Improve target item - incorporate bits from PR 313 (#1318)

Overhaul TargetItem based on @lesauxvi 's PR #313

.flake8

@@ -3,47 +3,5 @@ exclude = .git,.tox,__pycache__,doc,old,build,dist
 show_source = True
 statistics = True
 verbose = 2
-select =
+max-line-length = 88
-    E101,
+extend-ignore = E203, W503
-    E112,
-    E122,
-    E125,
-    E133,
-    E223,
-    E224,
-    E242,
-    E273,
-    E274,
-    E901,
-    E902,
-    W191,
-    W601,
-    W602,
-    W603,
-    W604,
-    E124,
-    E231,
-    E211,
-    E261,
-    E271,
-    E272,
-    E304,
-    F401,
-    F402,
-    F403,
-    F404,
-    E501,
-    E502,
-    E702,
-    E703,
-    E711,
-    E712,
-    E721,
-    F811,
-    F812,
-    F821,
-    F822,
-    F823,
-    F831,
-    F841,
-    W292

doc/source/graphicsItems/index.rst

@@ -46,3 +46,4 @@ Contents:
     uigraphicsitem
     graphicswidgetanchor
     dateaxisitem
+    targetitem

doc/source/graphicsItems/targetitem.rst

@@ -0,0 +1,17 @@
+TargetItem
+==========
+
+.. autoclass:: pyqtgraph.TargetItem
+    :members:
+    
+    .. automethod:: pyqtgraph.TargetItem.__init__
+
+
+TargetLabel
+==================
+
+.. autoclass:: pyqtgraph.TargetLabel
+    :members:
+
+    .. automethod:: pyqtgraph.TargetLabel.__init__
+        

examples/InfiniteLine.py

@@ -32,6 +32,53 @@ p1.addItem(inf1)
 p1.addItem(inf2)
 p1.addItem(inf3)
 
+targetItem1 = pg.TargetItem(
+    label=True,
+    symbol="crosshair",
+    labelOpts={
+        "angle": 0
+    }
+)
+
+targetItem2 = pg.TargetItem(
+    pos=(30, 5),
+    size=20,
+    label="vert={1:0.2f}",
+    symbol="star",
+    pen="#F4511E",
+    labelOpts={
+        "angle": 45,
+        "offset": QtCore.QPoint(15, 15)
+    }
+)
+
+targetItem3 = pg.TargetItem(
+    pos=(10, 10),
+    size=10,
+    label="Third Label",
+    symbol="x", 
+    pen="#00ACC1",
+    labelOpts={
+        "anchor": QtCore.QPointF(0.5, 0.5),
+        "offset": QtCore.QPointF(30, 0),
+        "color": "#558B2F",
+        "rotateAxis": (0, 1)
+    }
+)
+
+def callableFunction(x, y):
+    return f"Square Values: ({x**2:.4f}, {y**2:.4f})"
+
+targetItem4 = pg.TargetItem(
+    pos=(10, -10),
+    label=callableFunction
+)
+
+p1.addItem(targetItem1)
+p1.addItem(targetItem2)
+p1.addItem(targetItem3)
+p1.addItem(targetItem4)
+
 # Add a linear region with a label
 lr = pg.LinearRegionItem(values=[70, 80])
 p1.addItem(lr)

examples/utils.py

@@ -32,6 +32,7 @@ examples = OrderedDict([
     ('GraphicsItems', OrderedDict([
         ('Scatter Plot', 'ScatterPlot.py'),
         #('PlotItem', 'PlotItem.py'),
+        ('InfiniteLine', 'InfiniteLine.py'),
         ('IsocurveItem', 'isocurve.py'),
         ('GraphItem', 'GraphItem.py'),
         ('ErrorBarItem', 'ErrorBarItem.py'),

pyqtgraph/__init__.py

@@ -232,6 +232,7 @@ from .graphicsItems.FillBetweenItem import *
 from .graphicsItems.LegendItem import * 
 from .graphicsItems.ScatterPlotItem import * 
 from .graphicsItems.ItemGroup import *
+from .graphicsItems.TargetItem import * 
 
 from .widgets.MultiPlotWidget import * 
 from .widgets.ScatterPlotWidget import * 

pyqtgraph/graphicsItems/ScatterPlotItem.py

@@ -39,10 +39,22 @@ _USE_QRECT = QT_LIB not in ['PySide2', 'PySide6']
 
