Resizing one widget before another

Hi is there a way to resize one widget before another in a layout? For example, when I resize the window, I want one of the widgets to resize to zero first before resizing the other widget. Here is what I have so far:

from PySide2 import QtCore, QtWidgets, QtGui

class TestWindow(QtWidgets.QMainWindow):
    def __init__(self):
        super(TestWindow, self).__init__()
        wgt = QtWidgets.QWidget(self)
        mainLayout = QtWidgets.QHBoxLayout()
        wgt.setLayout(mainLayout)
        self.setWindowTitle("Test")

        l = QtWidgets.QFrame()
        r = QtWidgets.QFrame()

        l.setStyleSheet("background-color: blue;")
        r.setStyleSheet("background-color: green;")

        mainLayout.addWidget(l)
        mainLayout.addWidget(r)            

        self.setCentralWidget(wgt)


app = QtWidgets.QApplication()        
x = TestWindow()

x.show()
app.exec_()

Here are some pictures showing what I want:

Green disappears first, then blue

So in this example, I want the green box to get smaller first, before the blue one does when I resize the main window. How do I achieve this in QT?

Solution

I found a workaround for this. I decided to use a splitter for this solution.

class TitleSplitter(QtWidgets.QSplitter):

    def __init__(self):
        super(TitleSplitter, self).__init__()
        self.setStyleSheet("::handle {background-color: transparent}")
        self.setHandleWidth(0)

    def resizeEvent(self, event):
        """ Restrain the size """
        if self.count() > 1:
            w = self.widget(0).sizeHint().width()
            self.setSizes([w, self.width()-w])
        return super(TitleSplitter, self).resizeEvent(event)

    def addWidget(self, *args, **kwargs):
        """ Hide splitters when widgets added """
        super(TitleSplitter, self).addWidget(*args, **kwargs)
        self.hideHandles()

    def hideHandles(self):
        """ Hide our splitters """
        for i in range(self.count()):
            handle = self.handle(i)
            handle.setEnabled(False)
            handle.setCursor(QtCore.Qt.ArrowCursor)
            handle.setAttribute(QtCore.Qt.WA_TransparentForMouseEvents)

Usage:

split = TitleSplitter()
l = QtWidgets.QFrame()
r = QtWidgets.QFrame()

l.setStyleSheet("background-color: blue;")
r.setStyleSheet("background-color: green;")
split.addWidget(l)
split.addWidget(r)

It only works if you have 2 widgets added. Ideally I would probably use a layout for this, but this works well enough for me.