How to use the riffle.model.FilesystemSortProxy function in Riffle

self._bookmarksWidget = QtGui.QListView()
        self._contentSplitter.addWidget(self._bookmarksWidget)

        self._filesystemWidget = QtGui.QTableView()
        self._filesystemWidget.setSelectionBehavior(
            self._filesystemWidget.SelectRows
        )
        self._filesystemWidget.setSelectionMode(
            self._filesystemWidget.SingleSelection
        )
        self._filesystemWidget.verticalHeader().hide()

        self._contentSplitter.addWidget(self._filesystemWidget)

        proxy = riffle.model.FilesystemSortProxy(self)
        model = riffle.model.Filesystem(
            path=self._root, parent=self, iconFactory=self._iconFactory
        )
        proxy.setSourceModel(model)
        proxy.setDynamicSortFilter(True)

        self._filesystemWidget.setModel(proxy)
        self._filesystemWidget.setSortingEnabled(True)

        self._contentSplitter.setStretchFactor(1, 1)
        self.layout().addWidget(self._contentSplitter)

        self._footerLayout = QtGui.QHBoxLayout()
        self._footerLayout.addStretch(1)

        self._cancelButton = QtGui.QPushButton('Cancel')