How to stop execution of a running background thread from main thread on swift while using DispatchQueue

Question

 DispatchQueue.global(qos: .background).async {
    //This will run on the background queue
    self.writeValue(tag: GlobalData.WRITE_DATA, data: getDataForWrite(1) )
    self.writeValue(tag: GlobalData.WRITE_DATA, data: getDataForWrite(2) )
    self.writeValue(tag: GlobalData.WRITE_DATA, data: getDataForWrite(3) )
    self.writeValue(tag: GlobalData.WRITE_DATA, data: getDataForWrite(4) )
    self.writeValue(tag: GlobalData.WRITE_DATA, data: getDataForWrite(5) )
    // .....
    DispatchQueue.main.async {
        //This will run on the main queue, after the previous code in outer block
        print("done writing data")
    }
}

I need to stop executing this thread from the main thread. How is it possible

Solution 1

I think the best solution is to execute DispatchWorkItem in async:

DispatchWorkItem encapsulates work that can be performed. A work item can be dispatched onto a DispatchQueue and within a DispatchGroup

so at the end your code might be:

let workItem = DispatchWorkItem {
   //.... writing stuff in background ....

   DispatchQueue.main.async {
      //.... done writing stuff, updating ui ....
   }
}
DispatchQueue.global().async(execute: workItem)

when you need to stop the execution just call .cancel():

//.... but, if stuff goes wrong ....
DispatchQueue.main.async {
   workItem.cancel()
}

Solution 2

You can use a BlockOperation

let blockOperation = BlockOperation {
    self.writeValue(tag: GlobalData.WRITE_DATA, data: getDataForWrite(1) )
    self.writeValue(tag: GlobalData.WRITE_DATA, data: getDataForWrite(2) )
    self.writeValue(tag: GlobalData.WRITE_DATA, data: getDataForWrite(3) )
    self.writeValue(tag: GlobalData.WRITE_DATA, data: getDataForWrite(4) )
    self.writeValue(tag: GlobalData.WRITE_DATA, data: getDataForWrite(5) )
    //...
}

let queue = OperationQueue()
queue.addOperation(blockOperation)

And at some point in time from your main thread you can cancel the operation:

blockOperation.cancel()

More info on BlockOperation

More info on OperationQueue

Comments

[Shyam]

What if I want to run the above operation in background queue?

[Kai]

Please bear in mind that cancelling a work item will not cancel one that is already executing - developer.apple.com/documentation/dispatch/dispatchworkitem/‌​…