SwiftUI Global Queue Runs on Thread 1

Question

I am trying to sort a very large array in my mobile app. I am using SwiftUI and would like the array to be sorted once the user taps on a button. This sorting can take about 3 seconds, so I would like this to happen off the main queue. However even when calling the code to happen on a global async block it still seems to happen on thread 1 and slow down my UI. During this time CPU spikes to 100% and the UI is unresponsive.

Button("Sort List") {
    DispatchQueue.global(qos: .background).async {
        self.updateList()
    }
}

...

func updateList() {
    //have tried putting the queue here as well
    var sorted: [OBJ] = []
    for obj in self.allObjs {
       for i in 0..<self.selected.count {
           ...
           let i = sorted.insertionIndexOf(obj, isOrderedBefore: self.sorter)
           sorted.insert(obj, at: i)
        }
     }
     self.someStateVar = sorted
}

This seems correct to me, so something else (not shown in this code) might be happening — New Dev, Jun 08 '20 at 23:48

Asperi · Answer 1 · 2020-06-10T04:14:11.003

Try the below explicit redirection to main queue.

Note: more provided code would allow to perform better analyses

func updateList() {
    //have tried putting the queue here as well
    var sorted: [OBJ] = []
    for obj in self.allObjs {
       for i in 0..<self.selected.count {
           ...
           let i = sorted.insertionIndexOf(obj, isOrderedBefore: self.sorter)
           sorted.insert(obj, at: i)
        }
     }
     DispatchQueue.main.async {
        self.someStateVar = sorted   // update on main queue
     }
}

user3441734 · Answer 2 · 2020-06-09T18:49:51.410

Are you sure this part of code does what you would like to do?

var sorted: [OBJ] = []

for obj in self.allObjs {
   for i in 0..<self.selected.count {
       ...
       let i = sorted.insertionIndexOf(obj, isOrderedBefore: self.sorter)
       sorted.insert(obj, at: i)
    }
 }

For me, it looks a little bit strange :-)

What type is OBJ?
Is selected part of some model or state? Is it related to UI?
Is sorted an empty array?
insertionIndexOf(obj, isOrderedBefore: self.sorter)? Where is this function from? Is it part of a custom sorting algorithm?
Do you really would like to redefine i? If yes, where i (as a parameter in your loop) is used and for what?

I know this is not an answer but a lot of questions :-). If you need an answer, rewrite your question such a way, we can run and play within our environment.

SwiftUI Global Queue Runs on Thread 1

2 Answers2