app.py

import asyncio
import time
import concurrent.futures
from threading import Event
from h2o_wave import main, app, Q, ui


# This takes a lot of time (compute heavy).
def blocking_function(q: Q, loop: asyncio.AbstractEventLoop):
    count = 0
    total = 10
    future = None
    while count < total:
        # Check if cancelled.
        if q.client.event.is_set():
            asyncio.ensure_future(show_cancel(q), loop=loop)
            return 
        # This blocks the main thread and prevents any other execution.
        # This would be the compute in the real world.
        time.sleep(1)
        count += 1
        # If future is not done yet, skip the update to keep the correct order.
        if not future or future.done():
            # Assume you are able to emit some kind of progress.
            future = asyncio.ensure_future(update_ui(q, count / total), loop=loop)

    # Show a notification at the end.
    asyncio.ensure_future(show_notification(q), loop=loop)

async def show_cancel(q: Q):
    q.page['meta'].dialog.items[0].progress.caption = 'Cancelled'
    q.page['meta'].dialog.closable = True
    await q.page.save()

async def show_notification(q: Q):
    q.page['meta'].dialog = None
    q.page['meta'].notification_bar = ui.notification_bar(
        name='success_notification',
        text='Job done!',
        type='success',
        events=['dismissed']
    )
    await q.page.save()

async def update_ui(q: Q, value: int):
    q.page['meta'].dialog.items[0].progress.value = value
    await q.page.save()
    

@app('/')
async def serve(q: Q):
    # Unimportant, draw initial UI.
    if not q.client.initialized:
        q.page['meta'] = ui.meta_card(box='')
        q.page['form'] = ui.form_card(box='1 1 1 1', items=[
            ui.button(name='start_job', label='Start job')
        ])
        q.client.initialized = True
    
    # Handle start job button click.
    if q.args.start_job:
        q.page['meta'].dialog = ui.dialog(title='Blocking job', blocking=True, items=[
            ui.progress(label='Progress', value=0),
            ui.button(name='cancel', label='Cancel')
        ])
        # Do not run like this - will block the whole thread - freeze the app.
        # blocking_function(q, loop)

        # Get the current event loop - will be used for
        # running async functions within the blocking.
        loop = asyncio.get_event_loop()
        # Create an event to use for cancellation.
        q.client.event = Event()
        with concurrent.futures.ThreadPoolExecutor() as pool:
            await q.exec(pool, blocking_function, q, loop)

    if q.args.cancel:
        q.client.event.set()

    # Unimportant, just handle notification dismissal.
    if q.events.success_notification and q.events.success_notification.dismissed:
        q.page['meta'].notification_bar = None

    await q.page.save()