State

State is the source-of-truth domain data stored in Snapshot.

Use state for values that must survive from one transition to the next: records, selected ids, form fields, status flags, and domain collections.

Declare State in MEL

domain TodoApp {
  type Todo = {
    id: string,
    title: string,
    completed: boolean
  }

  state {
    todos: Array<Todo> = []
    filterMode: "all" | "active" | "completed" = "all"
    selectedTodoId: string | null = null
  }
}

Read State in TypeScript

const snapshot = app.snapshot();

console.log(snapshot.state.todos);
console.log(snapshot.state.filterMode);
console.log(snapshot.state.selectedTodoId);

Application code reads state through snapshot.state.

Change State with Actions

action setFilter(nextFilter: "all" | "active" | "completed")
  dispatchable when filterMode != nextFilter {
  onceIntent {
    patch filterMode = nextFilter
  }
}

State changes happen through patches declared by the domain, or through patches returned by effect handlers.

Common Mistake

Do not mutate snapshot.state in UI, server, or agent code. A snapshot is the read result. To request change, submit an action.

Next

Move derived labels, totals, and flags into Computed Values.