18. Using Plugin Layers
Hint
The code snippets on this page need the following imports if you’re outside the pyqgis console:
1from qgis.core import (
2 QgsPluginLayer,
3 QgsPluginLayerType,
4 QgsMapLayerRenderer,
5 QgsApplication,
6 QgsProject,
7)
8
9from qgis.PyQt.QtGui import QImage
If your plugin uses its own methods to render a map layer, writing your own
layer type based on QgsPluginLayer
might be the best way to implement that.
18.1. Subclassing QgsPluginLayer
Below is an example of a minimal QgsPluginLayer implementation. It is based on the original code of the Watermark example plugin.
The custom renderer is the part of the implement that defines the actual drawing on the canvas.
1class WatermarkLayerRenderer(QgsMapLayerRenderer):
2
3 def __init__(self, layerId, rendererContext):
4 super().__init__(layerId, rendererContext)
5
6 def render(self):
7 image = QImage("/usr/share/icons/hicolor/128x128/apps/qgis.png")
8 painter = self.renderContext().painter()
9 painter.save()
10 painter.drawImage(10, 10, image)
11 painter.restore()
12 return True
13
14class WatermarkPluginLayer(QgsPluginLayer):
15
16 LAYER_TYPE="watermark"
17
18 def __init__(self):
19 super().__init__(WatermarkPluginLayer.LAYER_TYPE, "Watermark plugin layer")
20 self.setValid(True)
21
22 def createMapRenderer(self, rendererContext):
23 return WatermarkLayerRenderer(self.id(), rendererContext)
24
25 def setTransformContext(self, ct):
26 pass
27
28 # Methods for reading and writing specific information to the project file can
29 # also be added:
30
31 def readXml(self, node, context):
32 pass
33
34 def writeXml(self, node, doc, context):
35 pass
The plugin layer can be added to the project and to the canvas as any other map layer:
plugin_layer = WatermarkPluginLayer()
QgsProject.instance().addMapLayer(plugin_layer)
When loading a project containing such a layer, a factory class is needed:
1class WatermarkPluginLayerType(QgsPluginLayerType):
2
3 def __init__(self):
4 super().__init__(WatermarkPluginLayer.LAYER_TYPE)
5
6 def createLayer(self):
7 return WatermarkPluginLayer()
8
9 # You can also add GUI code for displaying custom information
10 # in the layer properties
11 def showLayerProperties(self, layer):
12 pass
13
14
15# Keep a reference to the instance in Python so it won't
16# be garbage collected
17plt = WatermarkPluginLayerType()
18
19assert QgsApplication.pluginLayerRegistry().addPluginLayerType(plt)