TODO-App-Reducer.swift

func reduce(state: State, event: Event) -> State {
    switch event {
    case .addTodo(let todo):
        var newState = state
        newState.todos = state.todos + [todo]
        return newState
    case .toggleTodo(let todo):
        var newState = state
        newState.todos = state.todos.map { t in
            var newTodo = t
            if(todo.text == t.text) {
                newTodo.completed = !t.completed
            }
            return newTodo
        }
        return newState
    case .setVisibilityFilter(let filter):
        var newState = state
        newState.visibilityFilter = filter
        return newState
    }
}