 ## Build all symbol paths
 name_list = ['o', 's', 't', 't1', 't2', 't3', 'd', '+', 'x', 'p', 'h', 'star',
-             'arrow_up', 'arrow_right', 'arrow_down', 'arrow_left']
+             'arrow_up', 'arrow_right', 'arrow_down', 'arrow_left', 'crosshair']
 Symbols = OrderedDict([(name, QtGui.QPainterPath()) for name in name_list])
 Symbols['o'].addEllipse(QtCore.QRectF(-0.5, -0.5, 1, 1))
 Symbols['s'].addRect(QtCore.QRectF(-0.5, -0.5, 1, 1))
+
+def makeCrosshair(r=0.5, w=1, h=1):
+    path = QtGui.QPainterPath()
+    rect = QtCore.QRectF(-r, -r, r * 2, r * 2)
+    path.addEllipse(rect)
+    path.moveTo(-w, 0)
+    path.lineTo(w, 0)
+    path.moveTo(0, -h)
+    path.lineTo(0, h)
+    return path
+Symbols['crosshair'] = makeCrosshair()
+
 coords = {
     't': [(-0.5, -0.5), (0, 0.5), (0.5, -0.5)],
     't1': [(-0.5, 0.5), (0, -0.5), (0.5, 0.5)],

pyqtgraph/graphicsItems/TargetItem.py

@@ -1,132 +1,477 @@
+from math import atan2, pi
+
 from ..Qt import QtGui, QtCore
-import numpy as np
 from ..Point import Point
 from .. import functions as fn
 from .GraphicsObject import GraphicsObject
+from .UIGraphicsItem import UIGraphicsItem
 from .TextItem import TextItem
+from .ScatterPlotItem import Symbols, makeCrosshair
+from .ViewBox import ViewBox
+import string
+import warnings
 
 
-class TargetItem(GraphicsObject):
+class TargetItem(UIGraphicsItem):
     """Draws a draggable target symbol (circle plus crosshair).
 
     The size of TargetItem will remain fixed on screen even as the view is zoomed.
     Includes an optional text label.
     """
-    sigDragged = QtCore.Signal(object)
 
-    def __init__(self, movable=True, radii=(5, 10, 10), pen=(255, 255, 0), brush=(0, 0, 255, 100)):
+    sigPositionChanged = QtCore.Signal(object)
-        GraphicsObject.__init__(self)
+    sigPositionChangeFinished = QtCore.Signal(object)
-        self._bounds = None
+
-        self._radii = radii
+    def __init__(
-        self._picture = None
+        self,
+        pos=None,
+        size=10,
+        radii=None,
+        symbol="crosshair",
+        pen=None,
+        hoverPen=None,
+        brush=None,
+        hoverBrush=None,
+        movable=True,
+        label=None,
+        labelOpts=None,
+    ):
+        r"""
+        Parameters
+        ----------
+        pos : list, tuple, QPointF, QPoint, Optional
+            Initial position of the symbol.  Default is (0, 0)
+        size : int
+            Size of the symbol in pixels.  Default is 10.
+        radii : tuple of int
+            Deprecated.  Gives size of crosshair in screen pixels.
+        pen : QPen, tuple, list or str
+            Pen to use when drawing line. Can be any arguments that are valid
+            for :func:`~pyqtgraph.mkPen`. Default pen is transparent yellow.
+        brush : QBrush, tuple, list, or str
+            Defines the brush that fill the symbol. Can be any arguments that
+            is valid for :func:`~pyqtgraph.mkBrush`. Default is transparent
+            blue.
+        movable : bool
+            If True, the symbol can be dragged to a new position by the user.
+        hoverPen : QPen, tuple, list, or str
+            Pen to use when drawing symbol when hovering over it. Can be any
+            arguments that are valid for :func:`~pyqtgraph.mkPen`. Default pen
+            is red.
+        hoverBrush : QBrush, tuple, list or str
+            Brush to use to fill the symbol when hovering over it. Can be any
+            arguments that is valid for :func:`~pyqtgraph.mkBrush`. Default is
+            transparent blue.
+        symbol : QPainterPath or str
+            QPainterPath to use for drawing the target, should be centered at
+            ``(0, 0)`` with ``max(width, height) == 1.0``.  Alternatively a string
+            which can be any symbol accepted by
+            :func:`~pyqtgraph.ScatterPlotItem.setData`
+        label : bool, str or callable, optional
+            Text to be displayed in a label attached to the symbol, or None to
+            show no label (default is None). May optionally include formatting
+            strings to display the symbol value, or a callable that accepts x
+            and y as inputs.  If True, the label is ``x = {: >.3n}\ny = {: >.3n}``
+            False or None will result in no text being displayed
+        labelOpts : dict
+            A dict of keyword arguments to use when constructing the text
+            label. See :class:`TargetLabel` and :class:`~pyqtgraph.TextItem`
+        """
+        super().__init__(self)
         self.movable = movable
         self.moving = False
-        self.label = None
+        self._label = None
-        self.labelAngle = 0
+        self.mouseHovering = False
-        self.pen = fn.mkPen(pen)
-        self.brush = fn.mkBrush(brush)
 
