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在网上看了一些相关的教程，动手改动了其中的一些代码
没有用QtDesigner。

• 
  先记录几条学到的经验
  
  
  1.QPainter（）初始化中选择的画板一般都是pixmap或者主页面（self）
  
  2.如果想保存绘画痕迹的话，先画在pixmap上面，再把pixmap画出来
  
  3.如果想把绘画范围控制在一定区域内的话，可以重写QLabel类，重载其中的paintEvent和与鼠标有关的事件
• 
  上代码
  

import sys
from PyQt5.QtWidgets import QMainWindow,QApplication,QWidget,QMessageBox,QLineEdit,QInputDialog,QFileDialog,QColorDialog,QFontDialog,QLabel,QPushButton,QVBoxLayout,QStyle,QAction
from PyQt5.QtGui import QIcon,QPixmap,QPainter,QPen,QColor,QBrush,QFont
from PyQt5.QtCore import Qt,QRect


class mylable(QLabel):
    x0=0
    y0=0
    x1=0
    y1=0
    flag=False
    def __init__(self,parent):
        super(mylable,self).__init__(parent)
        self.pixmap = QPixmap(597, 497)#考虑边框的间距 减去px
        self.pixmap.fill(Qt.white)
        self.setStyleSheet("border: 2px solid red")
        self.Color=Qt.blue#pen color: defult:blue
        self.penwidth=4#pen width : default:4

    def paintEvent(self,event):
        #super().paintEvent(event)

        painter=QPainter(self.pixmap)
        painter.setPen(QPen(self.Color,self.penwidth,Qt.SolidLine))
        painter.drawLine(self.x0,self.y0,self.x1,self.y1)

        Label_painter=QPainter(self)
        Label_painter.drawPixmap(2,2,self.pixmap)

    def mousePressEvent(self, event):
        self.x1=event.x()
        self.y1=event.y()
        self.flag=True

    def mouseMoveEvent(self, event):
        if self.flag:
            self.x0 = self.x1
            self.y0 = self.y1
            self.x1 = event.x()
            self.y1 = event.y()
            self.update()

    def mouseReleaseEvent(self, event):
        self.flag=False


class cb(QMainWindow):
    def __init__(self):
        super(cb,self).__init__()
        self.initUi()
        
    def initUi(self):
        self.resize(800,600)
        vbox=QVBoxLayout()
        #选择绘画文件 按钮
        self.button_file=QPushButton(self)
        self.button_file.setGeometry(660,80,100,50)
        self.button_file.setText("File")
        self.button_file.setFont(QFont("Chiller",20))
        vbox.addWidget(self.button_file)
        vbox.addStretch()
        #选择画笔颜色 按钮
        self.button_color=QPushButton(self)
        self.button_color.setGeometry(660,270,100,50)
        self.button_color.setText("Color")
        self.button_color.setFont(QFont("Chiller",20))
        vbox.addWidget(self.button_color)
        vbox.addStretch()
        #选择画笔粗细 按钮
        self.button_width=QPushButton(self)
        self.button_width.setGeometry(660,460,100,50)
        self.button_width.setText("Width")
        self.button_width.setFont(QFont("Chiller",20))
        vbox.addWidget(self.button_width)
        vbox.addStretch()
        #设置画板
        self.lb=mylable(self)
        self.lb.setGeometry(20, 40, 601, 501)
        #橡皮
        eraser=QAction(QIcon("1239616.png"),"Eraser",self)
        eraser.setToolTip("Eraser")
        #工具栏
        self.menubar=self.addToolBar("ToolBar")
        self.menubar.addAction(eraser)
        #主页面
        self.setWindowIcon(QIcon("1216867.png"))
        self.setWindowTitle("Drawing Board")
        
        self.button_file.clicked.connect(self.openfile)
        self.button_color.clicked.connect(self.choose_color)
        eraser.triggered.connect(self.erase)
        self.button_width.clicked.connect(self.choose_width)

    def openfile(self):
        fname=QFileDialog.getOpenFileName(self,"选择图片文件",".")
        if fname[0]:
            self.lb.pixmap=QPixmap(fname[0])

    def choose_color(self):
        Color=QColorDialog.getColor()#color是Qcolor
        if Color.isValid():
            self.lb.Color=Color

    def erase(self):
        self.lb.Color=Qt.white
        self.lb.setCursor(Qt.CrossCursor)
        self.lb.penwidth=self.lb.penwidth+2

    def choose_width(self):
        width, ok = QInputDialog.getInt(self, '选择画笔粗细', '请输入粗细：',min=1,step=1)
        if ok:
            self.lb.penwidth=width

if __name__ == '__main__':
    app=QApplication(sys.argv)
    mainwindow=cb()
    mainwindow.show()
    sys.exit(app.exec_())