To add real-time functionality, we need to implement the subscriptions defined in the schema as well. We defined 2 subscriptions clients can use as well as specific events for each of them:
type PostEvent {
userId: ID!
post: Post!
}
type CommentEvent {
postUserId: ID!
postDate: AWSDateTime!
comment: Comment!
}
type Subscription {
post(userId: ID!): PostEvent
@aws_subscribe(mutations: ["notifyPost"])
comment(postUserId: ID!, postDateStarting: AWSDateTime!):
CommentEvent
@aws_subscribe(mutations: ["notifyComment"])
}Let's start with the simpler one: get new posts for a user. The PostEvent has the userId as a top-level field that allows filtering as we've discussed in the Event filtering chapter. And since we only need equality checking, the simpler arguments-based event matching is enough in this case.
The notifyPost mutation that triggers this subscription needs only the Post ID and it fetches everything else:
type Mutation {
notifyPost(postId: ID!): PostEvent!
}This is enough only because a Post can not be deleted in our example. We've covered a case when it is be needed in the removeTodo mutation chapter. While that provides a more robust implementation, the simpler way is enough for this example project.
In the previous chapter we've discussed how to call the notifyPost mutation. Now, let's see the resolver behind it!
The resolver gets the Post by ID and returns the structure defined in the type:
import {util} from "@aws-appsync/utils";
export function request(ctx) {
return {
version : "2018-05-29",
operation : "GetItem",
key : {
id: {S: ctx.args.postId}
},
};
}
export function response(ctx) {
if (ctx.error) {
return util.error(ctx.error.message, ctx.error.type);
}
return {
userId: ctx.result.userId,
post: ctx.result,
}
}Implementation here.