << i1; Q_ASSERT(i1 != l1.begin() && i1 != l1.end());
   ++i1; qDebug() << i1; Q_ASSERT(i1 != l1.begin() && i1 == l1.end());
   --i1; qDebug() << i1; Q_ASSERT(i1 != l1.begin() && i1 != l1.end());
   --i1; qDebug() << i1; Q_ASSERT(i1 != l1.begin() && i1 != l1.end());
   --i1; qDebug() << i1; Q_ASSERT(i1 == l1.begin() && i1 != l1.end());

   IterableLayoutAdapter<QLabel> l2(&l);
   auto i2 = l2.begin();
         qDebug() << i2; Q_ASSERT(i2 == l2.begin() && i2 != l2.end());
   ++i2; qDebug() << i2; Q_ASSERT(i2 != l2.begin() && i2 == l2.end());
   --i2; qDebug() << i2; Q_ASSERT(i2 == l2.begin() && i2 != l2.end());
}

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