Simple legend dock widget

gistfile1.py

from PyQt4.QtCore import *
from PyQt4.QtGui import *
from qgis.core import *


class LegendView(QLabel):
  def __init__(self, root, parent=None):
    QLabel.__init__(self, parent)
    self.root = root
    self.model = QgsLayerTreeModel(self.root)
    self.settings = QgsLegendSettings()
    self.settings.setTitle("")
    self.model.rowsInserted.connect(self.refreshLegend)
    self.model.rowsRemoved.connect(self.refreshLegend)
    self.model.dataChanged.connect(self.refreshLegend)
    self.refreshLegend()

  def refreshLegend(self):
    r = QgsLegendRenderer(self.model, self.settings)
    size = r.minimumSize()
    dpi = 96
    dpmm = dpi / 25.4
    s = QSize(size.width() * dpmm, size.height() * dpmm)
    img = QImage(s, QImage.Format_ARGB32_Premultiplied)
    img.fill(Qt.transparent)

    painter = QPainter(img)
    painter.scale(dpmm, dpmm)
    r.drawLegend(painter)
    painter.end()

    self.setPixmap( QPixmap.fromImage(img) )


def add_legend_dock(iface):
  dw = QDockWidget("Legend")
  dw.setWidget(LegendView(QgsProject.instance().layerTreeRoot()))
  iface.addDockWidget(Qt.LeftDockWidgetArea, dw)
  return dw