运行结果(GIF动图)：

widget.cpp

附代码：

xiaowen_QT_day25.pro

QT       += core guigreaterThan(QT_MAJOR_VERSION, 4): QT += widgetsCONFIG += c++11# The following define makes your compiler emit warnings if you use
# any Qt feature that has been marked deprecated (the exact warnings
# depend on your compiler). Please consult the documentation of the
# deprecated API in order to know how to port your code away from it.
DEFINES += QT_DEPRECATED_WARNINGS# You can also make your code fail to compile if it uses deprecated APIs.
# In order to do so, uncomment the following line.
# You can also select to disable deprecated APIs only up to a certain version of Qt.
#DEFINES += QT_DISABLE_DEPRECATED_BEFORE=0x060000    # disables all the APIs deprecated before Qt 6.0.0SOURCES += \main.cpp \widget.cppHEADERS += \widget.h# Default rules for deployment.
qnx: target.path = /tmp/$${TARGET}/bin
else: unix:!android: target.path = /opt/$${TARGET}/bin
!isEmpty(target.path): INSTALLS += target

widget.h

#ifndef WIDGET_H
#define WIDGET_H#include <QWidget>class Widget : public QWidget
{Q_OBJECTpublic:Widget(QWidget *parent = nullptr);~Widget();void paintEvent(QPaintEvent *);
public slots:void setBlackSlot();void updateBlueSlot();void repaintYellowSlot();
private:QColor _bgColor;
};
#endif // WIDGET_H

widget.cpp

#include "widget.h"
#include<QPainter>
#include<QDialog>
#include<QPushButton>
#include<QHBoxLayout>
Widget::Widget(QWidget *parent): QWidget(parent),_bgColor(Qt::red)
{/*布局控制窗口*/QDialog* ctrlDialog=new QDialog(this);QHBoxLayout* hBox=new QHBoxLayout(ctrlDialog);QPushButton* pb=new QPushButton("setBlackSlot");hBox->addWidget(pb);QPushButton* pb1=new QPushButton("updateblue");hBox->addWidget(pb1);QPushButton* pb2=new QPushButton("repaintYellow");hBox->addWidget(pb2);connect(pb,SIGNAL(clicked()),this,SLOT(setBlackSlot()));connect(pb1,SIGNAL(clicked()),this,SLOT(updateBlueSlot()));connect(pb2,SIGNAL(clicked()),this,SLOT(repaintYellowSlot()));ctrlDialog->show();}
void Widget::setBlackSlot()
{_bgColor=Qt::black;this->update();
}
void Widget::updateBlueSlot()
{_bgColor=Qt::blue;this->update();//调用update的时候，并不出马上触发painrEvent事件，//而是把绘画事件丢到消息循环中，等待消息循环调度，会多次update合并成一个。
}
void Widget::repaintYellowSlot()
{_bgColor=Qt::yellow;//this->update();this->repaint(0,0,150,100);//调用repaint的时候，马上触发paintEvent事件
}void Widget::paintEvent(QPaintEvent *)
{QPainter painter;painter.begin(this);painter.setBrush(_bgColor);painter.drawRect(this->rect());painter.end();
}Widget::~Widget()
{}

main.cpp

#include "widget.h"#include <QApplication>int main(int argc, char *argv[])
{QApplication a(argc, argv);Widget w;w.show();return a.exec();
}

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QT每日一练day25：触发绘画事件

运行结果(GIF动图)：

附代码：

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