解析嵌套完成处理程序

Question

我想在for循环中使用完成处理程序。问题在于它会在完成处理程序返回之前继续运行循环，因为它是异步调用。附件是我的代码。我需要使用GCD吗？我很新（很明显）是swift/ios。任何意见将不胜感激。鲍勃

for srcTerm in sFields { //search using all search fields 

     multiQuery (searchTerm: srcTerm) { 

      if srResult.count < self.lastValue { 
       self.lastValue = srResult.count 
       self.lastSearch = srcTerm 
      } 
     } 
     // Do more stuff 
    } 


    func multiQuery (searchTerm: String, completion: @escaping ([PFObject]) ->()) { 

     var exArray = [PFObject]() 
     let query = PFQuery(className: "searchLinks") 

     do { 

      query.whereKey("searchTerms", equalTo: searchTerm) 

      query.findObjectsInBackground (block: { (objects, error)-> Void in 
       if let error = error { 
        print("Error Generated: ",error) 
        return 
       } 
       if let objects = objects { 
        // do stuff 
       } 
       completion(self.srResult) 
      }) 
     } 
    }    // end of function 

Answer 1

你可以使用DispatchGroups，这里是一个例子（摘自https://medium.com/@wilson.balderrama/how-to-use-dispatchgroup-gdc-with-swift-3-35455b9c27e7类似GCD with nested Parse Queries但更新斯威夫特3 API）：

// Just a sample function to simulate async calls 
func run(after seconds: Int, closure: @escaping() -> Void) { 
    let queue = DispatchQueue.global(qos: .background) 
    queue.asyncAfter(deadline: .now() + .seconds(seconds)) { 
     closure() 
    } 
} 

let group = DispatchGroup() 

group.enter() 
run(after: 6) { 
    print("Hello after 6 seconds") 
    group.leave() 
} 

group.enter() 
run(after: 3) { 
    print("Hello after 3 seconds") 
    group.leave() 
} 

group.enter() 
run(after: 1) { 
    print("Hello after 1 second") 
    group.leave() 
} 


group.notify(queue: DispatchQueue.global(qos: .background)) { 
    print("All async calls were run!") 
} 

使用代码：

let group = DispatchGroup() 

for srcTerm in sFields { //search using all search fields 
    group.enter() 
    multiQuery (searchTerm: srcTerm) { 
     if srResult.count < self.lastValue { 
      self.lastValue = srResult.count 
      self.lastSearch = srcTerm 
     } 
     group.leave() 
    } 
} 

group.notify(queue: DispatchQueue.global(qos: .background)) { 
    // Do something after all async calls are done 
}