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如何使循环等待任务完成

dispatch asynchronous loops swift ios

Josch Hazard · 技术社区 · 7 年前

我知道这个话题已经有很多贡献了。我尝试了不同的变体 DispatchGroup

let names = ["peter", "susan", "john", "peter", "susan", "john"]
var holding = [String: [Double]]()


for i in 0...10 {

    for name in names {

        if holding[name] == nil {
            Alamofire.request("https://jsonplaceholder.typicode.com", parameters: parameters).responseJSON { responseData in

                    // do stuff here
                    holding[name] = result
            }

        } else {
            // do other stuff with existing "holding[name]"

        }

        // if if holding[name] == nil, the whole process should wait
    }
}

如果Im使用DispatchGroup,Alamofire请求将逐个执行,但整个循环无法识别 holding[name] 控股[名称] 总是 nil 因为循环不会等待。

编辑:

根据Mikess和VS answers,我尝试了以下方法:

var names = ["peter", "susan", "john", "peter", "susan", "john"]
var holding = [String: [Double]]()

let semaphore = DispatchSemaphore(value: 1)

for i in 0...10 {

    DispatchQueue.global().async { [unowned self] in
        self.semaphore.wait()

        for name in names {

            if holding[name] != nil {
                Alamofire.request("https://jsonplaceholder.typicode.com", parameters: parameters).responseJSON { responseData in

                    // do stuff here
                    holding[name] = result
                    semaphore.signal()
                }

            } else {
                // do other stuff with existing "holding[name]"
                semaphore.signal()

            }

            // if if holding[name] != nil, the wholeprocess should wait
        }
    }
}

但不幸的是,应用程序崩溃了。我做错了什么?

2 回复 | 直到 7 年前

Prashant Tukadiya 7 年前

你有两个选择

1 ) Semaphore

2 ) Operation Queues

Semaphores

你需要谨慎对待 semaphore.signal() semaphore.wait() 一对

信号量 可能会阻塞 主螺纹 Dispatch.global.async

即

semaphore.wait()

Alamofire.request("https://jsonplaceholder.typicode.com", parameters: parameters).responseJSON { responseData in

                     semaphore.signal()
                    holding[name] = result

 }

给你,封锁

问题是完成处理程序在主线程中执行,而主线程已经被信号量锁定。最初调用wait()。所以当完成时,信号量。信号()从未被调用

 DispatchQueue.global().async { [unowned self] in
        self.semaphore.wait()
        // And other statemetns 

    }

Versus 7 年前

我认为可以使用DispatchSemaphore来停止循环:

let semaphore = DispatchSemaphore(value: 1)
for i in 0...10 {

    for name in names {
        // if signal is 0, loop will be stopped forever,  
        // if signal > 0, loop will continue 
        semaphore.wait()
        if holding[name] == nil {
        Alamofire.request("https://jsonplaceholder.typicode.com", parameters: parameters).responseJSON { responseData in

                // do stuff here
                // release signal to continue current loop(signal + 1)
                semaphore.signal()
                holding[name] = result
            }

        } else {
        // do other stuff with existing "holding[name]"

        }

    // if if holding[name] == nil, the whole process should wait
    }
}