How can synchronous tasks be canceled in iOS?

There is no way to cancel a synchronous task that has already started execution without interrupting the thread.

To simulate cancellation of a synchronous task, the following approaches can be used:

1. Regular check of a cancellation flag: Add logic inside the synchronous task to check a boolean flag that is set externally when cancellation is needed. If the flag is set, the task exits early.

   var isCancelled = false
   
   func performSynchronousTask() {
       // Start of task
       for i in 0..<100000 {
           if isCancelled {
               print("Task cancelled")
               return // Early exit
           }
           // Perform part of the work
           // ... long computation or operation
       }
       print("Task completed")
   }
   
   func cancelTask() {
       isCancelled = true
   }
   
   // Usage example
   // performSynchronousTask() // Synchronous call
   // cancelTask() // Call from another thread to cancel
   

2. Using try? Task.checkCancellation() in an async context: If the synchronous task is running inside a Task in Swift Concurrency, you can periodically call try? Task.checkCancellation(). If the task is canceled, this call throws a CancellationError.

   func performCancellableSynchronousWork() throws {
       for i in 0..<100000 {
           try Task.checkCancellation() // Check for cancellation
           // Perform part of the work
           // ...
       }
   }
   
   // Usage example in an async context
   /*
   let task = Task {
       do {
           try performCancellableSynchronousWork()
           print("Task completed")
       } catch is CancellationError {
           print("Task canceled")
       } catch {
           print("Error occurred: \(error)")
       }
   }
   
   // To cancel the task
   // task.cancel()
    */