ios - Swift 中的 Grand Central Dispatch 仅在第一次运行时有效

Question

我得到了一个类似于复习的方法，它使用 GCD 模式，如下所示:

    func getStepsForTheWeek() {
    let concurrentQueue : dispatch_queue_t  = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0)

    dispatch_async(concurrentQueue, {

        // Create an array of Days.
        var days = [Day]()

        dispatch_sync(concurrentQueue, {
            print("first")

            for day in 0...7 {

                let date = self.getDate(day)

                // Get the date the day after that day.
                let endDate = self.getDateDayAfter(date)

                // Create a Day.
                var day = Day(date: date)

                self.pedometer.queryPedometerDataFromDate(date, toDate: endDate, withHandler: {numberOfSteps, error in
                    print("fetching")

                    if error != nil {
                        print("There was an error requesting data from the pedometer: \(error)")
                    } else {
                        day.steps = numberOfSteps!.numberOfSteps as Int
                        days.append(day)
                    }
                })
            }
        })

        dispatch_sync(dispatch_get_main_queue(), {
            print("second")

            self.historyViewController.days = days
            self.historyViewController.reloadHistory()
        })
    })
}

当应用程序启动时，该方法将按预期工作。但是当应用程序处于后台并且当我返回应用程序时，我得到了这个 Observer，它再次调用该方法来刷新其内容。

        NSNotificationCenter.defaultCenter().addObserver(self, selector: "appBecomeActive", name: UIApplicationWillEnterForegroundNotification, object: nil )

但是每次我这样做时，第二个代码块都会在第一个代码块之前运行。有帮助吗？

Answer 1

我认为您应该在计步器查询的完成处理程序中进行 UI 更新

    dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_BACKGROUND, 0), {

        self.pedometer.queryPedometerDataFromDate(date, toDate: endDate, withHandler: { numberOfSteps, error in
            if error != nil {
                print("There was an error requesting data from the pedometer: \(error)")
            } else {
                let numberOfStepsThisDay = numberOfSteps?.numberOfSteps as! Int

                day.steps = numberOfStepsThisDay
                days.append(day)
            }

            dispatch_async(dispatch_get_main_queue(), {
                self.historyViewController.days = self.days
                self.historyViewController.reloadHistory()
            })
        })
    })

你应该总是在方法的完成处理程序中更新 UI，如果它有一个，因为你不知道它是否可以是异步的(如果使用完成处理程序通常是异步的)，你可能会删除外部dispatch_async 如果除了计步器查询之外没有其他代码