python - QGraphicsItemGroup.removeFromGroup——子项目没有正确地重新定位到场景

Question

我正在尝试从 QGraphicsItemGroup 中删除 QGraphicsItem。当调用 removeFromGroup 时，该项目被删除(当然)。但是，它在场景中不再可见。我必须调用 Scene.addItem(item) 才能使其再次出现。这显然是你不应该做的事情(我收到了这样做的警告)。但我似乎找不到另一种解决方法。

这是一个最小的例子:

import sys 

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

class MainWindow(QMainWindow):

    def __init__(self, parent=None):
        super(MainWindow, self).__init__(parent)

        self.view = QGraphicsView()
        self.scene = QGraphicsScene()
        self.view.setScene(self.scene)

        self.setCentralWidget(self.view)


def add_group(scene):
    group = QGraphicsItemGroup()
    text = QGraphicsTextItem()
    text.setPlainText("I'm visible")
    group.addToGroup(text)
    scene.addItem(group)

    # After this, text is no longer in group. However, it is no longer visible.
    group.removeFromGroup(text)
    assert not text in group.childItems()

    # But text is still in scene. 
    assert text.scene() == scene

    # this works (i.e. text becomes visible again). However, it also produces a 
    # warning: QGraphicsScene::addItem: item has already been added to this scene. 
    # The docs also advice against it.
    scene.addItem(text)

    # According to the docs, I thought this might work, but it gives me a TypeError.
    # text.setParentItem(0)


if __name__ == '__main__':
    app = QApplication(sys.argv)
    main = MainWindow()
    add_group(main.scene)
    main.show()
    sys.exit(app.exec_())

非常欢迎提示和提示。

Answer 1

QGraphicsTextItem 永远不能作为场景的父级，因为它的父级必须是 QGraphicsItem(QGraphicsScene 不继承)。

创建 QGraphicsTextItem 时，它的父项是 None .它的父级设置为 group当它被添加到它时(QGraphicsItemGroup 是 QGraphicsItem 的子类)，然后设置回 None当它从 group 中删除时.

调用 scene.addItem()实际上是一个 NO-OP。 Qt 检查是否scene与text.scene()相同，如果是，它会打印警告并返回而不执行任何操作。

它在某些情况下似乎“有效”的事实，只是 python 垃圾收集机制的产物。

如果您的测试以更真实的方式重播，QGraphicsTextItem 在从组中移除后仍然可见:

import sys

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

class MainWindow(QMainWindow):
    def __init__(self, parent=None):
        super(MainWindow, self).__init__(parent)
        self.view = QGraphicsView(self)
        self.scene = QGraphicsScene(self.view)
        self.view.setScene(self.scene)
        self.setCentralWidget(self.view)
        self.group = QGraphicsItemGroup()
        self.text = QGraphicsTextItem()
        self.text.setPlainText("I'm visible")
        self.group.addToGroup(self.text)
        self.scene.addItem(self.group)
        self.group.removeFromGroup(self.text)

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