如何在Qt C ++中为自定义控件添加属性？

Question

How do I properly add a property to a custom control, which when changed triggers the paint event? The property only accepts an integer value of 0, 1 or 2.

The setter of the property would be called setPattern , so an example use would be setPattern(0) .

This property value will eventually be used to control the pattern drawn in the paint event.

labelheader.h

#ifndef LABELHEADER_H
#define LABELHEADER_H

#include <QLabel>

class LabelHeader : public QLabel
{
    Q_OBJECT
public:
    explicit LabelHeader(QWidget *parent = nullptr);
    explicit LabelHeader(const QString &text, QWidget *parent = nullptr);

signals:

public slots:

    // QWidget interface
protected:
    void paintEvent(QPaintEvent *event);
};

#endif // LABELHEADER_H

labelheader.cpp

#include "labelheader.h"
#include <QPainter>

LabelHeader::LabelHeader(QWidget *parent) :
    QLabel(parent)
{

}

LabelHeader::LabelHeader(const QString &text, QWidget *parent) :
    QLabel(text, parent)
{

}

void LabelHeader::paintEvent(QPaintEvent *event)
{
    // calculate font width
    QFontMetrics metrics(font());
    int text_width = metrics.boundingRect(text()).width();

    // calculate dimensions
    int y = height() * 0.5;
    int x = text_width + 4;

    // create pattern
    QPixmap px(4, 4);
    px.fill(Qt::transparent);

    QPainter pattern_painter(&px);

    // Create ashed 3 dots
    pattern_painter.setPen(Qt::NoPen);
    pattern_painter.setBrush(QBrush(palette().color(QPalette::WindowText), Qt::SolidPattern));
    pattern_painter.drawRect(0, 0, 1, 1);
    pattern_painter.drawRect(2, 2, 1, 1);

    // draw
    QPainter painter(this);

    // Draw dashed 3 dots
    painter.drawTiledPixmap(x, y-2, width()-x, 5, px);

    // Draw solid line
    //painter.drawLine(x,y,width(),y);

    QLabel::paintEvent(event);
}

Answer 1

You can simply call update() in the setter. If you want to only accept limited range of values, you can limit that in a setter or use an enum instead of an int.

#include <QLabel>

class LabelHeader : public QLabel
{
    Q_OBJECT
public:
    Q_PROPERTY(Pattern pattern READ(getPattern) WRITE(setPattern))

    enum Pattern { FIRST = 0, SECOND, THIRD };
    Q_ENUM(Pattern)

...

    void setPattern(Pattern value)
    {
        if (pattern != value)
        {
            pattern = value;
            update();
        }
    }

    Pattern getPattern() const { return pattern; }
...

private:
    Pattern pattern { FIRST };
};

Answer 2

This is how I do it. NOTE: I haven't written an actual custom control in a "library", I usually just use "promoted widgets".

Header File:

#ifndef MYWIDGET_H
#define MYWIDGET_H

#include <QObject>
#include <QWidget>

class MyWidget: public QWidget
{
    Q_OBJECT
    Q_PROPERTY(int value READ value WRITE setValue)

    private:
        int m_value;

    public:
        MyWidget(QWidget *parent=0);

        inline int value() const {return m_value;}
        void setValue(int value);

    signals:
        void valueChanged();
};

#endif // MYWIDGET_H

Source File:

#include "MyWidget.h"

MyWidget::MyWidget(QWidget *parent)
    :QWidget(parent)
{
    m_value = 0;
}

void MyWidget::setValue(int value)
{
    if (m_value == value) return;

    m_value = value;
    emit valueChanged();

    update();   // This will cause repaint
}

Answer 3

Solution

To accomplish what you need you do not actually need QProperty , but a private attribute of your LabelHeader class. Here is a step-by-step guide, covering all your requirements:

1. Create the enumeration type

In LabelHeader.h add:

enum PatternType {
    PatternOne = 0,
    PatternTwo,
    PatternThree
};

2. Declare a private attribute to hold the current pattern

In LabelHeader.h add:

private:
    PatternType m_pattern;

3. Right click m_pattern and select Refactor->Create Getter and Setter Member Functions

4. In the implementation of the setter invoke a paintEvent by calling update() , but only if the new pattern is different than the current one:

    if (m_pattern == pattern) return; m_pattern = pattern; update(); 

5. In the constructor add m_pattern to the initializer list to set its value by default

6. Use the value in the LabelHeader::paintEvent to draw the corresponding pattern

    // draw tiled pixmap QPainter painter(this); switch (m_pattern) { case PatternOne: painter.drawTiledPixmap(x, y-2, w, 5, px); break; case PatternTwo: painter.drawTiledPixmap(x, y, w, 5, px); break; default: painter.drawTiledPixmap(x, y+2, w, 5, px); } 

Example

I have extended the example from this answer to cover your current requirements as well. The full code is available on GitHub .

Remarks

Regarding the use of Q_PROPERTY , here is an important explanation kindly provided by @KubaOber:

Q_PROPERTY is a Qt idiom used to both convey a meaning to a human reader, and expose the property as such to the metatype system, making it easily scriptable from QML, etc.