如何在使用 DispatchQueue 时从 swift 上的主线程停止执行 运行 后台线程

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")
    }
}

我需要停止从主线程执行这个线程。怎么可能

Answer 1

我认为最好的解决方案是在async中执行DispatchWorkItem:

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

所以最后你的代码可能是：

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

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

当您需要停止执行时，只需调用 .cancel():

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

Answer 2

您可以使用 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)

并且在某个时间点您可以从您的主线程取消操作：

blockOperation.cancel()

有关 BlockOperation

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有关 OperationQueue

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