-    def setLabel(self, label):
+        if radii is not None:
-        if label is None:
+            warnings.warn(
-            if self.label is not None:
+                "'radii' is now deprecated, and will be removed in 0.13.0. Use 'size' "
-                self.label.scene().removeItem(self.label)
+                "parameter instead",
-                self.label = None
+                DeprecationWarning,
+                stacklevel=2,
+            )
+            symbol = makeCrosshair(*radii)
+            size = 1
+
+        if pen is None:
+            pen = (255, 255, 0)
+        self.setPen(pen)
+
+        if hoverPen is None:
+            hoverPen = (255, 0, 255)
+        self.setHoverPen(hoverPen)
+
+        if brush is None:
+            brush = (0, 0, 255, 50)
+        self.setBrush(brush)
+
+        if hoverBrush is None:
+            hoverBrush = (0, 255, 255, 100)
+        self.setHoverBrush(hoverBrush)
+
+        self.currentPen = self.pen
+        self.currentBrush = self.brush
+
+        self._shape = None
+
+        self._pos = Point(0, 0)
+        if pos is None:
+            pos = Point(0, 0)
+        self.setPos(pos)
+
+        if isinstance(symbol, str):
+            try:
+                self._path = Symbols[symbol]
+            except KeyError:
+                raise KeyError("symbol name found in available Symbols")
+        elif isinstance(symbol, QtGui.QPainterPath):
+            self._path = symbol
         else:
-            if self.label is None:
+            raise TypeError("Unknown type provided as symbol")
-                self.label = TextItem()
-                self.label.setParentItem(self)
-            self.label.setText(label)
-            self._updateLabel()
 
-    def setLabelAngle(self, angle):
+        self.scale = size
-        if self.labelAngle != angle:
+        self.setPath(self._path)
-            self.labelAngle = angle
+        self.setLabel(label, labelOpts)
-            self._updateLabel()
+
+    @property
+    def sigDragged(self):
+        warnings.warn(
+            "'sigDragged' has been deprecated and will be removed in 0.13.0.  Use "
+            "`sigPositionChanged` instead",
+            DeprecationWarning,
+            stacklevel=2,
+        )
+        return self.sigPositionChangeFinished
+
+    def setPos(self, pos):
+        """Method to set the position to ``(x, y)`` within the plot view
+
+        Parameters
+        ----------
+        pos : tuple, list, QPointF, QPoint, or pg.Point
+            Container that consists of ``(x, y)`` representation of where the
+            TargetItem should be placed
+
+        Raises
+        ------
+        TypeError
+            If the type of ``pos`` does not match the known types to extract
+            coordinate info from, a TypeError is raised
+        """
+        if isinstance(pos, Point):
+            newPos = pos
+        elif isinstance(pos, (tuple, list)):
+            newPos = Point(pos)
+        elif isinstance(pos, (QtCore.QPointF, QtCore.QPoint)):
+            newPos = Point(pos.x(), pos.y())
+        else:
+            raise TypeError
+        if self._pos != newPos:
+            self._pos = newPos
+            super().setPos(self._pos)
+            self.sigPositionChanged.emit(self)
+
+    def setBrush(self, *args, **kwargs):
+        """Set the brush that fills the symbol. Allowable arguments are any that
+        are valid for :func:`~pyqtgraph.mkBrush`.
+        """
+        self.brush = fn.mkBrush(*args, **kwargs)
+        if not self.mouseHovering:
+            self.currentBrush = self.brush
+            self.update()
+
+    def setHoverBrush(self, *args, **kwargs):
+        """Set the brush that fills the symbol when hovering over it. Allowable
+        arguments are any that are valid for :func:`~pyqtgraph.mkBrush`.
+        """
+        self.hoverBrush = fn.mkBrush(*args, **kwargs)
+        if self.mouseHovering:
+            self.currentBrush = self.hoverBrush
+            self.update()
+
+    def setPen(self, *args, **kwargs):
+        """Set the pen for drawing the symbol. Allowable arguments are any that
+        are valid for :func:`~pyqtgraph.mkPen`."""
+        self.pen = fn.mkPen(*args, **kwargs)
+        if not self.mouseHovering:
+            self.currentPen = self.pen
+            self.update()
+
+    def setHoverPen(self, *args, **kwargs):
+        """Set the pen for drawing the symbol when hovering over it. Allowable
+        arguments are any that are valid for
+        :func:`~pyqtgraph.mkPen`."""
+        self.hoverPen = fn.mkPen(*args, **kwargs)
+        if self.mouseHovering:
+            self.currentPen = self.hoverPen
+            self.update()
 
