UI not updating (in Swift) during intensive function on main thread-CodePudding

I wondered if anyone could provide advice on how I can ‘force’ the UI to update during a particularly intensive function (on the main thread) in Swift.

To explain: I am trying to add an ‘import’ feature to my app, which would allow a user to import items from a backup file (could be anything from 1 - 1,000,000 records, say, depending on the size of their backup) which get saved to the app’s CodeData database. This function uses a ‘for in’ loop (to cycle through each record in the backup file), and with each ‘for’ in that loop, the function sends a message to a delegate (a ViewController) to update its UIProgressBar with the progress so the user can see the live progress on the screen. I would normally try to send this intensive function to a background thread, and separately update the UI on the main thread… but this isn't an option because creating those items in the CoreData context has to be done on the main thread (according to Swift’s errors/crashes when I initially tried to do it on a background thread), and I think this therefore is causing the UI to ‘freeze’ and not update live on screen.

A simplified version of the code would be:

class CoreDataManager {
    
    var delegate: ProgressProtocol?
    
    // (dummy) backup file array for purpose of this example, which could contain 100,000's of items
    let backUp = [BackUpItem]()
    
    // intensive function containing 'for in' loop
    func processBackUpAndSaveData() {
        
        let totalItems: Float = Float(backUp.count)
        var step: Float = 0
        
        for backUpItem in backUp {

            // calculate Progress and tell delegate to update the UIProgressView
            step  = 1
            let calculatedProgress = step / totalItems
            delegate?.updateProgressBar(progress: calculatedProgress)
            
            // Create the item in CoreData context (which must be done on main thread)
            let savedItem = (context: context)
        }
        // loop is complete, so save the CoreData context
        try! context.save()
    }
}


// Meanwhile... in the delegate (ViewController) which updates the UIProgressView
class ViewController: UIViewController, ProgressProtocol  {
    
    let progressBar = UIProgressView()
    
    // Delegate function which updates the progress bar
    func updateProgressBar(progress: Float) {

        // Print statement, which shows up correctly in the console during the intensive task
        print("Progress being updated to \(progress)")

        // Update to the progressBar is instructed, but isn't reflected on the simulator
        progressBar.setProgress(progress, animated: false)
    }
}

One important thing to note: the print statement in the above code runs fine / as expected, i.e. throughout the long ‘for in’ loop (which could take a minute or two), the console continuously shows all the print statements (showing the increasing progress values), so I know that the delegate ‘updateProgressBar’ function is definitely firing correctly, but the Progress Bar on the screen itself simply isn’t updating / doesn’t change… and I’m assuming it’s because the UI is frozen and hasn’t got ‘time’ (for want of a better word) to reflect the updated progress given the intensity of the main function running.

I am relatively new to coding, so apologies in advance if I ask for clarification on any responses as much of this is new to me. In case it is relevant, I am using Storyboards (as opposed to SwiftUI).

Just really looking for any advice / tips on whether there are any (relatively easy) routes to resolve this and essentially 'force' the UI to update during this intensive task.

CodePudding user response：

You say "...Just really looking for any advice / tips on whether there are any (relatively easy) routes to resolve this and essentially 'force' the UI to update during this intensive task."

No. If you do time-consuming work synchronously on the main thread, you block the main thread, and UI updates will not take effect until your code returns.

You need to figure out how to run your code on a background thread. I haven't worked with CoreData in quite a while. I know it's possible to do CoreData queries on a background thread, but I no longer remember the details. That's what you're going to need to do.

As to your comment about print statements, that makes sense. The Xcode console is separate from your app's run loop, and is able to display output even if your code doesn't return. The app UI can't do that however.