在pyqt4中自定义new-style Signal

apt中搜索pyqt4,在pyqt5出来后，就被改名为python-qt4，一股淡淡的忧伤～

今天试着写一个自定义的新式信号（new-style Signal），却总是报错，错误的代码如下：

#!/usr/bin/python
#coding:utf-8
'''这是错误代码～～～～'''
import sys
from PyQt4.QtCore import QObject, pyqtSignal, QCoreApplication
from PyQt4.QtGui import QWidget
class A(QObject):
    def __init__(self,parent=None):
        QObject.__init__(self)
        self.mySignal = pyqtSignal()
    def useSignal(self):
        self.mySignal.emit()

class B(QObject):
    def __init__(self,parent=None):
        QObject.__init__(self)
        self.a = A()
        self.a.mySignal.connect(self.printASent)
        self.a.useSignal()
        
    def printASent(self):
        print 'Signal is called.'
        
if __name__ == '__main__':
    app = QCoreApplication(sys.argv)
    b = B()
    sys.exit(app.exec_())

出错代码：self.a.mySignal.connect(self.printASent)
AttributeError: 'PyQt4.QtCore.pyqtSignal' object has no attribute 'connect'

为什么会这样呢，在google了一下后得到解决方案，将mySignal作为类属性来定义，即为如下代码：

#!/usr/bin/python
#coding:utf-8
import sys
from PyQt4.QtCore import QObject, pyqtSignal, QCoreApplication
from PyQt4.QtGui import QWidget
class A(QObject):
    def __init__(self,parent=None):
        QObject.__init__(self)

    '''作为类属性来定义'''
    mySignal = pyqtSignal()
    
    def useSignal(self):
        self.mySignal.emit()

class B(QObject):
    def __init__(self,parent=None):
        QObject.__init__(self)
        self.a = A()
        self.a.mySignal.connect(self.printASent)
        self.a.useSignal()  def printASent(self):
        print 'Signal is called.'
        
if __name__ == '__main__':
    app = QCoreApplication(sys.argv)
    b = B()
    sys.exit(app.exec_())

此时运行就能得到正确结果。

为了一探究竟，又去翻了一下PyQt4的官方文档。

http://pyqt.sourceforge.net/Docs/PyQt4/new_style_signals_slots.html#unbound-and-bound-signals

“A signal (specifically an unbound signal) is an attribute of a class that is a sub-class of QObject. When a signal is referenced as an attribute of an instance of the class then PyQt4 automatically binds the instance to the signal in order to create a bound signal. This is the same mechanism that Python itself uses to create bound methods from class functions.”

上面一句话提出了这3个要点：

1.signal（明确的说是一个非绑定的signal）是QObject子类的一个属性.

2.singal被作为一个类实例对象的引用时，PyQt4自动的绑定这个实例对象至signal来创建一个绑定的signal（bound signal）.

3.这和，Python从类函数中创建绑定方法，是同样的机制

一句话，把mySignal定义为类属性,然后在使用时把他绑定为对象属性吧。