iosasync-awaituirefreshcontrolswift5

Swift 5.5: Async @objc didPullToRefresh selector crashes app with error EXC_BAD_ACCESS


I have a table to which I've added a refreshControl and when I pull down to refresh the content, I reset the array that feeds the table with data and then immediately request new data through an API call.

Until now, I have used completion handlers and protocols to get the data into the table view but I want to move the logic to async/await because of the complexity needed by the network calls and the pyramid of nested closures.

Populating the view in viewDidLoad works fine but with pullToRefresh selector I get an error:

Thread 1: EXC_BAD_ACCESS (code=1, address=0xbcf917df8160)

Implementation:

override func viewDidLoad() {
    super.viewDidLoad()
    setupView()
    setupTableView()
    setupTableRefreshControl()
    Task {
      await getBalances() //async network call
      myTable.reloadData()
    }
  } 
func setupTableRefreshControl() {
    myTable.refreshControl = UIRefreshControl()
    myTable.refreshControl?.addTarget(self, action: #selector(didPullToRefresh), for: .valueChanged)
  }

Code that crashes app:

   @objc func didPullToRefresh() async {
    balance.reset() // reset array to []
    Task {
      await getBalances() //async network call
      myTable.reloadData()
    }
  }

Solution

  • At the time of writing, @objc selectors and async methods don't play well together and can result in runtime crashes, instead of an error at compile-time.

    Here's a sample of how easy it is to inadvertently replicate this issue while converting our code to async/await: we mark the following method as async

    @objc func myFunction() async {
        //...
    

    not noticing that it is also marked as @objc and used as a selector

    NotificationCenter.default.addObserver(
        self,
        selector: #selector(myFunction),
        name: "myNotification",
        object: nil
    )
    

    while somewhere else, a notification is posted

    NotificationCenter.default.post(name: "myNotification", object: nil)
    

    Boom 💥 EXC_BAD_ACCESS

    Instead, we should provide a wrapper selector for our brand new async method

    @objc
    func myFunctionSelector() {
        Task {
            await myFunction()
        }
    }
    
    func myFunction() async { 
        //... 
    

    and use it for the selector

    NotificationCenter.default.addObserver(
        self,
        selector: #selector(myFunctionSelector),
        name: "myNotification",
        object: nil
    )