     def boundingRect(self):
-        if self._picture is None:
+        return self.shape().boundingRect()
-            self._drawPicture()
-        return self._bounds
 
-    def dataBounds(self, axis, frac=1.0, orthoRange=None):
+    def paint(self, p, *_):
-        return [0, 0]
+        p.setPen(self.currentPen)
+        p.setBrush(self.currentBrush)
+        p.drawPath(self.shape())
 
-    def viewTransformChanged(self):
+    def setPath(self, path):
-        self._picture = None
+        if path != self._path:
+            self._path = path
+            self._shape = None
+        return None
+
+    def shape(self):
+        if self._shape is None:
+            s = self.generateShape()
+            if s is None:
+                return self._path
+            self._shape = s
+
+            # beware--this can cause the view to adjust
+            # which would immediately invalidate the shape.
             self.prepareGeometryChange()
-        self._updateLabel()
+        return self._shape
 
-    def _updateLabel(self):
+    def generateShape(self):
-        if self.label is None:
+        dt = self.deviceTransform()
-            return
+        if dt is None:
-
+            self._shape = self._path
-        # find an optimal location for text at the given angle
+            return None
-        angle = self.labelAngle * np.pi / 180.
+        v = dt.map(QtCore.QPointF(1, 0)) - dt.map(QtCore.QPointF(0, 0))
-        lbr = self.label.boundingRect()
+        dti = fn.invertQTransform(dt)
-        center = lbr.center()
+        devPos = dt.map(QtCore.QPointF(0, 0))
-        a = abs(np.sin(angle) * lbr.height()*0.5)
+        tr = QtGui.QTransform()
-        b = abs(np.cos(angle) * lbr.width()*0.5)
+        tr.translate(devPos.x(), devPos.y())
-        r = max(self._radii) + 2 + max(a, b)
+        va = atan2(v.y(), v.x())
-        pos = self.mapFromScene(self.mapToScene(QtCore.QPointF(0, 0)) + r * QtCore.QPointF(np.cos(angle), -np.sin(angle)) - center)
+        tr.rotate(va * 180.0 / pi)
-        self.label.setPos(pos)
+        tr.scale(self.scale, self.scale)
-
+        return dti.map(tr.map(self._path))
-    def paint(self, p, *args):
-        if self._picture is None:
-            self._drawPicture()
-        self._picture.play(p)
-
-    def _drawPicture(self):
-        self._picture = QtGui.QPicture()
-        p = QtGui.QPainter(self._picture)
-        p.setRenderHint(p.Antialiasing)
-        
-        # Note: could do this with self.pixelLength, but this is faster.
-        o = self.mapToScene(QtCore.QPointF(0, 0))
-        dx = (self.mapToScene(QtCore.QPointF(1, 0)) - o).x()
-        dy = (self.mapToScene(QtCore.QPointF(0, 1)) - o).y()
-        if dx == 0 or dy == 0:
-            p.end()
-            self._bounds = QtCore.QRectF()
-            return
-        px = abs(1.0 / dx)
-        py = abs(1.0 / dy)
-        
-        r, w, h = self._radii
-        w = w * px
-        h = h * py
-        rx = r * px
-        ry = r * py
-        rect = QtCore.QRectF(-rx, -ry, rx*2, ry*2)
-        p.setPen(self.pen)
-        p.setBrush(self.brush)
-        p.drawEllipse(rect)
-        p.drawLine(Point(-w, 0), Point(w, 0))
-        p.drawLine(Point(0, -h), Point(0, h))
-        p.end()
-        
-        bx = max(w, rx)
-        by = max(h, ry)
-        self._bounds = QtCore.QRectF(-bx, -by, bx*2, by*2)
 
