Computed Values

Computed values are derived from current state and read from Snapshot.

Use computed for values that can be recalculated from state: counts, booleans, labels, totals, and UI-ready summaries.

Declare Computed Values

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

  state {
    todos: Array<Todo> = []
  }

  computed todoCount = len(todos)
  computed completedCount = len(filter(todos, $item.completed))
  computed activeCount = todoCount - completedCount
  computed hasCompleted = completedCount > 0
}

Computed values are not separate storage. They describe how to derive a value from the current domain state.

Read Them in TypeScript

const snapshot = app.snapshot();

console.log(snapshot.computed["todoCount"]);
console.log(snapshot.computed["activeCount"]);

Use Them in the Domain

action clearCompleted() available when hasCompleted {
  onceIntent {
    patch todos = filter(todos, !$item.completed)
  }
}

Common Mistake

Do not patch a computed value. If the value must be stored, put it in state. If it can be derived, keep it in computed.

Next

Learn how callers request transitions in Actions.