后台线程 - 两个网络调用[重复]

Question

我在 completeOnboarding 方法中有两个方法，它们都有网络操作，应该在后台线程中完成，如下所示。但是，我想知道我是否在做为什么首先调用completion(true)，我该如何处理这个问题？

DispatchQueue.global(qos: .background).async {
    self?.completeOnboarding( completion: { (success) in
      DispatchQueue.main.async {
        if success {
         print("success")
        } else {
         print("failed")
        }
     }
 })

func completeOnboarding(completion: @escaping(Bool) -> Void){

   // has network post operation
    classRegistration() {(success) in
      if !success {
        completion(false)
        return
      }
    }

    // has network post operation
    classLocation() { (success) in
      if !success {
        completion(false)
        return
      }
    }
  completion(true)
}

Answer 1

最终完成(true) 不等待 classLocation() 和 classRegistration() 调用。如果您有多个网络调用并且您想等待所有调用完成，您可以（一种方法）将它们添加到 DispatchGroup 并等待该调用完成：

func dispatchAndWait(completion: @escaping () -> Void) {
    func networkOne(completion: @escaping (_ success: Bool) -> Void) {
        print("[DEBUG] Enter \(#function)")

        DispatchQueue.main.asyncAfter(deadline: .now() + Double.random(in: 0...3)) {
            completion(Bool.random())
        }
        print("[DEBUG] Return \(#function)")
        
    }
    
    func networkTwo(completion: @escaping (_ success: Bool) -> Void) {
        print("[DEBUG] Enter \(#function)")
        
        DispatchQueue.main.asyncAfter(deadline: .now() + Double.random(in: 0...3)) {
            completion(Bool.random())
        }
        print("[DEBUG] Return \(#function)")
    }
    
    // Create a DispatchGroup and add both calls
    let dispatchGroup = DispatchGroup()
    // Enter first network call
    dispatchGroup.enter()
    networkOne { success in
        print("[DEBUG] Complete networkOne with success: \(success)")
        // Exit first network call
        dispatchGroup.leave()
    }
    
    // Enter second network call
    dispatchGroup.enter()
    networkTwo { success in
        print("[DEBUG] Complete networkTwo with success: \(success)")
        // Exit second network call
        dispatchGroup.leave()
    }
    
    // notify gets called when all tasks have exited
    dispatchGroup.notify(queue: DispatchQueue.main) {
        completion()
    }
}

还有一点建议：

classRegistration() {(success) in
  if !success {
    completion(false)
    return
  }
}

在success==true的情况下永远不会完成，你应该确保在每条路径上都调用完成

classRegistration() {(success) in
  // ... do whatever needs to be done here
  completion(success)
}

Answer 2

1. 假设 classRegistration 需要成功才能开始 classLocation --

又快又脏 --

func completeOnboarding(completion: @escaping(Bool) -> Void){

   // has network post operation
    classRegistration() {(success) in
      if success {
        
        // has network post operation
        classLocation() { (success) in
            completion(success)
        }

      } else {
        completion(false)
      }
    }
}

正确的方式（其他包括 - NSOperations with dependency, Dispatch group）

func completeOnboarding(completion: @escaping(Bool) -> Void){
   
   let serialQueue = DispatchQueue(label: "classname.serial")

   var proceedWithSuccess = true

   serialQueue.async {
       
       serialQueue.suspend() //run 1 operation at a time
   
       classRegistration() {(success) in
            proceedWithSuccess = success
            serialQueue.resume() //let next operation run
       }
   }
   
   serialQueue.async {
    
    guard proceedWithSuccess else { return } 

      serialQueue.suspend()

          classLocation() { (success) in

           proceedWithSuccess = success
           serialQueue.resume()
       }
   }
          
   serialQueue.async {
      completion(proceedWithSuccess)
   }
}

1. 如果您希望 classLocation() 即使注册失败也能触发 - 只需去掉上面的保护语句即可。

如果由我决定，我会为异步操作使用自定义 NSOperation 子类并明确提及操作之间的依赖关系，但它需要大量样板代码（也许稍后需要研究）；不过，在这种情况下，串行队列（或来自其他答案的调度组）对您来说应该足够了。