Swift-从闭包内部返回变量

Question

have the following function. I am looking to return the result from the function as an Int following the completion of the thread execution. It is querying a variable form an external device. When I call the function get variable, I immediately receive the result -1, then several seconds after this I receive the result from the completion thread. How can I re-work this so no result is returned until the real value is returned? still a noob with Swift3 and GCD..thanks

func getVariable(variableName: String) -> Int {
    var res: Int = -1
    print (deviceOK)
    if deviceOK {
        DispatchQueue.global(qos: .default).async {
            // logging in
            (self.deviceGroup).wait(timeout: DispatchTime.distantFuture)
            (self.deviceGroup).enter()

            self.myPhoton!.getVariable(variableName, completion: { (result:Any?, error:Error?) -> Void in
                if let _ = error  {
                    print("Failed reading variable " + variableName + " from device")
                } else {
                    if let res = result! as? Int {
                        print("Variable " + variableName + " value is \(res)")
                        self.deviceGroup.leave()
                    }
                }
            })
        }
    }

    return res
}

Answer 1

Maybe you can use completion block yourself:

func getVariable(variableName: String, onComplete: ((Int) -> ())) {
    var res: Int = -1
    print (deviceOK)
    if deviceOK {
        DispatchQueue.global(qos: .default).async {
            // logging in
            (self.deviceGroup).wait(timeout: DispatchTime.distantFuture)
            (self.deviceGroup).enter()

            self.myPhoton!.getVariable(variableName, completion: { (result:Any?, error:Error?) -> Void in
                if let _ = error  {
                    print("Failed reading variable " + variableName + " from device")
                } else {
                    if let res = result! as? Int {
                        onComplete(res)
                        print("Variable " + variableName + " value is \(res)")
                        self.deviceGroup.leave()
                    }
                }
            })
        }
    } else {
        onComplete(res)
    }
}

Another approach is to use Promises, take a look at this implementation: https://github.com/FutureKit/FutureKit

Answer 2

您应该使用完成处理程序来使getvariable函数而不是返回Int。