如何使用 QComboBox.setPlaceholderText? [英] How do i use QComboBox.setPlaceholderText?

问题描述

在 Qt 5.15 中引入了 placeholderText 属性 -

这是一个错误还是我错过了什么?我怎样才能做到这一点?

导入系统从 PyQt5 导入 QtWidgets从 PyQt5 导入 QtCore类 MainWindow(QtWidgets.QMainWindow):def __init__(self):超级().__init__()central_w = QtWidgets.QWidget()self.setCentralWidget(central_w)vbox = QtWidgets.QVBoxLayout()central_w.setLayout(vbox)self.combo_box = QtWidgets.QComboBox()self.combo_box.addItems([一"、二"、三"])self.combo_box.setPlaceholderText("这里有一些占位符文本")self.combo_box.setCurrentIndex(-1)vbox.addWidget(self.combo_box)如果 __name__ == __main__":应用程序 = QtWidgets.QApplication(sys.argv)主窗口 = 主窗口()main_window.show()app.exec_()

我没有找到显示占位符文本的方法.我试过在组合框中根本没有项目，但即使这样也不显示占位符文本

---

这些是我正在运行的版本:

• Qt:5.15.2
• PyQt(Python 模块)版本:5.15.2(这恰好与 Qt 版本相同，但有时可能略有不同)
• Python:3.8.5(默认，2020 年 7 月 28 日，12:59:40) [GCC 9.3.0]"
• 操作系统:Ubuntu 20.04.1 LTS(带 Xfce)

---

PS:如果你想达到类似的效果(即:在可点击区域上有一个文本，点击后显示带有不同选项的下拉菜单)你可以使用 QPushButtonsetMenu 函数.文档:https://doc.qt.io/qt-5/qpushbutton.html#setMenu

解决方案

查看Qt源码的修改，发现Qt 5.15.0增加placeHolderText的功能，修改了currentText():

//https://code.qt.io/cgit/qt/qtbase.git/tree/src/widgets/widgets/qcombobox.cpp?h=5.15.0#n2344QString QComboBox::currentText() 常量{Q_D(const QComboBox);if (d->lineEdit)return d->lineEdit->text();else if (d->currentIndex.isValid())返回 d->itemText(d->currentIndex);别的return d->placeholderText;}

但这会产生不良影响，QTBUG-86580在 Qt 5.15.2 中被删除:

QString QComboBox::currentText() const{Q_D(const QComboBox);if (d->lineEdit)return d->lineEdit->text();if (d->currentIndex.isValid())返回 d->itemText(d->currentIndex);返回 {};}

忘记纠正 placeHolder 不再可见的副作用.我已经报告了这个错误:QTBUG-90522.

考虑到上述情况，有以下备选方案:

• 使用 PyQt5/PySide2 5.15.0 或 5.15.1.

• 重写paintEvent方法将placeHolderText设置为currentText:

导入系统从 PyQt5 导入 QtCore、QtGui、QtWidgets# 或者# 从 PySide2 导入 QtCore、QtGui、QtWidgets类组合框(QtWidgets.QComboBox):# https://code.qt.io/cgit/qt/qtbase.git/tree/src/widgets/widgets/qcombobox.cpp?h=5.15.2#n3173defpaintEvent(自我，事件):画家 = QtWidgets.QStylePainter(self)Painter.setPen(self.palette().color(QtGui.QPalette.Text))# 绘制组合框框、focusrect 和 selected 等.opt = QtWidgets.QStyleOptionComboBox()self.initStyleOption(opt)Painter.drawComplexControl(QtWidgets.QStyle.CC_ComboBox，选择)如果 self.currentIndex() <0:opt.palette.setBrush(QtGui.QPalette.ButtonText，opt.palette.brush(QtGui.QPalette.ButtonText).color().lighter(),)如果 self.placeholderText():opt.currentText = self.placeholderText()# 绘制图标和文本Painter.drawControl(QtWidgets.QStyle.CE_ComboBoxLabel，选择)类 MainWindow(QtWidgets.QMainWindow):def __init__(self):超级().__init__()self.combo_box = 组合框()self.combo_box.addItems([一"、二"、三"])self.combo_box.setPlaceholderText("这里有一些占位符文本")self.combo_box.setCurrentIndex(-1)central_w = QtWidgets.QWidget()self.setCentralWidget(central_w)vbox = QtWidgets.QVBoxLayout(central_w)vbox.addWidget(self.combo_box)如果 __name__ == __main__":应用程序 = QtWidgets.QApplication(sys.argv)主窗口 = 主窗口()main_window.show()app.exec_()

In Qt 5.15 the placeholderText property was introduced - link to documentation

However using setPlaceholderText doesn't do anything for me. When running the code below i don't get any text in the QComboBox (unless of course i select one of the three items)

