Question

当我点击绘图按钮时，我想绘制一个网格（一系列线条），当我点击清除按钮时，我希望它们清除。

我让网格显示为一个独立的程序，但我无法弄清楚如何将它与QPushButton结合起来。

在程序运行时单击“绘图”按钮时，我收到以下消息。

“QPainter :: begin：Paint device返回引擎== 0，键入：1 QPainter :: setPen：画家不活跃“

谢谢

#include <QtGui> 
#include <QPainter>
#include "myqtapp.h"

// including <QtGui> saves us to include every class user, <QString>, <QFileDialog>,...

myQtApp::myQtApp(QWidget *parent)
{
    setupUi(this); // this sets up GUI

        // signals/slots mechanism in action
        connect( pushButton_draw, SIGNAL( clicked() ), this, SLOT( draw() ) );
    connect( pushButton_clear, SIGNAL( clicked() ), this, SLOT( clear() ) ); 
    connect( pushButton_about, SIGNAL( clicked() ), this, SLOT( about() ) ); 
}



void myQtApp::draw()
{
    //draw the grid

    int lineSpacing(30),// line spacing in pixels
            numberOfLines;

    QPen pen(Qt::black, 2, Qt::SolidLine);

    QPainter painter(this);
    painter.setPen(pen);

    //Grid takes up at most a 400x400 area starting at (right 150, down 50) from upper left
    numberOfLines = 400/lineSpacing; //Round down grid size to fit in 400x400
    for(int i = 0; i<numberOfLines; i++){
        painter.drawLine(150, 50+i*lineSpacing, 150+(numberOfLines-1)*lineSpacing, 50+i*lineSpacing);
        painter.drawLine(150+i*lineSpacing, 50, 150+i*lineSpacing, 50+(numberOfLines-1)*lineSpacing );
    }

}

Answer 1

您遇到的问题是因为您试图在窗口小部件的paintEvent（）调用之外使用QPainter绘制UI - 来自Qt文档：

QPainter的常见用法是在widget的paint事件中：Construct 并定制（例如设置笔或画笔）画家。然后画画。记得在绘图后销毁QPainter对象。

如果您尝试在paintEvent（）调用之外绘制窗口小部件，则结果是不可预测的。

这样做的正确方法是这样的：

   // myQtApp.h
   class myQtApp : public QWidget
   {
   Q_OBJECT
   public:
      myQtApp(QWidget *parent = 0); // Constructor as you have
   protected:
      void paintEvent(QPaintEvent *event); // This is re-implemented from QWidget
   protected slots:
      void draw();
   private:
      bool drawTheLines;
   }

和

   // myQtApp.cpp
   void myQtApp::paintEvent(QPaintEvent *event)
   {
      QPainter painter(this);

      if(drawTheLines)
      {
         // Do the drawing here - as in your current draw() function
      }

      QWidget::paintEvent(event); // call the base class so everything else is drawn OK
   }


   void draw();
   {
      drawTheLines = true;
      update(); // This forces a repaint of the widget with paintEvent()
   }

基本Qt GUI：用于绘制线的QPushButton

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