Qt4 如何在小部件内部绘图？

发布于 2024-09-26 21:30:50 字数 399 浏览 0 评论 0原文

Qt4，QtCreator

我试图在 Widget 内绘制：

void Widget::on_pushButton_clicked()
{
    QPainter painter;

    painter.begin(ui->label);

    QRectF rectangle(10.0, 20.0, 80.0, 60.0);
     int startAngle = 30 * 16;
     int spanAngle = 120 * 16;
     painter.drawArc(rectangle, startAngle, spanAngle);

    painter.end();
}

但是当我按下按钮时什么也没有发生。

如何正确地做到这一点？

原文

Qt4, QtCreator

I am trying to draw inside Widget:

void Widget::on_pushButton_clicked()
{
    QPainter painter;

    painter.begin(ui->label);

    QRectF rectangle(10.0, 20.0, 80.0, 60.0);
     int startAngle = 30 * 16;
     int spanAngle = 120 * 16;
     painter.drawArc(rectangle, startAngle, spanAngle);

    painter.end();
}

But when I press button nothing happens.

How to do this right way?

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翻了热茶 2024-10-03 21:30:50

您需要重写 paintEvent() 并在那里进行绘画。您实际上并不需要 begin() 和 end()。使用声明画家

QPainter painter(this);

构造函数将处理 begin()，并且当 painter 对象超出范围并被调用时，将调用 end()被毁了。

您也不需要单击事件来触发绘画。每当小部件需要绘制自身时，paintEvent()就会被调用。您可以使用按钮单击来切换一个布尔值，paintEvent() 检查该布尔值以确定是否应显示矩形和圆弧。只需确保在切换变量后调用 update() 即可。

void Widget::on_pushButton_clicked()
{
    drawShapes = !drawShapes;
    update();
}

void Widget::paintEvent(QPaintEvent *)
{
    QPainter painter(this);

    if(drawShapes)
    {
        QRectF rectangle(10.0, 20.0, 80.0, 60.0);
        int startAngle = 30 * 16;
        int spanAngle = 120 * 16;
        painter.drawArc(rectangle, startAngle, spanAngle);
    }
}

更新：

为了避免重写小部件的paintEvent()，您可以使用QLabel并为其分配一个像素图并绘制那。注意：据我所知，每次修改像素图时都需要设置它。

void MainForm::slot_buttonClick()
{
    QPixmap pixmap(100,100);
    pixmap.fill(QColor("transparent"));

    QPainter painter(&pixmap);
    painter.setBrush(QBrush(Qt::black));
    painter.drawRect(10, 10, 100, 100);

    label.setPixmap(pixmap);
}

You need to override paintEvent() and do your painting there. You don't really need the begin() and end(). Declare the painter with

QPainter painter(this);

The constructor will handle begin(), and end() will be called when the painter object goes out of scope and is destroyed.

You also won't need the click event to trigger the painting. paintEvent() will be called whenever the widget needs to draw itself. You could use the the button click to toggle a boolean that the paintEvent() checks to determine whether or not it should display the rectangle and arc. Just make sure you call update() after you toggle the variable.

void Widget::on_pushButton_clicked()
{
    drawShapes = !drawShapes;
    update();
}

void Widget::paintEvent(QPaintEvent *)
{
    QPainter painter(this);

    if(drawShapes)
    {
        QRectF rectangle(10.0, 20.0, 80.0, 60.0);
        int startAngle = 30 * 16;
        int spanAngle = 120 * 16;
        painter.drawArc(rectangle, startAngle, spanAngle);
    }
}

UPDATE:

To avoid having to override the paintEvent() of a widget, you could use a QLabel and assign a pixmap to it and paint to that. Note: As far as I can tell, you will need to set the pixmap each time you modify it.

void MainForm::slot_buttonClick()
{
    QPixmap pixmap(100,100);
    pixmap.fill(QColor("transparent"));

    QPainter painter(&pixmap);
    painter.setBrush(QBrush(Qt::black));
    painter.drawRect(10, 10, 100, 100);

    label.setPixmap(pixmap);
}

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