Is this a bug or am i missing something? How can i make this work?

import sys
from PyQt5 import QtWidgets
from PyQt5 import QtCore


class MainWindow(QtWidgets.QMainWindow):
    def __init__(self):
        super().__init__()

        central_w = QtWidgets.QWidget()
        self.setCentralWidget(central_w)
        vbox = QtWidgets.QVBoxLayout()
        central_w.setLayout(vbox)

        self.combo_box = QtWidgets.QComboBox()
        self.combo_box.addItems(["one", "two", "three"])
        self.combo_box.setPlaceholderText("Some placeholder text here")
        self.combo_box.setCurrentIndex(-1)
        vbox.addWidget(self.combo_box)


if __name__ == "__main__":
    app = QtWidgets.QApplication(sys.argv)
    main_window = MainWindow()
    main_window.show()
    app.exec_()

I have found no way of showing the placeholder text. I've tried having no items at all in the combo box but even this doesn't show the placeholder text

---

These are the versions i am running:

• Qt: 5.15.2
• PyQt (Python module) version: 5.15.2 (this happens to be the same as the Qt version but can sometimes differ slightly)
• Python: "3.8.5 (default, Jul 28 2020, 12:59:40) [GCC 9.3.0]"
• OS: Ubuntu 20.04.1 LTS (with Xfce)

---

PS: If you want to achieve a similar effect (that is: having a text on a clickable area which shows a drop-down with different options after being clicked) you can use a QPushButton with the setMenu function. Documentation: https://doc.qt.io/qt-5/qpushbutton.html#setMenu

解决方案

Reviewing the modifications of the Qt source code, it is observed that to add the functionality of the placeHolderText in Qt 5.15.0, the currentText() was modified:

// https://code.qt.io/cgit/qt/qtbase.git/tree/src/widgets/widgets/qcombobox.cpp?h=5.15.0#n2344
QString QComboBox::currentText() const
{
    Q_D(const QComboBox);
    if (d->lineEdit)
        return d->lineEdit->text();
    else if (d->currentIndex.isValid())
        return d->itemText(d->currentIndex);
    else
        return d->placeholderText;
}

But that generates an undesirable effect that was reported in QTBUG-86580 so that functionality was removed in Qt 5.15.2:

QString QComboBox::currentText() const
{
    Q_D(const QComboBox);
    if (d->lineEdit)
        return d->lineEdit->text();
    if (d->currentIndex.isValid())
        return d->itemText(d->currentIndex);
    return {};
}

Forgetting to correct the side effect which is that the placeHolder is no longer visible. I have already reported the bug: QTBUG-90522.

Considering the above, there are the following alternatives:

• Use PyQt5/PySide2 5.15.0 or 5.15.1.

• Override the paintEvent method to set the placeHolderText as currentText:

import sys
from PyQt5 import QtCore, QtGui, QtWidgets

# or
# from PySide2 import QtCore, QtGui, QtWidgets


class ComboBox(QtWidgets.QComboBox):
    # https://code.qt.io/cgit/qt/qtbase.git/tree/src/widgets/widgets/qcombobox.cpp?h=5.15.2#n3173
    def paintEvent(self, event):
        
        painter = QtWidgets.QStylePainter(self)
        painter.setPen(self.palette().color(QtGui.QPalette.Text))

        # draw the combobox frame, focusrect and selected etc.
        opt = QtWidgets.QStyleOptionComboBox()
        self.initStyleOption(opt)
        painter.drawComplexControl(QtWidgets.QStyle.CC_ComboBox, opt)

        if self.currentIndex() < 0:
            opt.palette.setBrush(
                QtGui.QPalette.ButtonText,
                opt.palette.brush(QtGui.QPalette.ButtonText).color().lighter(),
            )
            if self.placeholderText():
                opt.currentText = self.placeholderText()

        # draw the icon and text
        painter.drawControl(QtWidgets.QStyle.CE_ComboBoxLabel, opt)


class MainWindow(QtWidgets.QMainWindow):
    def __init__(self):
        super().__init__()

        self.combo_box = ComboBox()
        self.combo_box.addItems(["one", "two", "three"])
        self.combo_box.setPlaceholderText("Some placeholder text here")
        self.combo_box.setCurrentIndex(-1)

        central_w = QtWidgets.QWidget()
        self.setCentralWidget(central_w)
        vbox = QtWidgets.QVBoxLayout(central_w)
        vbox.addWidget(self.combo_box)


if __name__ == "__main__":
    app = QtWidgets.QApplication(sys.argv)
    main_window = MainWindow()
    main_window.show()
    app.exec_()

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