ListView is one of the most commonly used scrolling controls in Flet. It allows for the display of a list of items arranged linearly, providing an efficient way to present a large number of elements while maintaining smooth scrolling performance. Unlike Column or Row, ListView ensures seamless scrolling even with thousands of items.

Why Use ListView?

ListView is preferred over Column or Row for handling large lists due to its optimized rendering and scrolling capabilities. It efficiently manages memory usage and rendering performance, making it ideal for dynamic and extensive data sets.

Example: Auto-Scrolling ListView

The following example demonstrates how to create an auto-scrolling ListView in Python using Flet:

from time import sleep
import flet as ft

def main(page: ft.Page):
    page.title = "Auto-scrolling ListView"

    lv = ft.ListView(expand=1, spacing=10, padding=20, auto_scroll=True)
    count = 1

    for i in range(0, 60):
        lv.controls.append(ft.Text(f"Line {count}"))
        count += 1

    page.add(lv)

    for i in range(0, 60):
        sleep(1)
        lv.controls.append(ft.Text(f"Line {count}"))
        count += 1
        page.update()

ft.app(main)

This example initializes a ListView that automatically scrolls as new items are added dynamically over time.

Key Properties

1. auto_scroll

  • Enables automatic scrolling when new items are added.

  • Must be False for scroll_to() to work.

  • Default: False

2. build_controls_on_demand

  • Controls are created only when they are about to become visible, improving performance with large lists.

  • Default: True

3. cache_extent

  • Defines the extra area before and after the visible region that will be laid out in advance for smoother scrolling.

4. clip_behavior

  • Determines whether the content is clipped based on ClipBehavior settings.

  • Default: HARD_EDGE

5. controls

  • A list of child elements inside the ListView.

6. divider_thickness

  • Specifies the thickness of dividers between items.

  • Default: 0 (no divider).

7. first_item_prototype

  • Uses the dimensions of the first item as a reference for all items.

  • Default: False

8. horizontal

  • Arranges ListView items horizontally instead of vertically.

9. item_extent

  • Sets a fixed height or width for each item to improve rendering efficiency.

10. on_scroll_interval

  • Defines the interval (in milliseconds) for throttling on_scroll events.

  • Default: 10

11. padding

  • Defines spacing around ListView items.

12. reverse

  • Determines if the scrolling direction should be reversed.

  • Default: False

13. semantic_child_count

  • The number of children that contribute semantic information.

14. spacing

  • Specifies spacing between items.

Methods

scroll_to(offset, delta, key, duration, curve)

Moves the scroll position based on:

  • Absolute offset

  • Relative delta

  • A specific control with key

Events

on_scroll

  • Fires when the scroll position changes.

  • The event handler receives an instance of OnScrollEvent.

Conclusion

ListView in Flet is a powerful and efficient way to display large lists of data while ensuring smooth scrolling. By utilizing its properties and methods effectively, developers can build responsive and user-friendly interfaces for handling extensive datasets.

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