pyqgis - Adding attribute list to combobox in QGIS plugin?

I am developing a simple plugin at the moment which contains 3 combo boxes:

• comboBox_1


• comboBox_2


• comboBox_3

Combobox 1 & 2 lists the vector layers available, however I would like the 3rd combobox to list the attribute column that both layers have in common (from the attribute table).

    # run method that performs all the real work
def run(self):
    # show the dialog
    self.dlg.show()
    for layer in self.iface.legendInterface().layers():
        if layer.type() == QgsMapLayer.VectorLayer:
            self.dlg.comboBox.addItem(layer.name())
            self.dlg.comboBox_2.addItem(layer.name())    

Answer

You could achieve this based on the QGIS Combo Manager

Just create your own filter for the fields in a subclass of FieldCombo. The following code is untested (literally), so please bear with me if there's a typo or other minor problems, and think of it as a reference.

from qgiscombomanager import *

class MyFieldCombo( FieldCombo ):
    def __init__(self, widget, vectorLayerCombo1, vectorLayerCombo2, initField="", options={}):
        FieldCombo.__init__(self, widget, vectorLayerCombo1, vectorLayerCombo2, initField, options)
        self.vectorLayerCombo1 = vectorLayerCombo1
        self.vectorLayerCombo2 = vectorLayerCombo2


    # Overrides the filter from FieldCombo. Will be called for every field
    # and should return True, if the field should be shown.
    def __isFieldValid(self, idx):
        if not FieldCombo.__isFieldValid(self, idx):
            return False

        for f in self.vectorLayerCombo2.pendingFields():
            # adjust this line to reflect your idea of "common"
            if f.name() == self.vectorLayerCombo1.pendingFields()[idx].name():
                return True

        return False

# run method that performs all the real work
def run(self):
    # show the dialog
    self.dlg.show()
    self.vlc1 = VectorLayerCombo( self.dlg.comboBox )
    self.vlc2 = VectorLayerCombo( self.dlg.comboBox_2 )
    self.fc = MyFieldCombo( self.dlg.comboBox_3 )