     def mouseDragEvent(self, ev):
-        if not self.movable:
+        if not self.movable or int(ev.button() & QtCore.Qt.LeftButton) == 0:
             return
-        if ev.button() == QtCore.Qt.LeftButton:
-            if ev.isStart():
-                self.moving = True
-                self.cursorOffset = self.pos() - self.mapToParent(ev.buttonDownPos())
-                self.startPosition = self.pos()
         ev.accept()
+        if ev.isStart():
+            self.symbolOffset = self.pos() - self.mapToView(ev.buttonDownPos())
+            self.moving = True
 
         if not self.moving:
             return
+        self.setPos(self.symbolOffset + self.mapToView(ev.pos()))
 
-            self.setPos(self.cursorOffset + self.mapToParent(ev.pos()))
         if ev.isFinish():
             self.moving = False
-                self.sigDragged.emit(self)
+            self.sigPositionChangeFinished.emit(self)
+
+    def mouseClickEvent(self, ev):
+        if self.moving and ev.button() == QtCore.Qt.RightButton:
+            ev.accept()
+            self.moving = False
+            self.sigPositionChanged.emit(self)
+            self.sigPositionChangeFinished.emit(self)
+
+    def setMouseHover(self, hover):
+        # Inform the item that the mouse is(not) hovering over it
+        if self.mouseHovering is hover:
+            return
+        self.mouseHovering = hover
+        if hover:
+            self.currentBrush = self.hoverBrush
+            self.currentPen = self.hoverPen
+        else:
+            self.currentBrush = self.brush
+            self.currentPen = self.pen
+        self.update()
 
     def hoverEvent(self, ev):
-        if self.movable:
+        if self.movable and (not ev.isExit()) and ev.acceptDrags(QtCore.Qt.LeftButton):
-            ev.acceptDrags(QtCore.Qt.LeftButton)
+            self.setMouseHover(True)
+        else:
+            self.setMouseHover(False)
 
