QStateMachine procedural transition (i.e. Signal)

Question

I need to be able to change state of a QStateMachine from a Signal (or something equivalent) that was not generated by user interaction (i.e. by a process completing). This example redirectsa user generated signal to demonstrate the idea, but doesn't yet work. Here, the push button signals fire other signals that I want to use for the transition. The states here are B1 <-> B2 where Bi -> Bj on the redirected signal when button bi is pressed.

The QStateMachine documentation only seems to cover transitions based on direct signals from buttons and I haven't been able to extrapolate to the more general case of using any Signal (i.e. one that I create).

import sys
from PySide2.QtCore import Qt, QStateMachine, QState, Signal as QSignal
from PySide2.QtWidgets import QApplication, QMainWindow, QPushButton, QVBoxLayout, QWidget

class MainWindow(QMainWindow):
    
    def __init__(self):
        QMainWindow.__init__(self)

        self.w = QWidget()
        self.setCentralWidget(self.w)
        
        l1 = QVBoxLayout()
        self.b1 = QPushButton("B1", self.w)
        self.b1.pressed.connect(self.b1Pressed)
        l1.addWidget(self.b1)
        self.b2  = QPushButton("B2", self.w)
        self.b2.pressed.connect(self.b2Pressed)
        l1.addWidget(self.b2)
        
        self.w .setLayout(l1)
        
        self.setupStateMachine()
        self.show()

    def b1Pressed(self):
        print("b1 pressed")
        self._b1Pressed.emit()
        
    def b2Pressed(self):
        print("b2 pressed")
        self._b2Pressed.emit()
            
    def setupStateMachine(self):
        
        self._b1Pressed = QSignal()
        self._b2Pressed = QSignal()
        
        self.sm = QStateMachine()
        
        self.B1 = QState()
        self.B1.assignProperty(self.b1, "enabled", True)
        self.B1.assignProperty(self.b2, "enabled", False)
        self.sm.addState(self.B1)

        self.B2 = QState()
        self.B2.assignProperty(self.b1, "enabled", False)
        self.B2.assignProperty(self.b2, "enabled", True)
        self.sm.addState(self.B2)
        
        # The point of it all:
        # change states from a signal (or program generated event)
        self.B1.addTransition(self._b1Pressed, self.B2)
        self.B2.addTransition(self._b2Pressed, self.B1)

if __name__ == '__main__':

    app = QApplication(sys.argv)
    window = MainWindow()
    sys.exit(app.exec_())

Answer 1

Your code has the following problems:

• The signals are not declared as attributes of the class but at the same level of the methods.
• You have not established an initial state.
• You have not started the QStateMachine.

class MainWindow(QMainWindow):
    _b1Pressed = QSignal()
    _b2Pressed = QSignal()

    def __init__(self):
        QMainWindow.__init__(self)

        self.w = QWidget()
        self.setCentralWidget(self.w)

        l1 = QVBoxLayout()
        self.b1 = QPushButton("B1", self.w)
        self.b1.pressed.connect(self.b1Pressed)
        l1.addWidget(self.b1)
        self.b2 = QPushButton("B2", self.w)
        self.b2.pressed.connect(self.b2Pressed)
        l1.addWidget(self.b2)

        self.w.setLayout(l1)

        self.setupStateMachine()
        self.show()

    def b1Pressed(self):
        print("b1 pressed")
        self._b1Pressed.emit()

    def b2Pressed(self):
        print("b2 pressed")
        self._b2Pressed.emit()

    def setupStateMachine(self):

        self.sm = QStateMachine()

        self.B1 = QState()
        self.B1.assignProperty(self.b1, "enabled", True)
        self.B1.assignProperty(self.b2, "enabled", False)
        self.sm.addState(self.B1)

        self.B2 = QState()
        self.B2.assignProperty(self.b1, "enabled", False)
        self.B2.assignProperty(self.b2, "enabled", True)
        self.sm.addState(self.B2)

        # The point of it all:
        # change states from a signal (or program generated event)
        self.B1.addTransition(self._b1Pressed, self.B2)
        self.B2.addTransition(self._b2Pressed, self.B1)

        self.sm.setInitialState(self.B1)
        self.sm.start()