[GIS] PyQgis question: How to change or set active layer

Question

Developing a QGIS plugin, I found that if no layer is selected in the Layers panel I can iterate over all layers and do anything to them, but "activate" them.

Could someone give me a hint please?

Answer 1

You can set the active layer using the qgis.utils.iface object (i.e. QgisInterface):

qgis.utils.iface.setActiveLayer(QgsMapLayer)  # returns bool
# or
qgis.utils.iface.legendInterface().setCurrentLayer(QgsMapLayer)  # returns bool

Clearing the selection, or the active layer (underlined QTreeWidget item in QGIS 2.0), in the legend is a bit more involved:

from PyQt4.QtCore import *
from PyQt4.QtGui import *
lg = qgis.utils.iface.mainWindow().findChild(QTreeWidget, 'theMapLegend')
lg.selectionModel().clear()  # clear just selection
lg.setCurrentItem(None)  # clear selection and active layer

There are three ways to retrieve the current layers, relative to what you may want to do:

~ via QList< QgsMapLayer * > QgsLegendInterface::layers()

qgis.utils.iface.legendInterface().layers()  # returns [layer, ...]

~ via QList< QgsMapLayer * > QgsMapCanvas::layers():

qgis.utils.iface.mapCanvas().layers()  # returns [layer, ...]

~ via const QMap< QString, QgsMapLayer * > & QgsMapLayerRegistry::mapLayers():

QgsMapLayerRegistry.instance().mapLayers()  # returns {'layer_id': layer, ...}

Reference these answers for information about the differences in those layer collections.