iOS-你真的了解并发吗?

 @IBAction func didClickOnStart(sender: AnyObject) {            let serialQueue = dispatch_queue_create("com.appcoda.imagesQueue", DISPATCH_QUEUE_SERIAL)                  dispatch_async(serialQueue) { () -> Void in                    let img1 = Downloader .downloadImageWithURL(imageURLs[0])          dispatch_async(dispatch_get_main_queue(), {                            self.imageView1.image = img1          })                }      dispatch_async(serialQueue) { () -> Void in                    let img2 = Downloader.downloadImageWithURL(imageURLs[1])                    dispatch_async(dispatch_get_main_queue(), {                            self.imageView2.image = img2          })                }      dispatch_async(serialQueue) { () -> Void in                    let img3 = Downloader.downloadImageWithURL(imageURLs[2])                    dispatch_async(dispatch_get_main_queue(), {                            self.imageView3.image = img3          })                }      dispatch_async(serialQueue) { () -> Void in                    let img4 = Downloader.downloadImageWithURL(imageURLs[3])                    dispatch_async(dispatch_get_main_queue(), {                            self.imageView4.image = img4          })      }        }

public enum NSOperationQueuePriority : Int {     case VeryLow     case Low     case Normal     case High     case VeryHigh  }

• 3、可以取消特定的operation或者所有operation.operation添加到queue之后可以被取消， cancel() 方法被调用的时候取消已经完成。 当你取消任意一个operation，下面三个当中场景的其中一个会发生：

  • operation已经完成。这种情况cancel方法什么都不做。
  • operation正在执行.这种情况，系统不会强制停止operation，但会吧 cancelled 属性置为true
  • operation任然在等待执行。这种情况，operation将永远不会被执行。
  </li>

• 4、NSOperation有3个有用的bool属性 finished 、 cancelled 、 ready .

  • finished 在执行完成后设置为true
  • cancelled 在operation被调用 cancel() 方法后设置为true
  • ready 在operation即将执行的时候设置true
  </ul> </li>
  • 5、任何一个 NSOperation 有一个完成后被调用的block，这个block将被调用，当finished设置为true的时候
  </ul>

  现在让我们用NSOperationQueues重写刚刚那个的demo。首先声明属性，在ViewController里面:

  var queue = NSOperationQueue()

  </div>

  在 didClickOnstart() 方法里面修改代码：

   @IBAction func didClickOnStart(sender: AnyObject) {      queue = NSOperationQueue()         queue.addOperationWithBlock { () -> Void in                    let img1 = Downloader.downloadImageWithURL(imageURLs[0])             NSOperationQueue.mainQueue().addOperationWithBlock({              self.imageView1.image = img1          })      }            queue.addOperationWithBlock { () -> Void in          let img2 = Downloader.downloadImageWithURL(imageURLs[1])                    NSOperationQueue.mainQueue().addOperationWithBlock({              self.imageView2.image = img2          })         }            queue.addOperationWithBlock { () -> Void in          let img3 = Downloader.downloadImageWithURL(imageURLs[2])                    NSOperationQueue.mainQueue().addOperationWithBlock({              self.imageView3.image = img3          })         }            queue.addOperationWithBlock { () -> Void in          let img4 = Downloader.downloadImageWithURL(imageURLs[3])                    NSOperationQueue.mainQueue().addOperationWithBlock({              self.imageView4.image = img4          })         }  }

  </div>

  使用 addOperationWithBlock 创建了一个operation，很简单是不？为了在主线程执行，替代了使用GCD中调用的 dispatch_async() ,我们可以用 NSOperationQueue.mainQueue() 实现相同的功能。

  上面的例子使用 addOperationWithBlock 来添加operation在queue。让我们来看看 NSBlockOperation 是如何实现的

  @IBAction func didClickOnStart(sender: AnyObject) {            queue = NSOperationQueue()      let operation1 = NSBlockOperation(block: {          let img1 = Downloader.downloadImageWithURL(imageURLs[0])          NSOperationQueue.mainQueue().addOperationWithBlock({              self.imageView1.image = img1          })      })            operation1.completionBlock = {          print("Operation 1 completed")      }      queue.addOperation(operation1)            let operation2 = NSBlockOperation(block: {          let img2 = Downloader.downloadImageWithURL(imageURLs[1])          NSOperationQueue.mainQueue().addOperationWithBlock({              self.imageView2.image = img2          })      })            operation2.completionBlock = {          print("Operation 2 completed")      }      queue.addOperation(operation2)                  let operation3 = NSBlockOperation(block: {          let img3 = Downloader.downloadImageWithURL(imageURLs[2])          NSOperationQueue.mainQueue().addOperationWithBlock({              self.imageView3.image = img3          })      })            operation3.completionBlock = {          print("Operation 3 completed")      }      queue.addOperation(operation3)            let operation4 = NSBlockOperation(block: {          let img4 = Downloader.downloadImageWithURL(imageURLs[3])          NSOperationQueue.mainQueue().addOperationWithBlock({              self.imageView4.image = img4          })      })            operation4.completionBlock = {          print("Operation 4 completed")      }      queue.addOperation(operation4)  }

