[Concept] Accumulator

[muukii]

Compared to sink method, the Accumulator retains the selected value for itself not using Changes object.

sink method often drops the previous value due to receiving unordered events that happen by committing concurrently.
it's safe, but it's not happy for UI components that update themselves only when the value is updated.

Furthermore, the strategy of using the Changes structure may gain the cost of copying the state struct itself by growing state complexity. Verge might stop using this strategy. Each branch retaining the value from the root state should be affordable rather than that.

    let sub = store.accumulate { [weak self] in

     // here is escaping closure
      
      // subscribe count
      $0.ifChanged(\.count).do { value in
        // escaping closure but will be released after it runs.
        let _ = self
        exp.fulfill()
      }
      // subscribe name
      $0.ifChanged(\.name).do { value in
          // escaping closure but will be released after it runs.
        let _ = self

      }

    }

• For MainQueue
• For background queue