from kivy.app import App
from kivy.properties import StringProperty, ObjectProperty
from kivy.lang import Builder
from kivy.event import EventDispatcher

KV = '''
#:import A kivy.animation.Animation

BoxLayout:
    orientation: 'vertical'
    Label:
        size_hint_y: None
        height: 50
        text: 'An Awesome App'

    ScreenManager:
        id: screenmanager
        LoginScreen:
        WelcomeScreen:

<AutoLabel@Label>:
    size_hint: None, None
    size: self.texture_size
    valign: 'center'

<FormTextInput@TextInput>:
    size_hint: None, None
    multiline: False
    height: self.minimum_height
    width: 200

<FormButton@Button>:
    size_hint_y: None
    height: 50

<LoginScreen@Screen>:
    name: 'login'

    BoxLayout:
        id: box
        orientation: 'vertical'
        pos_hint: {'center': (.5, .5)}
        size_hint: None, None
        size: self.minimum_size
        xoff: 0

        # this is just to shake the box if login fails
        canvas.before:
            PushMatrix:
            Translate:
                x: self.xoff or 0

        canvas.after:
            PopMatrix:

        GridLayout:
            size_hint: None, None
            size: self.minimum_size
            cols: 2

            AutoLabel:
                text: 'username'
            FormTextInput:
                id: username
            AutoLabel:
                text: 'password'
            FormTextInput:
                id: password
                password: True

        FormButton:
            text: 'login'
            on_release:
                success = app.login_manager.login(username.text, password.text)
                if success: app.goto_screen('welcome')
                else: A.cancel_all(box), (
                A(xoff=-50, t='out_quad', d=.1)
                + A(xoff=0, t='out_elastic', d=.4)
                ).start(box)


<WelcomeScreen@Screen>:
    name: 'welcome'

    BoxLayout:
        orientation: 'vertical'

        Label:
            text: 'welcome {username}'.format(username=app.login_manager.username)

        FormButton:
            text: 'logout'
            on_release:
                app.login_manager.logout()
                app.goto_screen('login')
'''

class LoginManager(EventDispatcher):
    username = StringProperty()

    def login(self, username, password):
        # here one would do a proper check
        if username == 'admin' and password == 'admin':
            self.username = username
            return True
        else:
            return False

    def logout(self):
        self.username = ''


class MyApp(App):
    login_manager = ObjectProperty()

    def build(self):
        self.login_manager = LoginManager()
        return Builder.load_string(KV)

    def goto_screen(self, name):
        self.root.ids.screenmanager.current = name


if __name__ == '__main__':
    app = MyApp()
    app.run()