from PyQt4.QtCore import *
from PyQt4.QtGui import *

layer = iface.activeLayer()

class A(QWidget):
    mySignal = pyqtSignal([int])
    
    def __init__(self):
        super(A, self).__init__()
        
        self.progressbar = QProgressBar()
        layout = QGridLayout()
        self.button = QPushButton('Start')
        self.button.clicked.connect(self.iterator)
        layout.addWidget(self.button, 0, 0)
        layout.addWidget(self.progressbar, 0, 1)
        self.setLayout(layout)
        self.mySignal.connect(self.status)
        self.cnt = layer.featureCount()
        
    @pyqtSlot(int)
    def status(self, element):
        perc = element*100/float(self.cnt)
        self.progressbar.setValue(perc)
#        print "Progress {0:.{1}f} % element: {2}"
#                .format(element*100/float(self.cnt), 2, element)

    def iterator(self):
        i = 0
        for f in layer.getFeatures():
            i += 1
            a.mySignal.emit(i)

a = A()
a.show()