QGraphicsView鼠标拖动setSceneRect失灵问题

问题

场景：实现放大缩小功能的同时，能够通过鼠标自由拖动。

问题：当放大时拖动异常，缩小的时候能够正常拖动

可执行版本

import sys
from PyQt5 import QtCore, QtWidgets, QtGui
from PyQt5.QtGui import QFont, QFontMetrics
from PyQt5.QtWidgets import QApplication, QWidget, QLabel, QHBoxLayout, QGraphicsScene, QGraphicsView
from PyQt5.QtCore import QRectF


class ImageView(QtWidgets.QGraphicsItem):
    def __init__(self, parent=None):
        super().__init__(parent)
        self.__image = None

    def load_image(self, image:QtGui.QPixmap):
        self.__image = image

    def boundingRect(self) -> QtCore.QRectF:
        if self.__image is not None:
            w = self.__image.width()
            h = self.__image.height()
            return QRectF(0,0,w,h)
        else:
            return QtWidgets.QGraphicsItem.boundingRect()

    def paint(self, painter: QtGui.QPainter, option, widget) -> None:
        if self.__image is not None:
            w = self.__image.width()
            h = self.__image.height()
            painter.drawPixmap(0,0,w,h, self.__image)

class InfoItem(QtWidgets.QGraphicsItem):
    def __init__(self, parent=None):
        super().__init__(parent)
        self.__text = "123"
        self.fm = QFontMetrics(QFont(self.__text))

    def boundingRect(self) -> QtCore.QRectF:
        self.fm.boundingRect(self.__text)
        return QtCore.QRectF(self.fm.boundingRect(self.__text))

    def updateInfo(self, pos):
        self.__text = "pos:({}, {})".format(pos.x(), pos.y())
        font = QFont(self.__text, 24)
        self.fm = QFontMetrics(font)
        self.update()

    def paint(self, painter: QtGui.QPainter, option, widget) -> None:
        if self.scene() is not None and self.__text == '123':
            self.scene().sigMouseMoved.connect(self.updateInfo, QtCore.Qt.AutoConnection)

        painter.drawText(QtCore.QPoint(0, 0), self.__text)


class Scene(QGraphicsScene):
    sigMouseMoved = QtCore.pyqtSignal(object)
    def __init__(self, parent=None):
        super().__init__(parent)

    def mouseMoveEvent(self, event: 'QGraphicsSceneMouseEvent') -> None:
        self.sigMouseMoved.emit(event.scenePos())
        event.accept()

class View(QGraphicsView):
    def __init__(self, parent=None):
        super().__init__(parent)
        self.startPos = None
        self.mode = 0


    def wheelEvent(self, event: QtGui.QWheelEvent) -> None:
        print(self.transform().m11())
        sensitivity = 2  # 灵敏度
        factor = event.angleDelta().y() / 240 / 5 * sensitivity + 1.0
        if self.transform().m11() > 4 and factor > 1:
            return
        self.scale(factor, factor)
        event.accept()

    def mousePressEvent(self, event: QtGui.QMouseEvent) -> None:
        self.mode = 1

    def mouseMoveEvent(self, event: QtGui.QMouseEvent) -> None:
        if self.mode == 1:
            delta = QtCore.QPointF(self.startPos) - event.pos()

            transform = self.transform()

            deltaX = delta.x() / transform.m11()
            deltaY = delta.y() / transform.m22()

            self.setSceneRect(self.sceneRect().translated(deltaX, deltaY))

            self.update()
        self.startPos = event.pos()
        super().mouseMoveEvent(event)

    def mouseReleaseEvent(self, event: QtGui.QMouseEvent) -> None:
        self.mode = 0




class sceneDemo(QWidget):
    def __init__(self):
        super().__init__()
        self.setWindowTitle('SceneDemo')

        self.view = View(self)
        self.scene = Scene()
        self.scene.setSceneRect(QRectF(0,0,100,100))
        self.view.resize(600,600)
        self.view.setScene(self.scene)
        self.view.setDragMode(QGraphicsView.NoDrag)

        self.view.setMouseTracking(True)
        self.view.setTransformationAnchor(QGraphicsView.AnchorUnderMouse)
        self.view.setHorizontalScrollBarPolicy(QtCore.Qt.ScrollBarAlwaysOff)
        self.view.setVerticalScrollBarPolicy(QtCore.Qt.ScrollBarAlwaysOff)
        self.view.setViewportUpdateMode(QGraphicsView.FullViewportUpdate)


        pixItem = ImageView()
        pixItem.load_image(QtGui.QPixmap("bg.jpg"))
        self.scene.addItem(pixItem)

        info = InfoItem()
        self.scene.addItem(info)




if __name__ == "__main__":
    app = QtWidgets.QApplication(sys.argv)

    win = sceneDemo()
    win.show()

    sys.exit(app.exec())

问题一

缺少语句

self.scene.setSceneRect(QRectF(0,0,100,100))

问题二

mouseMoveEvent中判断按钮状态

# if self.mode == 1:
if event.button() == QtCore.Qt.LeftButton: