[gql_link] controlling/cancelling streams about gql HOT 7 OPEN

Hi
The WebSocketLink has dispose(); I suggest adding dispose() to the Link interface class and implement it on other links.
If this is OK I will do it.

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@agent3bood dispose() is for disposing the connection to the server, not individual requests.

I think in the subscription case, we should add an unsubscribe or stop callback to the response context that sends a send a GQL_STOP message with the original id. We could also extend Stream with an unsubscribe callback.

One thing I considered but decided against: Having identical subscription requests map to the same id, and then a StopSubscription request context to cancel. The problem is that that results in potentially unrelated code cancelling each other's subscription.

Anyhow, I don't think graphql/client.dart doesn't have this ability either right now either, so I'm not too concerned

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A single steam at the WebSocketLink might be used by multiple listeners at the client. So I think StreamSubscription should be used to cancel a stream at the client and the link should follow the state of it's stream to know if the subscription should be closed.

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Thanks for the feedback.
To close a graphQL subscription the client has to send this message

{
"type": "stop",
"id": "123"
}

The current code will handle closing the stream once it is confirmed by the server.

Thinking of a pretty way to implement this!

1. The code that request closing the connection must have the id of the operation (can be added to the response.context).
2. Then we can do myLink.stop(id).

PS:
The client has to send the stop command so that the server will not be sending unnecessary updates.

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Another approach could be to cancel a subscription automatically once nobody is listening anymore.

Ferry currently does not support this, but my PR https://github.com/gql-dart/ferry/pull/52/files would enable ferry the send the "onCancel" event to the link, which could then clean up its resources.

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Update:

We have 3 links and each link should have its own implementation for cancelling.

1. gql_htt_link
   This issue is not relevant to this link, because the network request is not a stream.
   https://github.com/gql-dart/gql/blob/master/links/gql_http_link/lib/src/link.dart#L168

2. gql_websocket_link
   Tis feature is already implemented.
   https://github.com/gql-dart/gql/blob/master/links/gql_websocket_link/lib/src/link.dart#L186

3. gal_dio_link
   There is an issue open related to this
   #180

@smkhalsa Please provide your feedback if we can close this issue, as Ferry is the main user of the links.

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I haven't been involved in the development of the link packages, so I'll defer to the others on this thread.

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