  </div>

  创建了一个 NSBlockOperation 实例把任务封装到block。 使用 NSBlockOperation ，允许设置完成的回调(completion handler). Operation完成后，回调会被调用。运行demo后，控制台上输出:

  Operation 1 completed  Operation 3 completed  Operation 2 completed  Operation 4 completed

  </div>

  取消Operations

  如上面提到的， NSBlockOperation 允许我们管理operations。现在我们创建一个cancel按钮放到导航栏,实现cancel这个事件。 为了说明cancel operation,添加依赖在 #2 和 #1 之间, 然后在添加 #3 和 #2 之间的依赖。 这就说明 #2 会在 #1 完成时候开始， #3 会在 #2 完成后开始。 #4 没有依赖会并发工作。

  取消所有operation的方法是 cancelAllOperations() ,在ViewController类里面添加。

  @IBAction func didClickOnCancel(sender: AnyObject) {                  self.queue.cancelAllOperations()     }

  </div>

  修改后的 didClickOnStart() 代码是

  @IBAction func didClickOnStart(sender: AnyObject) {        queue = NSOperationQueue()                let operation1 = NSBlockOperation(block: {            let img1 = Downloader.downloadImageWithURL(imageURLs[0])            NSOperationQueue.mainQueue().addOperationWithBlock({                self.imageView1.image = img1            })        })                operation1.completionBlock = {            print("Operation 1 completed, cancelled:\(operation1.cancelled)")        }        queue.addOperation(operation1)                let operation2 = NSBlockOperation(block: {            let img2 = Downloader.downloadImageWithURL(imageURLs[1])            NSOperationQueue.mainQueue().addOperationWithBlock({                self.imageView2.image = img2            })        })        operation2.addDependency(operation1)        operation2.completionBlock = {            print("Operation 2 completed, cancelled:\(operation2.cancelled)")        }        queue.addOperation(operation2)                        let operation3 = NSBlockOperation(block: {            let img3 = Downloader.downloadImageWithURL(imageURLs[2])            NSOperationQueue.mainQueue().addOperationWithBlock({                self.imageView3.image = img3            })        })        operation3.addDependency(operation2)          operation3.completionBlock = {            print("Operation 3 completed, cancelled:\(operation3.cancelled)")        }        queue.addOperation(operation3)                let operation4 = NSBlockOperation(block: {            let img4 = Downloader.downloadImageWithURL(imageURLs[3])            NSOperationQueue.mainQueue().addOperationWithBlock({                self.imageView4.image = img4            })        })                operation4.completionBlock = {            print("Operation 4 completed, cancelled:\(operation4.cancelled)")        }        queue.addOperation(operation4)            }

  </div>

  执行的结果：

  Operation 3 completed, cancelled:true  Operation 2 completed, cancelled:true  Operation 1 completed, cancelled:true  Operation 4 completed, cancelled:true

  </div>

  按下start按钮后点击取消按钮. 这些operation将被取消在operation #1 完成之后。

  在这里发生了什么？

  • 1、当 #1 已经执行，cancel不执行任何事情。第一张图片任然显示

  • 2、点击cancel按钮足够快， #2 被取消. cancelAllOperations() 阻止 #2 执行，所以image2没有显示

  • 3、 #3 依赖 #2 完成，因为 #2 取消了，所以 #2 也不能被执行

  • 4、 #4 没有依赖，并发执行，所以下载了图片.

  

  总结

  1、了解并发的概念、解释了GCD、创建串行和并发队列

  2、检验了NSOperationQueues的执行顺序

  3、学习了NSOperationQueues和GCD的优缺点

  参考

  完整的 Demo

  深入了解: https://developer.apple.com/library/ios/documentation/General/Conceptual/ConcurrencyProgrammingGuide/Introduction/Introduction.html

  </div>

  来自： http://www.liuchendi.com/2016/01/05/iOS/29_GCD_Operation/

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