pyqt5：qgraphicsscene：单击和掉落的鼠标项目（无需按住）

Question

qgraphicsscene中的默认mouseevent函数允许通过按下，按下，移动和发布来移动项目。我正在尝试覆盖qgraphicsscene mouseevent（）函数，以使用按下启动键（选择项目），移动（不举行按下），按下，按下，按下 - 释放扭曲（Drop item）。尽管我将很简单：

self.moving = False
def mousePressEvent(self, event):
    if event.button() == Qt.LeftButton:
        if self.moving == False: # first click, pick up, start moving
            super().mousePressEvent(event)
            self.moving = True
        else:                    # second click, drop, end moving item
            super().mouseReleaseEvent(event)
            self.moving = False    

def mouseMoveEvent(self, event):      
    if self.moving == True:
        super().mousePressEvent(event)
        super().mouseMoveEvent(event)

def mouseReleaseEvent(self, event):      
    pass

到目前为止，我无法捡起并移动该物品，但有人发现任何错误吗？另外，我在哪里可以找到qgraphicsscene Mouseevent函数的原始实现？

谢谢你！

Answer 1

首先，您将永远不要 调用具有错误参数类型的事件处理程序的基本实现。
使用鼠标移动事件来调用MousePressEvent，因为其参数在概念上是错误的，虽然在图形项目上完成时通常并不“危险”，但在窗口小部件上完成时通常会导致致命的崩溃。

也就是说，如果您想在鼠标上按下鼠标按下和释放，这显然意味着您必须在Mouserelealeaseevent（） handler中更改“关注状态”，不是MousePressEvent（）一个。

Furthermore, you must ensure that:

1. the item becomes the mouse grabber 已发布鼠标按钮；
2. 该项目还接受悬停事件 /em>悬停事件（最重要的是，悬停移动事件）总是由该项目接收和处理；

大约在第一个点，一般而言，当按下任何鼠标按钮并释放该按钮时，通常说一个小部件（或图形项）将变成鼠标抓取。成为鼠标抓手意味着将所有鼠标事件都派遣到该项目，直到释放鼠标为止。由于您希望能够在释放鼠标按钮后能够接收鼠标事件，因此在释放按钮时必须明确抓取（或删除）鼠标。

在下面的示例中，我将显示如何实现上述内容，并提供对已经移动的项目的支持（通过itemismobleable flag）。显示“可移动标志”文本的项目也可以在按下左键时移动，否则仅提供其他项目的按下/发布支持。

请注意，基于鼠标事件的移动项目必须考虑项目转换：事件的pos（）始终在本地坐标中映射，因此，如果您单击矩形的左上角，项目旋转，您将始终获得左上角的位置。由于setPos（）使用 parent 坐标系统，因此我们必须将这些位置映射到父级，以实现正确的运动。这也意味着复杂的转换（考虑父母）可能会使事情变得更加困难，因此请注意。对于更复杂的方案，您可能必须进一步实施目标位置的计算，或者坚持默认行为。

from PyQt5.QtGui import *
from PyQt5.QtWidgets import *
from PyQt5.QtCore import *

class NoMouseButtonMoveRectItem(QGraphicsRectItem):
    moving = False
    def mousePressEvent(self, event):
        super().mousePressEvent(event)
        if event.button() == Qt.LeftButton:
            # by defaults, mouse press events are not accepted/handled,
            # meaning that no further mouseMoveEvent or mouseReleaseEvent
            # will *ever* be received by this item; with the following,
            # those events will be properly dispatched
            event.accept()
            self.pressPos = event.screenPos()

    def mouseMoveEvent(self, event):
        if self.moving:
            # map the position to the parent in order to ensure that the
            # transformations are properly considered:
            currentParentPos = self.mapToParent(
                self.mapFromScene(event.scenePos()))
            originParentPos = self.mapToParent(
                self.mapFromScene(event.buttonDownScenePos(Qt.LeftButton)))
            self.setPos(self.startPos + currentParentPos - originParentPos)
        else:
            super().mouseMoveEvent(event)

    def mouseReleaseEvent(self, event):
        super().mouseReleaseEvent(event)
        if (event.button() != Qt.LeftButton 
            or event.pos() not in self.boundingRect()):
                return

        # the following code block is to allow compatibility with the
        # ItemIsMovable flag: if the item has the flag set and was moved while
        # keeping the left mouse button pressed, we proceed with our
        # no-mouse-button-moved approach only *if* the difference between the
        # pressed and released mouse positions is smaller than the application
        # default value for drag movements; in this way, small, involuntary
        # movements usually created between pressing and releasing the mouse
        # button will still be considered as candidates for our implementation;
        # if you are *not* interested in this flag, just ignore this code block
        distance = (event.screenPos() - self.pressPos).manhattanLength()
        if (not self.moving and distance > QApplication.startDragDistance()):
            return
        # end of ItemIsMovable support

        self.moving = not self.moving
        # the following is *mandatory*
        self.setAcceptHoverEvents(self.moving)
        if self.moving:
            self.startPos = self.pos()
            self.grabMouse()
        else:
            self.ungrabMouse()


if __name__ == '__main__':
    import sys
    from random import randrange, choice
    app = QApplication(sys.argv)
    scene = QGraphicsScene()
    view = QGraphicsView(scene)
    view.resize(QApplication.primaryScreen().size() * 2 / 3)
    # create random items that support click/release motion
    for i in range(10):
        item = NoMouseButtonMoveRectItem(0, 0, 100, 100)
        item.setPos(randrange(500), randrange(500))
        item.setPen(QColor(*(randrange(255) for _ in range(3))))
        if choice((0, 1)):
            item.setFlags(item.ItemIsMovable)
            QGraphicsSimpleTextItem('Movable flag', item)
        else:
            item.setBrush(QColor(*(randrange(255) for _ in range(3))))
        scene.addItem(item)
    view.show()
    sys.exit(app.exec_())