MVVM：使用闭包，无主或弱自我将View与ViewModel绑定？

Question

I'm implementing a simple Master-Detail app where the Master viewController manages a table view which shows the results of a call to a REST service. The Detail viewController manages a view where I show more information about the item selected in the Master . Common scenario.

I'm trying to apply MVVM pattern. In the Master viewController , I create and initialize its viewModel this way:

lazy private var viewModel: ListViewModel = {
    return ListViewModel()
}()

override func viewDidLoad() {
    super.viewDidLoad()

    initViewModel()
}

private func initViewModel() {
    viewModel.onModelChange = { [weak self] () in
        DispatchQueue.main.async {
            self?.tableView.reloadData()
        }
    }

    viewModel.fetchData()
}

My question is: in the closure provided to the viewModel , should self be weak or unowned instead? I found an example implementing an scenario similar to mine that was setting it to weak , and another one setting it to unowned , so I'm not completely clear.

Answer 1

[unowned self] . This tells your model not to hold a strong reference to ViewController

Apple document states it clearly that:

“Like weak references, an unowned reference does not keep a strong hold on the instance it refers to. Unlike a weak reference, however, an unowned reference is assumed to always have a value”.

In your case , there is always be ViewController . So benefit of unowned reference is it's nonoptional . No unwrap required each time it is used

Answer 2

The difference between unowned and weak is that weak is declared as an Optional while unowned isn't. If you know that self will not be nil use unowned , if you don't know: Use weak

Answer 3

Unowned is used when you 100% sure the object won't be deallocated.

weak you then need to take care of its reference count issues.

viewModel.onModelChange = { [weak self] () in
    guard let strongSelf = self else { return }
    strongSelf.tableView.reloadData()
}

I generally do it like this. You then hold a strong reference of self to avoid it being allocated during the block is running.