Qt控件焦点切换

们日常切换控件，例如QQ登陆的账号和密码输入框就可以通过Tab键切换焦点。 
 
图1 qq切换焦点 
Qt中QWidget提供了一个静态方式实现该效果 
其中也包含介绍使用

[static] void QWidget::setTabOrder(QWidget *first, QWidget *second)
Puts the second widget after the first widget in the focus order.
Note that since the tab order of the second widget is changed, you should order a chain like this:

  setTabOrder(a, b); // a to b
  setTabOrder(b, c); // a to b to c
  setTabOrder(c, d); // a to b to c to d

not like this:

  // WRONG
  setTabOrder(c, d); // c to d
  setTabOrder(a, b); // a to b AND c to d
  setTabOrder(b, c); // a to b to c, but not c to d

If first or second has a focus proxy, setTabOrder() correctly substitutes the proxy.

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实现效果如下 
 
图2 Tab切换焦点 
主要部分源码

    QPushButton *p = new QPushButton("按钮");
    QLineEdit *l = new QLineEdit("输入框");
    QCheckBox *c =  new QCheckBox("复选框");
    QComboBox *b = new QComboBox;
    setTabOrder(p, l);
    setTabOrder(l, c);
    setTabOrder(c, b);

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当然也可以让控件屏蔽焦点的使用，或者接受响应其他类型的焦点方式 
主要使用

void setFocusPolicy(Qt::FocusPolicy policy)

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其中Qt::FocusPolicy 这个枚举类型定义了一个控件可以用来获取键盘焦点的各种策略。

Constant	Value	Description
Qt::TabFocus	0x1	the widget accepts focus by tabbing.
Qt::ClickFocus	0x2	the widget accepts focus by clicking.
Qt::StrongFocus	TabFocus | ClickFocus | 0x8	he widget accepts focus by both tabbing and clicking. On macOS this will also be indicate that the widget accepts tab focus when in ‘Text/List focus mode’.
Qt::WheelFocus	StrongFocus | 0x4	like Qt::StrongFocus plus the widget accepts focus by using the mouse wheel.
Qt::NoFocus	0	the widget does not accept focus.

例如设置 
QCheckBox为NoFouse

c->setFocusPolicy(Qt::NoFocus);

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效果如下 
 
图3 复选框设置NoFocuse 
函数

[slot] void QWidget::setFocus() //可通过信号槽方式设置
This is an overloaded function. 
Gives the keyboard input focus to this widget (or its focus proxy) if this widget or one of its parents is the active window.

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可直接设置焦点

接下来可以看一下Qt是怎么实现焦点切换的，查看qwidget.cpp源码

void QWidget::setTabOrder(QWidget* first, QWidget *second)
{
    //如果这两个控件都设置为没有焦点则不进行焦点设置，如图3中所示
    if (!first || !second || first->focusPolicy() == Qt::NoFocus || second->focusPolicy() == Qt::NoFocus)
        return;

    /******************************************************************
            QWidget *QWidget::window() const
        {
            QWidget *w = const_cast<QWidget *>(this);
            QWidget *p = w->parentWidget();
            while (!w->isWindow() && p) {
                w = p;
                p = p->parentWidget();
            }
            return w;
        }
    ******************************************************************/
    //查看当前控件“祖先”(ancestor widget)窗口，见上面代码
    if (Q_UNLIKELY(first->window() != second->window())) {
        qWarning("QWidget::setTabOrder: 'first' and 'second' must be in the same window");
        return;
    }

    //找到first或其子类中焦点的控件(不是很懂)
    QWidget *fp = first->focusProxy();
    if (fp) {
        QList<QWidget *> l = first->findChildren<QWidget *>();
        for (int i = l.size()-1; i >= 0; --i) {
            QWidget * next = l.at(i);
            if (next->window() == fp->window()) {
                fp = next;
                if (fp->focusPolicy() != Qt::NoFocus)
                    break;
            }
        }
        first = fp;
    }

    if (fp == second)
        return;

    if (QWidget *sp = second->focusProxy())
        second = sp;

    //双向链表存储焦点触发控件顺序
//    QWidget *fp = first->d_func()->focus_prev;
    QWidget *fn = first->d_func()->focus_next;

    if (fn == second || first == second)
        return;

    QWidget *sp = second->d_func()->focus_prev;
    QWidget *sn = second->d_func()->focus_next;

    fn->d_func()->focus_prev = second;
    first->d_func()->focus_next = second;

    second->d_func()->focus_next = fn;
    second->d_func()->focus_prev = first;

    sp->d_func()->focus_next = sn;
    sn->d_func()->focus_prev = sp;

    //查错
    Q_ASSERT(first->d_func()->focus_next->d_func()->focus_prev == first);
    Q_ASSERT(first->d_func()->focus_prev->d_func()->focus_next == first);

    Q_ASSERT(second->d_func()->focus_next->d_func()->focus_prev == second);
    Q_ASSERT(second->d_func()->focus_prev->d_func()->focus_next == second);
}

http://blog.csdn.net/gx864102252/article/details/72731992