[GIS] Adding dynamic text feedback for QGIS plugin

pyqgisqgis-plugins

I am building a plugin using Qt Designer which will initially allow users to select features for a chosen polygon layer and then when a feature is selected on the canvas, the area is printed out in some sort of display widget.

For comparison, the Coordinate Capture tool (shown on the left in the image below) allows users to select on canvas and then print the coordinates in the boxes shown.

How could I connect selecting features on canvas to displaying the area in the text browser?

I am using the following guides for reference:

The following is what I have in code so far. If I move the code from print_area() to run(), I can print the total area in the text browser. But can't seem to connect it correctly so that whenever a layer is chosen from the combo box, the text browser get's updated.

def run(self):
    layers = self.iface.legendInterface().layers()
    layer_list = []
    for layer in layers:
        self.dockwidget.comboBox.clear()
        self.dockwidget.show()
        layer_list.append(layer.name())
        self.dockwidget.comboBox.addItems(layer_list)
        selectedLayerIndex = self.dockwidget.comboBox.currentIndex()
        selectedLayer = layers[selectedLayerIndex]

def print_area(self):
    for feat in selectedLayer.getFeatures():
        geom = feat.geometry()
        self.dockwidget.textBrowser.setText(str(geom.area()))

    selectedLayerIndex.selectionChanged.connect(run)

Best Answer

I would use a Line Edit widget.

Inside your run() function put something like:

self.selectedLayer = self.iface.activeLayer() # Adjust this to your situation
self.selectedLayer.selectionChanged.connect( self.printSelectedArea )

Then define your printSelectedArea function:

def printSelectedArea( self ):
  area=0
  for f in self.selectedLayer.selectedFeatures(): 
    area += f.geometry().area()
  self.dockwidget.txtArea.setText( str( area ) )

Note that this will print the selected area of your currently selected layer. You would need to adjust the code if you want to add areas across layers, which I didn't see from your question.

When a user chooses a layer in your comboBox you should disconnect the printSelectedArea slot from the previous selected layer and connect it to the new one. You could also disconnect the printSelectedArea slot from any SIGNAL when your plugin is closed or unload. Otherwise, you would end up messing printSelectedArea calls up.

Additionally, you would need to add conditionals to make sure there is a selectedLayer before calling selectedFeatures().

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QGIS Plugins – Enabling a Keyboard Shortcut for a QGIS Plugin

I would suggest getting rid of the default "Ok/Cancel" buttons and creating your own push buttons/event listeners (button may be created in Qt Designer):

code e.g:

def doSomething(self):
    # process data

def closeForm(self):
    self.dlg.close()

def initGui(self):
    # more stuff here not shown
    self.dlg.pushButton1.clicked.connect(self.doSomething)
    self.dlg.pushButton2.clicked.connect(self.closeForm)
    self.dlg.pushButton1.setShortcut("A")

When the user clicks the pushButton1 the data will be processed in the doSomething function and the form will stay open. When the user clicks the pushButton2 the form will close.

Update, added statement above when you click the "A" key it will call clicked event of pushButton1 which will call the doSomething function.

pyqgis – Adding Fields to Layer in QGIS Using Python Plugin

One method (which you are currently using) is the standard QComboBox. First we need to import some more classes so add this before importing PyQt4:

from qgis.core import QgsMapLayerRegistry, QgsField, QgsExpression
from PyQt4.QtCore import QVariant, QSettings, QTranslator, qVersion, QCoreApplication

Then in your run(self) function, we can use the following (I've added some comments which hopefully will help you understand what's going on):

def run(self):
    # here I populate a combo box in a form
    layers = self.iface.legendInterface().layers()
    layer_list = []
    for layer in layers:
        layer_list.append(layer.name())
    self.dlg.Cmb_Capa.addItems(layer_list)

    self.dlg.show()
    result = self.dlg.exec_()
    if result:
        # Read current text from combo box
        combo_layer = self.dlg.Cmb_Capa.currentText()
        # Identify layer from table of contents by matching name with the combo box text
        selected_layer = QgsMapLayerRegistry.instance().mapLayersByName(combo_layer)[0]
        # Add a "real" field type with the name "Area"
        selected_layer.dataProvider().addAttributes( [ QgsField("Area", QVariant.Double) ] )
        # Update the attribute table
        selected_layer.updateFields()
        # Allow editing of the selected layer
        selected_layer.startEditing()
        # Find index of the newly-created "Area" field
        idx = selected_layer.fieldNameIndex( "Area" )
        # Use standard QgsExpression to calculate area for each feature
        e = QgsExpression( """ $area """ )
        e.prepare( selected_layer.pendingFields() )
        for f in selected_layer.getFeatures():
            f[idx] = e.evaluate(f)
            selected_layer.updateFeature(f)
        # Save the selected layer
        selected_layer.commitChanges()

Alternatively, you could use QGIS custom widget: QgsMapLayerComboBox. This is automatically updated when any changes are made to the layer list (i.e. layers added/removed, names changed etc) without you having to refresh the plugin. To use this, you will need to import an additional class:

from qgis.gui import QgsMapLayerProxyModel

Now your run(self) function can look like this:

 def run(self):
    # Here, QgsMapLayerComboBox automatically reads the relevant layers
    self.dlg.Cmb_Capa.setFilters( QgsMapLayerProxyModel.PolygonLayer )

    self.dlg.show()
    result = self.dlg.exec_()
    if result:
        combo_layer = self.dlg.Cmb_Capa.currentText()
        selected_layer = QgsMapLayerRegistry.instance().mapLayersByName(combo_layer)[0]
        selected_layer.dataProvider().addAttributes( [ QgsField("Area", QVariant.Double) ] )
        selected_layer.updateFields()
        selected_layer.startEditing()
        idx = selected_layer.fieldNameIndex( "Area" )
        e = QgsExpression( """ $area """ )
        e.prepare( selected_layer.pendingFields() )
        for f in selected_layer.getFeatures():
            f[idx] = e.evaluate(f)
            selected_layer.updateFeature(f)
        selected_layer.commitChanges()

Note: When using QGIS custom widgets, you may need to edit your "plugin_name_dialog_base.ui" file, find the QgsMapLayerComboBox class and replace the <header> to qgis.gui.h.

For example, it might look like this:

<customwidget>
 <class>QgsMapLayerComboBox</class>
 <extends>QComboBox</extends>
 <header>qgsmaplayercombobox.h</header>
</customwidget>

And it needs to look like this:

<customwidget>
 <class>QgsMapLayerComboBox</class>
 <extends>QComboBox</extends>
 <header>qgis.gui.h</header>
</customwidget>

Best Answer

Related Solutions

QGIS Plugins – Enabling a Keyboard Shortcut for a QGIS Plugin

pyqgis – Adding Fields to Layer in QGIS Using Python Plugin

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