python - 更改事件键上的光标形状

Question

我正在尝试更改关键事件上的光标形状:

当我按“C”时，我想显示一个 LineCursor，

当我按“S”时，我想显示一个十字光标，和

当我按“N”时，我想显示标准的箭头光标。

光标只有在离开 Canvas 并返回 Canvas 时才会改变，
但如果光标停留在 Canvas 上，则不会。
Canvas 上的 self.update() 不起作用

这里是重现问题的代码:

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

class MainWindow(QMainWindow):
    def __init__(self, parent=None):
        super(MainWindow, self).__init__(parent)
        self.setObjectName("MainWindow")
        self.resize(942, 935)
        self.centralwidget = QWidget(self)
        self.centralwidget.setObjectName("centralwidget")
        self.horizontalLayout = QHBoxLayout(self.centralwidget)
        self.horizontalLayout.setObjectName("horizontalLayout")
        self.MainView = QGraphicsView(self.centralwidget)
        self.MainView.setObjectName("MainView")
        self.horizontalLayout.addWidget(self.MainView)
        self.setCentralWidget(self.centralwidget)
        self.setWindowTitle("MainWindow")

        self.scene = QGraphicsScene( 0.,0., 1240., 1780. )
        self.canvas = Canvas()

        self.widget = QWidget()
        box_layout = QVBoxLayout()
        self.widget.setLayout(box_layout)
        box_layout.addWidget(self.canvas)
        self.scene.addWidget(self.widget)
        self.MainView.setScene(self.scene)
        self.MainView.setRenderHints(QPainter.Antialiasing)
        self.MainView.fitInView(0, 0, 45, 55, Qt.KeepAspectRatio)

        self.show()

        empty = QPixmap(1240, 1748)
        empty.fill(QColor(Qt.white))
        self.canvas.newPixmap(empty)

    def keyPressEvent(self, e):
        key = e.key()
        if key == Qt.Key_C:
            self.canvas.setCutCursor()
        elif key == Qt.Key_N:
            self.canvas.setNormalCursor()
        elif key == Qt.Key_S:
            self.canvas.setSelectionCursor()


class Canvas(QLabel):
    def __init__(self):
        super().__init__()
        sizePolicy = QSizePolicy(QSizePolicy.Fixed, QSizePolicy.Fixed)
        self.setSizePolicy(sizePolicy)
        self.setAlignment(Qt.AlignLeft)
        self.setAlignment(Qt.AlignTop)

    def newPixmap(self, pixmap):
        self.setPixmap(pixmap)

    def setCutCursor(self):
        newCursor = QPixmap(500,3)
        newCursor.fill(QColor("#000000"))
        self.setCursor(QCursor(newCursor))

    def setSelectionCursor(self):
        self.setCursor(Qt.CrossCursor)

    def setNormalCursor(self):
        self.setCursor(QCursor(Qt.ArrowCursor))


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

Answer 1

这似乎是一个从 Unresolved 老错误:setCursor on QGraphicsView don't work when add QWidget on the QGraphicsScene

有一种可能的解决方法，但它远非完美。
首先，您必须考虑，在处理鼠标事件和小部件代理时，处理 QGraphicsScene 及其 View [s] 并不容易，主要是因为事件的多个嵌套级别以及实际 View (及其parent，直到顶层窗口)和代理本身，它是您添加到场景中的小部件的抽象。虽然 Qt 开发人员做了大量工作以使其尽可能透明，但在某些时候，您可能会面临一些通常难以修复或解决的意外或不受欢迎的行为，这也是因为图形场景可能在不止一个 View 。

除了前面提到的错误，您还必须考虑到图形 View 使用 QWidget.setCursor每当它的任何项目调用 setCursor 时在内部因为 View 是一个非常复杂的小部件，在某些时候它甚至可能会尝试“恢复”光标，如果它认为它应该(即使它不应该)。
最后，一些与焦点有关的事件可能会成为这一切的障碍。

