pyqgis correct activate signal for combo box list

Question

I am a pyqgis user and I am trying to create a QGIS plugin where that plugin have two tabs widgets and two combo box where first combo box have a list from layers and second combo box have list of fields for current layer in combo box one.

That combo box is inside in tab widget two, and I want to to activate any time user start plugin.

My code works but not correctly - I have curious results.

In the first way in my _ init _ the combo box 1 work fine every time to start the user the plugin then combo box 1 activate correct, but the combo box 2 insert only six fields for some layers (that layers have many fields 19).

In the second way combo box 1 and combo box 2 work fine have correct list of layers and fields, but to activate combo box 1 must be the user to move some layers in panel and after this activate. I don't want this .

Where I have wrong ?

Who is correct signal activate for combo box 1 ?

Maybe some signal to activate when the user press tab 2 from tab widget?

in _ _init __ :

#first way
registry = QgsMapLayerRegistry.instance()
registry.layersAdded.connect(self.change_layers)
#second way
self.iface.legendInterface().groupRelationsChanged.connect(self.change_layers)

#field change
self.ui.comboBox_2.currentIndexChanged.connect(self.updateFields)

layer function :

def change_layers(self):
   self.ui.comboBox_2.clear()
   legendInterface = self.iface.legendInterface()
   listLayerName = [i.name() for i in legendInterface.layers() if i.type() == QgsMapLayer.VectorLayer]
   # add all these layer names to combo box comboLayerList
   self.ui.comboBox_2.addItems(listLayerName)

field function :

    def layer_field(self):
        self.mapCanvas.refresh()
        layers = self.iface.legendInterface().layers()
        # Identify selected layer by its index
        selectedLayerIndex = self.ui.comboBox_2.currentIndex()
        print selectedLayerIndex
        selectedLayer = layers[selectedLayerIndex]
        # Identify fields of the selected layer
        fields = selectedLayer.pendingFields()
        # Get field names of the fields
        fieldnames = [field.name() for field in fields]
        # Clear comboBox_5
        self.ui.comboBox_3.clear()
        # Add field names to comboBox_5
        self.ui.comboBox_3.addItems(fieldnames)

Answer 1

When user select layer in comboBox1, you could use combo.changeCurrentIndex.connect to trigger function in cascade and load fields name to comboBox2.

Example :

def updateFields(self):
   fields_array = []
   layers = self.iface.legendInterface().layers()
   layer_name = self.ui.comboBox_1.currentText()
   for layer in layers:
       if el.name() == layer_name: # find layer from combo text
           fields = layer.dataProvider().fields() # get fields
           fields_array= [fields.name() for field in fields]    
   return self.ui.comboBox_2.addItems(fields_array) # populate combo

self.ui.comboBox_1.currentIndexChanged.connect(self.updateFields) # on select