+    def viewTransformChanged(self):
+        GraphicsObject.viewTransformChanged(self)
+        self._shape = None  # invalidate shape, recompute later if requested.
+        self.update()
+
+    def pos(self):
+        """Provides the current position of the TargetItem
+
+        Returns
+        -------
+        Point
+            pg.Point of the current position of the TargetItem
+        """
+        return self._pos
+
+    def label(self):
+        """Provides the TargetLabel if it exists
+
+        Returns
+        -------
+        TargetLabel or None
+            If a TargetLabel exists for this TargetItem, return that, otherwise
+            return None
+        """
+        return self._label
+
+    def setLabel(self, text=None, labelOpts=None):
+        """Method to call to enable or disable the TargetLabel for displaying text
+
+        Parameters
+        ----------
+        text : Callable or str, optional
+            Details how to format the text, by default None
+            If None, do not show any text next to the TargetItem
+            If Callable, then the label will display the result of ``text(x, y)``
+            If a fromatted string, then the output of ``text.format(x, y)`` will be
+            displayed
+            If a non-formatted string, then the text label will display ``text``, by
+            default None
+        labelOpts : dictionary, optional
+            These arguments are passed on to :class:`~pyqtgraph.TextItem`
+        """
+        if not text:
+            if self._label is not None and self._label.scene() is not None:
+                # remove the label if it's already added
+                self._label.scene().removeItem(self._label)
+            self._label = None
+        else:
+            # provide default text if text is True
+            if text is True:
+                # convert to default value or empty string
+                text = "x = {: .3n}\ny = {: .3n}"
+
+            labelOpts = {} if labelOpts is None else labelOpts
+            if self._label is not None:
+                self._label.scene().removeItem(self._label)
+            self._label = TargetLabel(self, text=text, **labelOpts)
+
+    def setLabelAngle(self, angle):
+        warnings.warn(
+            "TargetItem.setLabelAngle is deprecated and will be removed in 0.13.0."
+            "Use TargetItem.label().setAngle() instead",
+            DeprecationWarning,
+            stacklevel=2,
+        )
+        if self.label() is not None and angle != self.label().angle():
+            self.label().setAngle(angle)
+        return None
+
+
+class TargetLabel(TextItem):
+    """A TextItem that attaches itself to a TargetItem.
+
+    This class extends TextItem with the following features :
+    * Automatically positions adjacent to the symbol at a fixed position.
+    * Automatically reformats text when the symbol location has changed.
+
+    Parameters
+    ----------
+    target : TargetItem
+        The TargetItem to which this label will be attached to.
+    text : str or callable, Optional
+        Governs the text displayed, can be a fixed string or a format string
+        that accepts the x, and y position of the target item; or be a callable
+        method that accepts a tuple (x, y) and returns a string to be displayed.
+        If None, an empty string is used.  Default is None
+    offset : tuple or list or QPointF or QPoint
+        Position to set the anchor of the TargetLabel away from the center of
+        the target in pixels, by default it is (20, 0).
+    anchor : tuple, list, QPointF or QPoint
+        Position to rotate the TargetLabel about, and position to set the
+        offset value to see :class:`~pyqtgraph.TextItem` for more inforation.
+    kwargs : dict of arguments that are passed on to
+        :class:`~pyqtgraph.TextItem` constructor, excluding text parameter
+    """
+
+    def __init__(
+        self,
+        target,
+        text="",
+        offset=(20, 0),
+        anchor=(0, 0.5),
+        **kwargs,
+    ):
+        super().__init__(anchor=anchor, **kwargs)
+        self.setParentItem(target)
+        self.target = target
+        self.setFormat(text)
+
+        if isinstance(offset, Point):
+            self.offset = offset
+        elif isinstance(offset, (tuple, list)):
+            self.offset = Point(*offset)
+        elif isinstance(offset, (QtCore.QPoint, QtCore.QPointF)):
+            self.offset = Point(offset.x(), offset.y())
+        else:
+            raise TypeError("Offset parameter is the wrong data type")
+        self.target.sigPositionChanged.connect(self.valueChanged)
+        self.valueChanged()
+
+    def format(self):
+        return self._format
+
+    def setFormat(self, text):
+        """Method to set how the TargetLabel should display the text.  This
+        method should be called from TargetItem.setLabel directly.
+
+        Parameters
+        ----------
+        text : Callable or str
+            Details how to format the text.
+            If Callable, then the label will display the result of ``text(x, y)``
+            If a fromatted string, then the output of ``text.format(x, y)`` will be
+            displayed
+            If a non-formatted string, then the text label will display ``text``
+        """
+        if not callable(text):
+            parsed = list(string.Formatter().parse(text))
+            if parsed and parsed[0][1] is not None:
+                self.setProperty("formattableText", True)
+            else:
+                self.setText(text)
+                self.setProperty("formattableText", False)
+        else:
+            self.setProperty("formattableText", False)
+        self._format = text
+        self.valueChanged()
+
+    def valueChanged(self):
+        x, y = self.target.pos()
+        if self.property("formattableText"):
+            self.setText(self._format.format(float(x), float(y)))
+        elif callable(self._format):
+            self.setText(self._format(x, y))
+
+    def viewTransformChanged(self):
+        viewbox = self.getViewBox()
+        if isinstance(viewbox, ViewBox):
+            viewPixelSize = viewbox.viewPixelSize()
+            scaledOffset = QtCore.QPointF(
+                self.offset.x() * viewPixelSize[0], self.offset.y() * viewPixelSize[1]
+            )
+            self.setPos(scaledOffset)
+        return super().viewTransformChanged()
+
+    def mouseClickEvent(self, ev):
+        return self.parentItem().mouseClickEvent(ev)
+
+    def mouseDragEvent(self, ev):
+        targetItem = self.parentItem()
+        if not targetItem.movable or int(ev.button() & QtCore.Qt.LeftButton) == 0:
+            return
+        ev.accept()
+        if ev.isStart():
+            targetItem.symbolOffset = targetItem.pos() - self.mapToView(
+                ev.buttonDownPos()
+            )
+            targetItem.moving = True
+
+        if not targetItem.moving:
+            return
+        targetItem.setPos(targetItem.symbolOffset + self.mapToView(ev.pos()))
+
+        if ev.isFinish():
+            targetItem.moving = False
+            targetItem.sigPositionChangeFinished.emit(self)