无法在场景中正确定位QGraphicsRectItem

我这辈子都搞不懂,但我把它归结为一个自我控制的问题。

我想做的是画一个 QGraphicsRectItem QGraphicsScene 。绘制矩形后,可以将所有项目一起移动。我已经调查过了 QGraphicsItemGroup 已经决定在我的实际用例中不可行。

问题是: 我已经完成了上面提到的所有事情, 除了 我无法正确定位矩形项目,即它的大小合适,通过移动它,所有项目都会移动,但它没有与所选项目的统一边界矩形对齐。我已经尝试将所有内容保持在场景坐标中,所以我不确定为什么会有偏移。

为什么会出现抵消,如何缓解?

#####The line where the position of the rect item is set is marked like this#####

from PyQt4.QtGui import *
from PyQt4.QtCore import *
import sys


class DiagramScene(QGraphicsScene):
    def __init__(self, parent=None):
        super().__init__(parent)

        self.selBox = None
        self.selectionChanged.connect(self.onSelectionChange)

    @pyqtSlot()
    def onSelectionChange(self):
        count = 0
        items = self.selectedItems()

        # Get bounding rect of all selected Items
        for item in self.selectedItems():
            if count == 0:
                rect = item.mapRectToScene(item.boundingRect())
            else:
                rect = rect.unite(item.mapRectToScene(item.boundingRect()))
            count += 1

        if count > 0:
            if self.selBox:
                # Update selBox if items are selected and already exists
                self.selBox.setRect(rect)
                self.selBox.items = items
            else:
                # Instantiate selBox if items are selected and does not already exist
                self.selBox = DiagramSelBox(rect, items)
                ##### Set position of selBox to topLeft corner of united rect #####
                self.selBox.setPos(rect.topLeft())
                self.addItem(self.selBox)
        elif self.selBox:
            # Remove selBox from scene if no items are selected and box is drawn
            self.removeItem(self.selBox)
            del self.selBox
            self.selBox = None


class DiagramSelBox(QGraphicsRectItem):
    def __init__(self, bounds, items, parent=None, scene=None):
        super().__init__(bounds, parent, scene)

        self.setFlag(QGraphicsItem.ItemIsSelectable, True)
        self.pressPos = None
        self.items = items

    def paint(self, painter, option, widget=None):
        pen = QPen(Qt.DashLine)
        painter.setPen(pen)
        painter.drawRect(self.rect())

    def mousePressEvent(self, e):
        # Get original position of selBox when clicked
        self.pressPos = self.pos()
        # mouseEvent is not passed on to scene so item selection
        # does not change

    def mouseMoveEvent(self, e):
        super().mouseMoveEvent(e)
        if self.pressPos:
            # Move selBox is original position is set
            newPos = self.mapToScene(e.pos()) - self.rect().center()
            self.setPos(newPos)

    def mouseReleaseEvent(self, e):
        # Update position of all selected items
        change = self.scenePos() - self.pressPos
        for item in self.items:
            item.moveBy(change.x(), change.y())

        super().mouseReleaseEvent(e)


if __name__ == "__main__":

    app = QApplication(sys.argv)

    view = QGraphicsView()
    view.setDragMode(QGraphicsView.RubberBandDrag)

    scene = DiagramScene()

    scene.setSceneRect(0, 0, 500, 500)

    rect1 = scene.addRect(20, 20, 100, 50)
    rect2 = scene.addRect(80, 80, 100, 50)
    rect3 = scene.addRect(140, 140, 100, 50)

    rect1.setFlag(QGraphicsItem.ItemIsSelectable, True)
    rect2.setFlag(QGraphicsItem.ItemIsSelectable, True)
    rect3.setFlag(QGraphicsItem.ItemIsSelectable, True)

    view.setScene(scene)
    view.show()

    sys.exit(app.exec_())