ios - 如何在 Swift4 中返回受线程影响的值

Question

 func getAll(completion: (([Foods]?, Error?) -> Void)?){      
        var foodWithEname: [Foods]?
        let food: Food? = Food()
        var sortedFood: [Foods]?
        food?.getFoods(){ (foodsArray, error) in
            sortedFood = foodsArray?.sorted{ (left, right) in   
                if let lh = left.hits  {  
                    if let rh = right.hits {
                        return left.hits! > right.hits!
                    }
                    return true
                } else {
                    if let rh = right.hits {
                        return false
                    }
                    return false
                }
            }


            for fa in foodsArray! {

                var f = Foods(fid: fa.fid, fname: fa.fname, hits: fa.hits, addr: fa.addr, ename: nil)

                food?.getEffectsById(fa.fid){ (effectsArray, error) in
                    //foodWithEname?.ename = effectsArray
                    print ("11111")

                    DispatchQueue.global().async {
                        f.ename = effectsArray
                        print ("2222")
                        foodWithEname?.append(f)
                    }
                }

                DispatchQueue.global().async (execute: {
                    print ("00000")

                })

            }
            print("3333")
            completion?(foodWithEname, nil)
        }
        print ("4444")
    }

我需要向主视图返回 foodWithEname 值，但由于线程流，它没有返回值...

我不知道如何控制闭包流。

我在 MainView 中调用了函数:

let food: Food? = Food()
food?.getAll(){ (foodsArray, error) in

        }

结果:444433330000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000111112222111112222111112222111112222111112222111112222111112222111112222111112222111112222111112222111112222111112222111112222111112222111112222111112222111112222111112222111112222111112222111112222111112222111112222111112222111112222

我想要如下结果:

2222 00000 11111 2222 00000 11111 2222 00000 ... 3333

执行 completion?(foodWithEname, nil) 时，数组值应该在 foodWithEname 中。

请帮忙解决这个问题，谢谢!

Answer 1

在您的第二个循环中，您在一个循环中发送多个异步调用，在等待结果的同时您调用了完成处理程序，因此结果为空。

您需要在调度组中调用这些异步调用，并在所有执行完成后将结果返回给完成处理程序。

在第二个循环中做这样的事情:

// Create dispatch group
let group = DispatchGroup()

for fa in foodsArray! {

    var f = Foods(fid: fa.fid, fname: fa.fname, hits: fa.hits, addr: fa.addr, ename: nil)

    // Enter in group just before the call
    group.enter()

    food?.getEffectsById(fa.fid){ (effectsArray, error) in
        //foodWithEname?.ename = effectsArray
        print ("11111")

        DispatchQueue.global().async {
            f.ename = effectsArray
            print ("2222")

            // Leave the group when task is completed
            group.leave()

            foodWithEname?.append(f)
        }
    }

    DispatchQueue.global().async (execute: {
        print ("00000")

    })
}
print("3333")

// Wait for all tasks to complete, and call the completion handler
group.notify(queue: DispatchQueue.main) {
    completion?(foodWithEname, nil)
}