9. Using the Map Canvas

Sugestie

Fragmentele de cod de pe această pagină necesită următoarele importuri, dacă vă aflați în afara consolei pyqgis:

from qgis.PyQt.QtGui import (
    QColor,
)

from qgis.PyQt.QtCore import Qt, QRectF

from qgis.PyQt.QtWidgets import QMenu

from qgis.core import (
    QgsVectorLayer,
    QgsPoint,
    QgsPointXY,
    QgsProject,
    QgsGeometry,
    QgsMapRendererJob,
    QgsWkbTypes,
)

from qgis.gui import (
    QgsMapCanvas,
    QgsVertexMarker,
    QgsMapCanvasItem,
    QgsMapMouseEvent,
    QgsRubberBand,
)

The Map canvas widget is probably the most important widget within QGIS because it shows the map composed from overlaid map layers and allows interaction with the map and layers. The canvas always shows a part of the map defined by the current canvas extent. The interaction is done through the use of map tools: there are tools for panning, zooming, identifying layers, measuring, vector editing and others. Similar to other graphics programs, there is always one tool active and the user can switch between the available tools.

The map canvas is implemented with the QgsMapCanvas class in the qgis.gui module. The implementation is based on the Qt Graphics View framework. This framework generally provides a surface and a view where custom graphics items are placed and user can interact with them. We will assume that you are familiar enough with Qt to understand the concepts of the graphics scene, view and items. If not, please read the overview of the framework.

Whenever the map has been panned, zoomed in/out (or some other action that triggers a refresh), the map is rendered again within the current extent. The layers are rendered to an image (using the QgsMapRendererJob class) and that image is displayed on the canvas. The QgsMapCanvas class also controls refreshing of the rendered map. Besides this item which acts as a background, there may be more map canvas items.

Typical map canvas items are rubber bands (used for measuring, vector editing etc.) or vertex markers. The canvas items are usually used to give visual feedback for map tools, for example, when creating a new polygon, the map tool creates a rubber band canvas item that shows the current shape of the polygon. All map canvas items are subclasses of QgsMapCanvasItem which adds some more functionality to the basic QGraphicsItem objects.

Pentru a rezuma, arhitectura suportului pentru hartă constă în trei concepte:

• suportul de hartă — pentru vizualizarea hărții

• map canvas items — additional items that can be displayed on the map canvas

• map tools — for interaction with the map canvas

9.1. Încapsularea suportului de hartă

Map canvas is a widget like any other Qt widget, so using it is as simple as creating and showing it.

canvas = QgsMapCanvas()
canvas.show()

This produces a standalone window with map canvas. It can be also embedded into an existing widget or window. When using .ui files and Qt Designer, place a QWidget on the form and promote it to a new class: set QgsMapCanvas as class name and set qgis.gui as header file. The pyuic5 utility will take care of it. This is a very convenient way of embedding the canvas. The other possibility is to manually write the code to construct map canvas and other widgets (as children of a main window or dialog) and create a layout.

În mod implicit, canevasul hărții are un fundal negru și nu utilizează anti-zimțare. Pentru a seta fundalul alb și pentru a permite anti-zimțare pentru o redare mai bună

canvas.setCanvasColor(Qt.white)
canvas.enableAntiAliasing(True)

(In case you are wondering, Qt comes from PyQt.QtCore module and Qt.white is one of the predefined QColor instances.)

Now it is time to add some map layers. We will first open a layer and add it to the current project. Then we will set the canvas extent and set the list of layers for the canvas.

vlayer = QgsVectorLayer('testdata/airports.shp', "Airports layer", "ogr")
if not vlayer.isValid():
    print("Layer failed to load!")

# add layer to the registry
QgsProject.instance().addMapLayer(vlayer)

# set extent to the extent of our layer
canvas.setExtent(vlayer.extent())

# set the map canvas layer set
canvas.setLayers([vlayer])

După executarea acestor comenzi, suportul ar trebui să arate stratul pe care le-ați încărcat.

9.2. Benzile elastice și marcajele nodurilor

To show some additional data on top of the map in canvas, use map canvas items. It is possible to create custom canvas item classes (covered below), however there are two useful canvas item classes for convenience: QgsRubberBand for drawing polylines or polygons, and QgsVertexMarker for drawing points. They both work with map coordinates, so the shape is moved/scaled automatically when the canvas is being panned or zoomed.

To show a polyline:

r = QgsRubberBand(canvas, QgsWkbTypes.LineGeometry)  # line
points = [QgsPoint(-100, 45), QgsPoint(10, 60), QgsPoint(120, 45)]
r.setToGeometry(QgsGeometry.fromPolyline(points), None)

Pentru a afișa un poligon

r = QgsRubberBand(canvas, QgsWkbTypes.PolygonGeometry)  # polygon
points = [[QgsPointXY(-100, 35), QgsPointXY(10, 50), QgsPointXY(120, 35)]]
r.setToGeometry(QgsGeometry.fromPolygonXY(points), None)

Rețineți că punctele pentru poligon nu reprezintă o simplă listă: în fapt, aceasta este o listă de inele conținând inele liniare ale poligonului: primul inel reprezintă granița exterioară, în plus (opțional) inelele corespund găurilor din poligon.

