a信号连接b信号，b信号连接槽函数

# a信号与b信号连接
# 使得一个信号是间接触发槽函数，
# #另一个信号仍然是直接触发槽函数， 
# #这里增加了信号与信号之间的联系以及先后顺序的问题，以及传递关系
# 多个信号都与一个槽函数连接

class Demo(QWidget):
    def __init__(self):
        super(Demo,self).__init__()
        self.label = QLabel('label')
        self.btn = QPushButton('button')
        # 连接函数，这里有两个信号
        self.btn.pressed.connect(self.btn.released) # 注意注意这里~~~
        self.btn.released.connect(self.ChangeText)


        self.v_layout = QVBoxLayout()
        self.v_layout.addWidget(self.label)
        self.v_layout.addWidget(self.btn)
        self.setLayout(self.v_layout)
        self.count = 0

    # 定义槽函数
    def ChangeText(self):
        self.count = self.count + 1
        if self.count % 2 == 1: # pressed
            self.label.setText('haha, i have pressed this button')
        else: # released
            self.label.setText('xixi, i have released this button')
            print('I have clicked this button for :', self.count/2)