第一个解决方法是将光标设置到 View 本身(或者更好的是 View 的视口(viewport)，它是显示场景内容的实际小部件)。为了确保这一点，我们显然需要检查光标是否在 Canvas 内。

不幸的是，由于上面写的事件处理，这可能会变得有点困惑，因为一些事件甚至在主 Qt 事件循环中至少延迟了一个周期；结果是，虽然第一次设置光标可能会起作用，但再次设置它可能不会，即使这样，光标也可能在鼠标移动至少一个像素之前不会被应用。
作为第二种解决方法，我们需要一个事件过滤器来绕过所有这些，并在鼠标在视口(viewport)边缘内移动时检查光标。

class MainWindow(QMainWindow):
    def __init__(self, parent=None):
        super(MainWindow, self).__init__(parent)
        # ...
        self.show()

        empty = QPixmap(1240, 1748)
        empty.fill(QColor(Qt.darkGray))
        self.canvas.newPixmap(empty)

        # install an event filter on the view's viewport;
        # this is very, *VERY* important: on the *VIEWPORT*!
        # if you install it on the view, it will *not* work
        self.MainView.viewport().installEventFilter(self)

    def insideCanvasRect(self, pos):
        canvasRect = self.canvas.rect()
        # translate the canvas rect to its top level window to get the actual
        # geometry according to the scene; we can't use canvas.geometry(), as
        # geometry() is based on the widget's parent coordinates, and that
        # parent could also have any number of parents in turn;
        canvasRect.translate(self.canvas.mapTo(self.canvas.window(), QPoint(0, 0)))
        # map the geometry to the view's transformation, which probably uses
        # some scaling, but also translation *and* shearing; the result is a
        # polygon, as with shearing you could transform a rectangle to an
        # irregular quadrilateral
        polygon = self.MainView.mapFromScene(QRectF(canvasRect))
        # tell if the point is within the resulting polygon
        return polygon.containsPoint(pos, Qt.WindingFill)

    def eventFilter(self, source, event):
        if source == self.MainView.viewport() and (
            (event.type() == QEvent.MouseMove and not event.buttons()) or
            (event.type() == QEvent.MouseButtonRelease)
            ):
                # process the event
                super(MainWindow, self).eventFilter(source, event)
                if self.insideCanvasRect(event.pos()):
                    source.setCursor(self.canvas.cursor())
                else:
                    source.unsetCursor()
                # usually a mouse move event within the view's viewport returns False,
                # but in that case the event would be propagated to the parents, up
                # to the top level window, which might reset the *previous* cursor
                # at some point, no matter if we try to avoid that; to prevent that
                # we return True to avoid propagation.
                # Note that this will prevent any upper-level filtering and *could*
                # also create some issues for the drag and drop framework
                if event.type() == QEvent.MouseMove:
                    return True
        return super(MainWindow, self).eventFilter(source, event)

    def keyPressEvent(self, e):
        # send the canvas a fake leave event
        QApplication.sendEvent(self.canvas, QEvent(QEvent.Leave))
        key = e.key()
        if key == Qt.Key_C:
            self.canvas.setCutCursor()
        elif key == Qt.Key_N:
            self.canvas.setNormalCursor()
        elif key == Qt.Key_S:
            self.canvas.setSelectionCursor()
        pos = self.canvas.rect().center()
        event = QEnterEvent(pos, self.canvas.mapTo(self.canvas.window(), pos), self.canvas.mapToGlobal(pos))
        # send a fake enter event (mapped to the center of the widget, just to be sure)
        QApplication.sendEvent(self.canvas, event)
        # if we're inside the widget, set the view's cursor, otherwise it will not
        # be set until the mouse is moved
        if self.insideCanvasRect(self.MainView.viewport().mapFromGlobal(QCursor.pos())):
            self.MainView.viewport().setCursor(self.canvas.cursor())