Compatibility with Sanic 19.6.2

[tng10]

Hi,

Describe the bug

I just bumped into an error when upgrading to newest version of Sanic using ASGI interface.

Since ASGI lifespan protocol has support for only two server events, we no longer can make usage of before_server_start and after_server_stop server events.

Sanic OpenAPI uses before_server_start server event and this give us the following warning:

UserWarning: You have set a listener for "before_server_start" in ASGI mode. It will be executed as early as possible, but not before the ASGI server is started.

To Reproduce
Run Sanic using an ASGI interface

Expected behavior
Make sure this warning doesn't happen.

Environment:

• ubuntu 18.04
• Version [0.5.3]

On the decorator of the method build_spec at the file swagger.py I suggest the following change:

before

@blueprint.listener('before_server_start')

after

@blueprint.listener('after_server_start')

What do you think about it?

[chenjr0719]

Sounds good. Would you wanna submit a PR for this?

[tng10]

Yes, I'll do it.

[tng10]

@chenjr0719 I just created PR #130 for it

[ahopkins]

👍 Nice catch.

[stale]

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