我可以不在循环中使用调度组吗? [英] Can I use dispatch group not in a loop?

问题描述

说在调用某些函数之前，我要完成3个不同的异步任务.我知道如果这些任务处于循环状态，可以使用调度组来执行此类操作，

Say I have 3 different asynchronous tasks I want to get done before calling some function. I'm aware of using dispatch groups to do such a thing if those tasks were in a loop,

    var dispatchGroup = DispatchGroup()

    for task in tasks {
        dispatchGroup.enter()

        DoSomeTask { (error, ref) -> Void in
            dispatchGroup.leave()
        }
    }

    dispatchGroup.notify(queue: DispatchQueue.main, execute: {
        DoSomeFunction()
    })

但是，如果这些任务都具有相同的功能，但又不必彼此关联，例如将3个不同的值推送到数据库中，我会感到困惑.像这样:

However, I'm confused on how you'd do this if those tasks were all in the same function but didn't have to do with eachother, like pushing 3 different values to your database. Something like this:

   updateDatabase() {
        var dispatchGroup = DispatchGroup()

        DoTask1 { (error, ref) -> Void in           
        }

        DoTask2 { (error, ref) -> Void in           
        }

        DoTask3 { (error, ref) -> Void in        
        }
   }

在这种情况下，您将leave和enter语句放在哪里?

Where would you put the leave and enter statement in such a case as this?

推荐答案

在这种情况下，您的updateDatabase()函数需要在完成所有更新后调用的完成块:

In this case your updateDatabase() function needs a completion block that you call after all update are done:

updateDatabase(onSccess completion:() -> Void) {
    var dispatchGroup = DispatchGroup()

    dispatchGroup.enter()
    DoTask1 { (error, ref) -> Void in   
        dispatchGroup.leave()        
    }

    dispatchGroup.enter()
    DoTask2 { (error, ref) -> Void in           
        dispatchGroup.leave()
    }

    dispatchGroup.enter()
    DoTask3 { (error, ref) -> Void in        
        dispatchGroup.leave()
    }

    dispatchGroup.notify(queue: DispatchQueue.main) {
        completion()
    }
}

您不应该这样称呼它:

updateDatabase() {
    //do somthing after updating
}

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