How to require a button to be clicked on the first tab before going to other tabs in Kivy/KivyMD

Question

The code below is a demo I wrote. What I want to accomplish is that I have to click the button in the first tab before using other tabs. If the button is not clicked and I try to go to other tabs, a message will be shown and I'll be lead to the first tab. The problem is that the message will be shown, but I have to click one more time to go back to the first tab. Any idea how to fix it? Thanks in advance!

from kivy.lang import Builder

from kivymd.app import MDApp
from kivymd.uix.floatlayout import MDFloatLayout
from kivymd.uix.button import MDRaisedButton
from kivymd.uix.tab import MDTabsBase
from kivymd.toast import toast


KV = '''
MDBoxLayout:
    orientation: "vertical"

    MDToolbar:
        title: "Example Tabs"

    MDTabs:
        id: tabs
        on_tab_switch: app.on_tab_switch(*args)


<Tab>

    MDLabel:
        id: label
        text: "Tab 0"
        halign: "center"

<Tab4test>
    MDLabel:
        id: label
        text: "Tab 0"
        halign: "center"        
    MDRaisedButton: 
        text: "Click me!!!"
        on_release: root.change_flag()
'''


class Tab(MDFloatLayout, MDTabsBase):
    '''Class implementing content for a tab.'''

class Tab4test(MDFloatLayout, MDTabsBase):
    '''Class implementing content for a tab.'''
    def change_flag(self):
        MDApp.get_running_app().flag =1
        toast("flag value changed to 1!")

class Example(MDApp):
    flag = 0
    def build(self):
        return Builder.load_string(KV)

    def on_start(self):
        self.root.ids.tabs.add_widget(Tab4test(title=f"Tab 0"))
        for i in range(1, 3):
            self.root.ids.tabs.add_widget(Tab(title=f"Tab {i}"))
        print(self.flag)

    def on_tab_switch(
        self, instance_tabs, instance_tab, instance_tab_label, tab_text
    ):
        '''Called when switching tabs.

        :type instance_tabs: <kivymd.uix.tab.MDTabs object>;
        :param instance_tab: <__main__.Tab object>;
        :param instance_tab_label: <kivymd.uix.tab.MDTabsLabel object>;
        :param tab_text: text or name icon of tab;
        '''

        instance_tab.ids.label.text = tab_text
        print(self.flag)
        title = instance_tab.title

        if title != f"Tab 0" and self.flag == 0:
            toast("Please click the button in Tab 0 first!", duration=3)
            #print(instance_tabs.carousel.slides)
            #instance_tabs.carousel.load_slide(instance_tabs.carousel.slides[0])
            instance_tabs.switch_tab("Tab 0", search_by="title")

Example().run()

Answer 1

After searching what's inside of this MDTabs, I found MDTabsBar and i decited to disabled this on opening. After clicking button on first tab, i activate it:

test.py:

from kivymd.app import MDApp
from kivymd.uix.floatlayout import MDFloatLayout
from kivymd.uix.tab import MDTabsBase
from kivy.uix.screenmanager import Screen,ScreenManager
from kivy.properties import StringProperty,ObjectProperty
from kivy.clock import Clock
class S1(Screen):
    mdtabs= ObjectProperty()
    def on_enter(self, *args):
        Clock.schedule_once(self.settings)
    def settings(self,*args):
        print(self.mdtabs.children[0])
        self.mdtabs.children[0].disabled = True
    def fu(self):
        self.mdtabs.children[0].disabled = False
class SM(ScreenManager):
    pass
class Tab(MDFloatLayout, MDTabsBase):
    pass
class test(MDApp):
    def build(self):
        return SM()
if __name__ == '__main__':
    test().run()

test.kv:

<SM>:
    S1:
        name: 'p1'
<S1>:
    mdtabs:mdtabs
    MDTabs:
        id: mdtabs
        Tab:
            title: 'Tab1'
            Button:
                text: 'Click me'
                on_release: root.fu()
        Tab:
            title: 'Tab2'