View and MethodView classes do not support async handlers

[miguelgrinberg]

Regular views can now be written as async functions, but instances of the View and MethodView classes did not get updated to support this functionality. Consider the following example:

import asyncio
from flask import Flask
from flask.views import View, MethodView

app = Flask(__name__)

class Test(View):
    async def dispatch_request(self):
        await asyncio.sleep(1)
        return 'test'

class Test2(MethodView):
    async def get(self):
        await asyncio.sleep(1)
        return 'test2'

app.add_url_rule('/test', view_func=Test.as_view('test'))
app.add_url_rule('/test2', view_func=Test2.as_view('test2'))

The expectation is that you should be able to send requests to /test and /test2 and that their respective handlers should be wrapped with the app.ensure_sync() method, as function-based handlers are.

Instead sending requests to these URLs cause the following error:

TypeError: The view function did not return a valid response. The return type must be a string, dict, tuple, Response instance, or WSGI callable, but it was a coroutine.

Environment:

• Python version: >= 3.7
• Flask version: >= 2