Higher-level support for projections

[johanhaleby]

Maybe allow creating a projection in this style that also creates a subscription under the hood?

function IsUsernameClaimedProjection(username) {
  return createProjection({
    initialState: false,
    handlers: {
      AccountRegistered: (state, event) => true,
      AccountClosed: (state, event) => false,
      UsernameChanged: (state, event) => event.data.newUsername === username,
    },
    tagFilter: [`username:${username}`],
  })
}