问如何在QTreeWidget的空域点击时执行函数

EN

from PySide2 import QtCore, QtWidgets

class TreeWidget(QtWidgets.QTreeWidget):
    emptyClicked = QtCore.Signal()

    def __init__(self, parent=None):
        super(TreeWidget, self).__init__(parent)
        for i in range(2):
            it = QtWidgets.QTreeWidgetItem(self, ["item-{}".format(i)])
            for j in range(3):
                child_it = QtWidgets.QTreeWidgetItem(it, ["item-{}-{}".format(i, j)])
        self.expandAll()

    def mousePressEvent(self, event):
        super(TreeWidget, self).mousePressEvent(event)
        if not self.indexAt(event.pos()).isValid():
            self.emptyClicked.emit()

@QtCore.Slot()
def on_empty_clicked():
    print("clicked")

if __name__ == '__main__':
    import sys
    app = QtWidgets.QApplication(sys.argv)
    w = TreeWidget()
    w.emptyClicked.connect(on_empty_clicked)
    w.resize(640, 480)
    w.show()
    sys.exit(app.exec_())

from PySide2 import QtCore, QtGui, QtWidgets

from design import Ui_MainWindow

class MainWindow(QtWidgets.QMainWindow, Ui_MainWindow):
    emptyClicked = QtCore.Signal()

    def __init__(self, parent=None):
        super(MainWindow, self).__init__(parent)
        self.setupUi(self)
        self.treeWidget_A.viewport().installEventFilter(self)
        self.emptyClicked.connect(self.on_emptyClicked)

    def eventFilter(self, obj, event):
        if obj is self.treeWidget_A.viewport() and event.type() == QtCore.QEvent.MouseButtonPress:
            if not self.treeWidget_A.indexAt(event.pos()).isValid():
                self.emptyClicked.emit()
        return super(MainWindow, self).eventFilter(obj, event)

    @QtCore.Slot()
    def on_emptyClicked(self):
        print("empty")

if __name__ == "__main__":
    import sys
    app = QtWidgets.QApplication(sys.argv)
    w = MainWindow()
    w.show()
    sys.exit(app.exec_())