如果通过 Rx 数据源绑定在屏幕外加载数据，则导航栏大标题会缩小

Question

When using large titles in the navigation bar along with a UITableView that is bound to an Rx driver data source I've noticed that if the binding and initial data load happens while the view is offscreen, when you navigate to that view it will be scrolled such that the large title is shrunk to the “minimized” position.

Overall setup is a UITableViewController with prefersLargeTitles = true set. Tableview is setup and subsequently bound to the Rx datasource within viewDidLoad .

Example code:

override func viewDidLoad() {
  super.viewDidLoad()
  setupTableView()
  bindToTableView()

  // ...
}

private func setupTableView() {
  tableView.register(cellType: Cell.self)
  tableView.tableFooterView = UIView()
  tableView.separatorStyle = .none

  // ...

  // We are required to first reset the data source and delegate to allow
  // for RxCocoa to take over control.
  tableView.dataSource = nil
  tableView.delegate = nil
  tableView.rx.setDelegate(self)
    .disposed(by: bag)

  // ...
}

private func bindToModel() {
  viewModel.modelDriver
    .drive(tableView.rx.items) { tableView, row, model in
      let indexPath = IndexPath(row: row, section: 0)
      let cell: Cell = tableView.dequeueReusableCell(for: indexPath)
      cell.prepare(with: model)
      return cell
    }.disposed(by: bag)
}

And by "shrunk" I mean the titles switch to this style:

Has anyone else encountered this issue?

---

Solved: As @daniel-t mentions below the problem is not caused by Rx specifically, but by the timing of when prefersLargeTitles = true . If this property is not set before the call to tableView.reloadData() occurs then the table will load in the data and scroll appropriately for non-large titles . Then after large titles is set the tableview doesn't reset it's scroll position to compensate for the new, larger navigation bar area.

The reason this manifests somewhat weirdly, even when using something like .skipUntil(...viewWillAppear) , is because the act of binding triggers an initial Rx update which reloads the tableview.

Answer 1

Your problem is elsewhere, not in the code you presented or anything to do with Rx. The following works as intended:

class ViewController: UITableViewController {

    private let bag = DisposeBag()

    override func viewDidLoad() {
        super.viewDidLoad()

        title = "Title"
        navigationController?.navigationBar.prefersLargeTitles = true

        tableView.register(UITableViewCell.self, forCellReuseIdentifier: "Cell")
        tableView.tableFooterView = UIView()
        tableView.separatorStyle = .none
        tableView.dataSource = nil
        tableView.delegate = nil
        tableView.rx.setDelegate(self)
            .disposed(by: bag)

        let modelDriver = Driver.just(Array<String>(repeating: "Hello world", count: 30))
        modelDriver
            .drive(tableView.rx.items) { tableView, row, model in
                let indexPath = IndexPath(row: row, section: 0)
                let cell: UITableViewCell = tableView.dequeueReusableCell(withIdentifier: "Cell", for: indexPath)
                cell.textLabel?.text = model
                return cell
            }
            .disposed(by: bag)
    }
}

Maybe the problem has to do with how you are constructing your cells? Or maybe you are scrolling to the bottom of the list somewhere?