Benzile elastice acceptă unele personalizări, și anume schimbarea culorii și a lățimii liniei

r.setColor(QColor(0, 0, 255))
r.setWidth(3)

The canvas items are bound to the canvas scene. To temporarily hide them (and show them again), use the hide() and show() combo. To completely remove the item, you have to remove it from the scene of the canvas

canvas.scene().removeItem(r)

(În C + + este posibilă ștergerea doar a elementului, însă în Python del r ar șterge doar referința iar obiectul va exista în continuare, acesta fiind deținut de suport)

Rubber band can be also used for drawing points, but the QgsVertexMarker class is better suited for this (QgsRubberBand would only draw a rectangle around the desired point).

You can use the vertex marker like this:

m = QgsVertexMarker(canvas)
m.setCenter(QgsPointXY(10,40))

This will draw a red cross on position [10,45]. It is possible to customize the icon type, size, color and pen width

m.setColor(QColor(0, 255, 0))
m.setIconSize(5)
m.setIconType(QgsVertexMarker.ICON_BOX) # or ICON_CROSS, ICON_X
m.setPenWidth(3)

For temporary hiding of vertex markers and removing them from canvas, use the same methods as for rubber bands.

9.3. Folosirea instrumentelor în suportul de hartă

The following example constructs a window that contains a map canvas and basic map tools for map panning and zooming. Actions are created for activation of each tool: panning is done with QgsMapToolPan, zooming in/out with a pair of QgsMapToolZoom instances. The actions are set as checkable and later assigned to the tools to allow automatic handling of checked/unchecked state of the actions – when a map tool gets activated, its action is marked as selected and the action of the previous map tool is deselected. The map tools are activated using setMapTool() method.

from qgis.gui import *
from qgis.PyQt.QtWidgets import QAction, QMainWindow
from qgis.PyQt.QtCore import Qt

class MyWnd(QMainWindow):
    def __init__(self, layer):
        QMainWindow.__init__(self)

        self.canvas = QgsMapCanvas()
        self.canvas.setCanvasColor(Qt.white)

        self.canvas.setExtent(layer.extent())
        self.canvas.setLayers([layer])

        self.setCentralWidget(self.canvas)

        self.actionZoomIn = QAction("Zoom in", self)
        self.actionZoomOut = QAction("Zoom out", self)
        self.actionPan = QAction("Pan", self)

        self.actionZoomIn.setCheckable(True)
        self.actionZoomOut.setCheckable(True)
        self.actionPan.setCheckable(True)

        self.actionZoomIn.triggered.connect(self.zoomIn)
        self.actionZoomOut.triggered.connect(self.zoomOut)
        self.actionPan.triggered.connect(self.pan)

        self.toolbar = self.addToolBar("Canvas actions")
        self.toolbar.addAction(self.actionZoomIn)
        self.toolbar.addAction(self.actionZoomOut)
        self.toolbar.addAction(self.actionPan)

        # create the map tools
        self.toolPan = QgsMapToolPan(self.canvas)
        self.toolPan.setAction(self.actionPan)
        self.toolZoomIn = QgsMapToolZoom(self.canvas, False) # false = in
        self.toolZoomIn.setAction(self.actionZoomIn)
        self.toolZoomOut = QgsMapToolZoom(self.canvas, True) # true = out
        self.toolZoomOut.setAction(self.actionZoomOut)

        self.pan()

    def zoomIn(self):
        self.canvas.setMapTool(self.toolZoomIn)

    def zoomOut(self):
        self.canvas.setMapTool(self.toolZoomOut)

    def pan(self):
        self.canvas.setMapTool(self.toolPan)

You can try the above code in the Python console editor. To invoke the canvas window, add the following lines to instantiate the MyWnd class. They will render the currently selected layer on the newly created canvas

w = MyWnd(iface.activeLayer())
w.show()

9.3.1. Select a feature using QgsMapToolIdentifyFeature

You can use the map tool QgsMapToolIdentifyFeature for asking to the user to select a feature that will be sent to a callback function.

def callback(feature):
  """Code called when the feature is selected by the user"""
  print("You clicked on feature {}".format(feature.id()))

canvas = iface.mapCanvas()
feature_identifier = QgsMapToolIdentifyFeature(canvas)

# indicates the layer on which the selection will be done
feature_identifier.setLayer(vlayer)

# use the callback as a slot triggered when the user identifies a feature
feature_identifier.featureIdentified.connect(callback)

# activation of the map tool
canvas.setMapTool(feature_identifier)

9.3.2. Add items to map canvas contextual menu

Interaction with map canvas can also be done through entries you may add to its contextual menu using the contextMenuAboutToShow signal.

