Checking the to-do item

When the user of the app taps the Done button, our app has to tell the to-do item store to change the item's status to Done. Follow these steps to implement that feature:

1. Add the following test method to ToDoItemDetailsViewControllerTests:

   // ToDoItemDetailsViewControllerTest.swift
   func test_sendingButtonAction_shouldCheckItem() {
     let toDoItem = ToDoItem(title: "dummy title")
     sut.toDoItem = toDoItem
     let storeMock = ToDoItemStoreProtocolMock()
     sut.toDoItemStore = storeMock
   }

ToDoItemDetailsViewController doesn't have a property for toDoItemStore. This means we have to pause writing this test and add this property first.

2. Go to ToDoItemDetailsViewController and add the toDoItemStore property:

   // ToDoItemDetailsViewController.swift
   var toDoItemStore: ToDoItemStoreProtocol?

3. Now we can finish the test:

   // ToDoItemDetailsViewControllerTests.swift
   func test_sendingButtonAction_shouldCheckItem...