如何使用QGraphicsView::RubberBandDrag？

Question

有人可以提供一个解释，或者最好是一个简短的例子，说明如何在 QGraphicsView 中使用 RubberBandDrag 枚举值吗？ PyQt5 会很棒，但我可以从 C++ 版本翻译，如果有人愿意提供有用的示例。

NoDrag和ScrollHandDrag比较容易理解（NoDrag使鼠标成为指针，可以捕获某些位置的点击，ScrollHandDrag使鼠标成为手，你可以实现点击和拖动来滚动），但我不清楚 RubberBandDrag 可以用于什么。

在有人说 "go read the docs" 之前，这里是提供的信息

https://doc.qt.io/qt-5/qgraphicsview.html

enum QGraphicsView::DragMode

QGraphicsView::RubberBandDrag

A rubber band will appear. Dragging the mouse will set the rubber band geometry, and all items covered by the rubber band are selected. This mode is disabled for non-interactive views.

这很清楚，但我不确定如何实际使用 RubberBandDrag。有没有办法在初始放置后使用它来拖动 QPolygon 的点？那真的很有用。

Answer 1

QGraphicsView::RubberBandDrag flag only serves to activate the internal QRubberBand:

而 QRubberBand 通常只旨在可视化 selected 区域，在 QGraphicsView select 的情况下，该区域下方的项目如果 select 可用（启用 QGraphicsItem::ItemIsSelectable旗帜）。

根据你的最后一个问题：有没有办法在初始放置后使用它来拖动 QPolygon 的点？那真的很有用，在我看来你有一个 XY problem 因为在术语中使用 drag 似乎让你认为它用于拖动元素，因为不，拖动是指创建橡皮筋的方式。

~~稍后我将展示如何实现顶点的拖动来修改 QPolygon。~~

下面展示如何通过鼠标拖动修改顶点的位置：

import math

from PyQt5 import QtCore, QtGui, QtWidgets


class GripItem(QtWidgets.QGraphicsPathItem):
    circle = QtGui.QPainterPath()
    circle.addEllipse(QtCore.QRectF(-10, -10, 20, 20))
    square = QtGui.QPainterPath()
    square.addRect(QtCore.QRectF(-15, -15, 30, 30))

    def __init__(self, annotation_item, index):
        super(GripItem, self).__init__()
        self.m_annotation_item = annotation_item
        self.m_index = index

        self.setPath(GripItem.circle)
        self.setBrush(QtGui.QColor("green"))
        self.setPen(QtGui.QPen(QtGui.QColor("green"), 2))
        self.setFlag(QtWidgets.QGraphicsItem.ItemIsSelectable, True)
        self.setFlag(QtWidgets.QGraphicsItem.ItemIsMovable, True)
        self.setFlag(QtWidgets.QGraphicsItem.ItemSendsGeometryChanges, True)
        self.setAcceptHoverEvents(True)
        self.setZValue(11)
        self.setCursor(QtGui.QCursor(QtCore.Qt.PointingHandCursor))

    def hoverEnterEvent(self, event):
        self.setPath(GripItem.square)
        self.setBrush(QtGui.QColor("red"))
        super(GripItem, self).hoverEnterEvent(event)

    def hoverLeaveEvent(self, event):
        self.setPath(GripItem.circle)
        self.setBrush(QtGui.QColor("green"))
        super(GripItem, self).hoverLeaveEvent(event)

    def mouseReleaseEvent(self, event):
        self.setSelected(False)
        super(GripItem, self).mouseReleaseEvent(event)

    def itemChange(self, change, value):
        if change == QtWidgets.QGraphicsItem.ItemPositionChange and self.isEnabled():
            self.m_annotation_item.movePoint(self.m_index, value)
        return super(GripItem, self).itemChange(change, value)


class PolygonAnnotation(QtWidgets.QGraphicsPolygonItem):
    def __init__(self, parent=None):
        super(PolygonAnnotation, self).__init__(parent)
        self.m_points = []
        self.setZValue(10)
        self.setPen(QtGui.QPen(QtGui.QColor("green"), 2))
        self.setAcceptHoverEvents(True)

        self.setFlag(QtWidgets.QGraphicsItem.ItemIsSelectable, True)
        self.setFlag(QtWidgets.QGraphicsItem.ItemIsMovable, True)
        self.setFlag(QtWidgets.QGraphicsItem.ItemSendsGeometryChanges, True)

        self.setCursor(QtGui.QCursor(QtCore.Qt.PointingHandCursor))

        self.m_items = []

    def addPoint(self, p):
        self.m_points.append(p)
        self.setPolygon(QtGui.QPolygonF(self.m_points))
        item = GripItem(self, len(self.m_points) - 1)
        self.scene().addItem(item)
        self.m_items.append(item)
        item.setPos(p)

    def movePoint(self, i, p):
        if 0 <= i < len(self.m_points):
            self.m_points[i] = self.mapFromScene(p)
            self.setPolygon(QtGui.QPolygonF(self.m_points))

    def move_item(self, index, pos):
        if 0 <= index < len(self.m_items):
            item = self.m_items[index]
            item.setEnabled(False)
            item.setPos(pos)
            item.setEnabled(True)

    def itemChange(self, change, value):
        if change == QtWidgets.QGraphicsItem.ItemPositionHasChanged:
            for i, point in enumerate(self.m_points):
                self.move_item(i, self.mapToScene(point))
        return super(PolygonAnnotation, self).itemChange(change, value)

    def hoverEnterEvent(self, event):
        self.setBrush(QtGui.QColor(255, 0, 0, 100))
        super(PolygonAnnotation, self).hoverEnterEvent(event)

    def hoverLeaveEvent(self, event):
        self.setBrush(QtGui.QBrush(QtCore.Qt.NoBrush))
        super(PolygonAnnotation, self).hoverLeaveEvent(event)


if __name__ == "__main__":
    import sys

    app = QtWidgets.QApplication(sys.argv)
    scene = QtWidgets.QGraphicsScene()
    w = QtWidgets.QGraphicsView(scene)

    polygon_item = PolygonAnnotation()
    scene.addItem(polygon_item)
    r = 100
    sides = 10

    for i in range(sides):
        angle = 2 * math.pi * i / sides
        x = r * math.cos(angle)
        y = r * math.sin(angle)
        p = QtCore.QPointF(x, y) + QtCore.QPointF(200, 200)
        polygon_item.addPoint(p)

    w.resize(640, 480)
    w.show()
    sys.exit(app.exec_())

如何使用QGraphicsView::RubberBandDrag？

how to use QGraphicsView::RubberBandDrag?

python

pyqt

pyqt5

qgraphicsview