Dynamically adding Widget to ScrollView - Kivy

Question

I am trying to add widgets to a kivy scrollview. The scrollview is working properly but when i try to add a widget at the bottom of the scrollview, the scrollview automatically adjusts its scroll_y. This looks so bad since the scrollview jumps! It seems to me that this is a usual behavior of the scrollview. How can i edit or set the scrollview so that the user may continue scrolling without the scrollview changing position. Thanks in advance!

Answer 1

Yes, that is the normal ScrollView behavior. The behavior is because the ScrollView does not adjust the scroll_y value, which causes scrolling in most cases. You can avoid that behavior by calculating a new scroll_y value designed to keep the same portion of the viewport visible in the ScrollView. This is fairly easily done, provided that the added Widget has a known height. Here is an example that does that:

from functools import partial

from kivy.app import App
from kivy.clock import Clock
from kivy.lang import Builder
from kivy.uix.label import Label

kv = '''
BoxLayout:
    orientation: 'vertical'
    Button:
        text: 'add'
        on_release: app.add_new_widget()
    ScrollView:
        id: scroll
        BoxLayout:
            id: box
            orientation: 'vertical'
            size_hint_y: None
            height: self.minimum_height
'''

class TestApp(App):
    def build(self):
        self.count = 0
        return Builder.load_string(kv)

    def add_new_widget(self):
        vp_height = self.root.ids.scroll.viewport_size[1]
        sv_height = self.root.ids.scroll.height

        # add a new widget (must have preset height)
        label = Label(text='Widget #' + str(self.count), size_hint=(1, None), height=50)
        self.root.ids.box.add_widget(label)
        self.count += 1

        if vp_height > sv_height:  # otherwise there is no scrolling
            # calculate y value of bottom of scrollview in the viewport
            scroll = self.root.ids.scroll.scroll_y
            bottom = scroll * (vp_height - sv_height)

            # use Clock.schedule_once because we need updated viewport height
            # this assumes that new widgets are added at the bottom
            # so the current bottom must increase by the widget height to maintain position
            Clock.schedule_once(partial(self.adjust_scroll, bottom+label.height), -1)

    def adjust_scroll(self, bottom, dt):
        vp_height = self.root.ids.scroll.viewport_size[1]
        sv_height = self.root.ids.scroll.height
        self.root.ids.scroll.scroll_y = bottom / (vp_height - sv_height)

TestApp().run()

The add_new_widget() method adds another Label each time it is called, and the new Label has a specified height (50 in this case). The adjust_scroll() method is called, if necessary, and calculates a new scroll_y that prevents the ScrollView from scrolling.