Add support for message call

[codeofandrin]

Summary

This PR adds support for the message call object.

Related PR:

• add message.call field discord/discord-api-docs#6864

Checklist

• If code changes were made then they have been tested.
  • I have updated the documentation to reflect the changes.
• This PR fixes an issue.
• This PR adds something new (e.g. new method or parameters).
• This PR is a breaking change (e.g. methods or parameters removed/renamed)
• This PR is not a code change (e.g. documentation, README, ...)

[Rapptz]

Mostly okay, I'd prefer consistency with the previous version.

The previous name was CallMessage and not MessageCall. I still think that name makes more sense than the current one because Call is the adjective for Message, similar to PartialMessage rather than MessagePartial.

[MCausc78]

I think Message.system_content needs update to handle MessageType.call.

[codeofandrin]

I think Message.system_content needs update to handle MessageType.call.

yes you're right, it currently returns an empty string.

[codeofandrin]

The problem about this is: I don't know e.g. when Discord uses the next hour. When is it an hour and when is it 2 hours. I would assume that 91min is 2 hours and 89min is an hour but it could also be that 60..115min is an hour and the rest is 2 hours.

[Rapptz]

You can see the previous implementation:

discord.py/discord/message.py

Lines 319 to 330 in 1863a1c

	if self.type is MessageType.call:
	# we're at the call message type now, which is a bit more complicated.
	# we can make the assumption that Message.channel is a PrivateChannel
	# with the type ChannelType.group or ChannelType.private
	call_ended = self.call.ended_timestamp is not None
	if self.channel.me in self.call.participants:
	return '{0.author.name} started a call.'.format(self)
	elif call_ended:
	return 'You missed a call from {0.author.name}'.format(self)
	else:
	return '{0.author.name} started a call \N{EM DASH} Join the call.'.format(self)

However it's pretty out-of-date. From the client, the strings are as follows:

SYSTEM_MESSAGE_CALL_STARTED_HOOK: $[!!{username}!!](usernameHook) started a call.,
SYSTEM_MESSAGE_CALL_STARTED: [!!{username}!!](usernameOnClick) started a call.,
SYSTEM_MESSAGE_CALL_STARTED_WITH_DURATION_HOOK: $[!!{username}!!](usernameHook) started a call that lasted !!{callDuration}!!.,
SYSTEM_MESSAGE_CALL_MISSED_HOOK: You missed a call from $[!!{username}!!](usernameHook).,
SYSTEM_MESSAGE_CALL_MISSED_WITH_DURATION_HOOK: You missed a call from $[!!{username}!!](usernameHook) that lasted !!{callDuration}!!.,
SYSTEM_MESSAGE_JOIN_CALL: Join the call

The client formats the duration string like this...

m = function(e) {
    let t = null != e.call ? e.call.duration : null;
    return null != t ? t.humanize() : null
}(t);

Which isn't very helpful. We can just take your liberty here and just round, so 2.5 hours => 3 hours but 2.3 hours => 2 hours. Less than 1 hour would be in minutes (so 59 minutes is possible instead of rounding to an hour).

The skeleton should be something like this:

if self.type is MessageType.call:
    call_ended = self.call.ended_timestamp is not None
    missed = self._state.user not in self.call.participants

    if call_ended:
        duration = ...
        if missed:
            return 'You missed a call from {0.author.name} that lasted {1}'.format(self, duration)
        else:
            return '{0.author.name} started a call that lasted {1}.'.format(self, duration)
    else:
        if missed:
            return '{0.author.name} started a call \N{EM DASH} Join the call.'.format(self)
        else:
            return '{0.author.name} started a call.'.format(self)

[codeofandrin]

I've just noted that mentions doesn't contain the users anymore:

self._message.mentions=[]

So we have to retrieve the users directly from the cache.

[codeofandrin]

I've added the system call message. The "calculation" is based on how Discord determines it (approx.). I've manipulated the message object in the client to see what the output is. I also made some unit tests, which can be found here:

https://github.com/Puncher1/discord.py/blob/message-call-unittest/tests/test_call_system_content.py