The following code adds My menu ► My Action action next to default entries when you right-click over the map canvas.

# a slot to populate the context menu
def populateContextMenu(menu: QMenu, event: QgsMapMouseEvent):
    subMenu = menu.addMenu('My Menu')
    action = subMenu.addAction('My Action')
    action.triggered.connect(lambda *args:
                             print(f'Action triggered at {event.x()},{event.y()}'))

canvas.contextMenuAboutToShow.connect(populateContextMenu)
canvas.show()

9.4. Dezvoltarea instrumentelor personalizate pentru suportul de hartă

You can write your custom tools, to implement a custom behavior to actions performed by users on the canvas.

Map tools should inherit from the QgsMapTool, class or any derived class, and selected as active tools in the canvas using the setMapTool() method as we have already seen.

Iată un exemplu de instrument pentru hartă, care permite definirea unei limite dreptunghiulare, făcând clic și trăgând cursorul mouse-ului pe canevas. După ce este definit dreptunghiul, coordonatele sale sunt afișate în consolă. Se utilizează elementele benzii elastice descrise mai înainte, pentru a arăta dreptunghiul selectat, așa cum a fost definit.

class RectangleMapTool(QgsMapToolEmitPoint):
  def __init__(self, canvas):
    self.canvas = canvas
    QgsMapToolEmitPoint.__init__(self, self.canvas)
    self.rubberBand = QgsRubberBand(self.canvas, QgsWkbTypes.PolygonGeometry)
    self.rubberBand.setColor(Qt.red)
    self.rubberBand.setWidth(1)
    self.reset()

  def reset(self):
    self.startPoint = self.endPoint = None
    self.isEmittingPoint = False
    self.rubberBand.reset(QgsWkbTypes.PolygonGeometry)

  def canvasPressEvent(self, e):
    self.startPoint = self.toMapCoordinates(e.pos())
    self.endPoint = self.startPoint
    self.isEmittingPoint = True
    self.showRect(self.startPoint, self.endPoint)

  def canvasReleaseEvent(self, e):
    self.isEmittingPoint = False
    r = self.rectangle()
    if r is not None:
      print("Rectangle:", r.xMinimum(),
            r.yMinimum(), r.xMaximum(), r.yMaximum()
           )

  def canvasMoveEvent(self, e):
    if not self.isEmittingPoint:
      return

    self.endPoint = self.toMapCoordinates(e.pos())
    self.showRect(self.startPoint, self.endPoint)

  def showRect(self, startPoint, endPoint):
    self.rubberBand.reset(QgsWkbTypes.PolygonGeometry)
    if startPoint.x() == endPoint.x() or startPoint.y() == endPoint.y():
      return

    point1 = QgsPointXY(startPoint.x(), startPoint.y())
    point2 = QgsPointXY(startPoint.x(), endPoint.y())
    point3 = QgsPointXY(endPoint.x(), endPoint.y())
    point4 = QgsPointXY(endPoint.x(), startPoint.y())

    self.rubberBand.addPoint(point1, False)
    self.rubberBand.addPoint(point2, False)
    self.rubberBand.addPoint(point3, False)
    self.rubberBand.addPoint(point4, True)    # true to update canvas
    self.rubberBand.show()

  def rectangle(self):
    if self.startPoint is None or self.endPoint is None:
      return None
    elif (self.startPoint.x() == self.endPoint.x() or \
          self.startPoint.y() == self.endPoint.y()):
      return None

      return QgsRectangle(self.startPoint, self.endPoint)

  def deactivate(self):
    QgsMapTool.deactivate(self)
    self.deactivated.emit()

9.5. Dezvoltarea elementelor personalizate pentru suportul de hartă

Here is an example of a custom canvas item that draws a circle:

class CircleCanvasItem(QgsMapCanvasItem):
  def __init__(self, canvas):
    super().__init__(canvas)
    self.center = QgsPoint(0, 0)
    self.size   = 100

  def setCenter(self, center):
    self.center = center

  def center(self):
    return self.center

  def setSize(self, size):
    self.size = size

  def size(self):
    return self.size

  def boundingRect(self):
    return QRectF(self.center.x() - self.size/2,
      self.center.y() - self.size/2,
      self.center.x() + self.size/2,
      self.center.y() + self.size/2)

  def paint(self, painter, option, widget):
    path = QPainterPath()
    path.moveTo(self.center.x(), self.center.y());
    path.arcTo(self.boundingRect(), 0.0, 360.0)
    painter.fillPath(path, QColor("red"))


# Using the custom item:
item = CircleCanvasItem(iface.mapCanvas())
item.setCenter(QgsPointXY(200,200))
item.setSize(80)