Swift 3 - 异步显示警报控制器，长功能在后台运行

Question

我正在使用 Swift 3。

我试图做的行为是：用户点击一个按钮，一个旋转的齿轮警报控制器显示，同时它启动一个长时间运行的功能。一旦该函数执行完毕，旋转的齿轮就会消失，视图控制器也会消失。

下面的代码启动doProcessing 函数，但直到视图关闭前大约一秒钟才显示旋转齿轮。所以这不太对。

func displaySpinningGear() {

    print("display spinning gear")
    // show the alert window box
    let activityAlertController = UIAlertController(title: "Processing", message: "Please wait while the photo is being processed.", preferredStyle: .alert)

    //create an activity indicator
    let indicator = UIActivityIndicatorView(frame: activityAlertController.view.bounds)
    indicator.autoresizingMask = [.flexibleWidth, .flexibleHeight]
    indicator.hidesWhenStopped = true
    indicator.activityIndicatorViewStyle = UIActivityIndicatorViewStyle.gray

    //add the activity indicator as a subview of the alert controller's view
    activityAlertController.view.addSubview(indicator)
    indicator.isUserInteractionEnabled = false // required otherwise if there buttons in the UIAlertController you will not be able to press them
    indicator.startAnimating()

    print("start animating")

    self.present(activityAlertController, animated: true, completion: nil)
}

func onButtonClick() {
    self.displaySpinningGear()
    DispatchQueue.main.async {
        self.doProcessing() // long running function
    }

    if let viewController = presentingViewController {
        // This block will dismiss both current and a view controller presenting current
        viewController.dismiss(animated: true, completion: nil)
    }
    else {
        // This block will dismiss only current view controller
        self.dismiss(animated: true, completion: nil)
    }
}

下面的代码启动了doProcessing 函数，但视图立即消失，我可以从控制台得知我的doProcessing 函数仍在运行。这也不对。

function onButtonClick() {
    DispatchQueue.global(qos: .background).async {
        print("Processing")
        self.doProcessing() // run in background

        DispatchQueue.main.async {
            self.displaySpinningGear()
        }
    }

    if let viewController = presentingViewController {
        // This block will dismiss both current and a view controller presenting current
        viewController.dismiss(animated: true, completion: nil)
    }
    else {
        // This block will dismiss only current view controller
        self.dismiss(animated: true, completion: nil)
    }
}

如何在显示旋转齿轮时启动背景功能并在后台功能完成运行时（而不是之前）关闭视图和警报控制器？

编辑

按照@Honey 在评论中的建议，尝试移动代码以在背景块之外旋转齿轮，但无济于事。当进程函数仍在处理时，视图会立即关闭（我可以通过打印语句来判断）。

func onButtonClick() {
    DispatchQueue.main.async {
        self.displaySpinningGear()
    }
    DispatchQueue.global(qos: .background).async {
        print("Processing")
        self.doProcessing() // run in background
    }

    if let viewController = presentingViewController {
        // This block will dismiss both current and a view controller presenting current
        viewController.dismiss(animated: true, completion: nil)
    }
    else {
        // This block will dismiss only current view controller
        self.dismiss(animated: true, completion: nil)
    }
}

Answer 1

从长时间运行的函数中进行回调，以便在它结束时返回一个值并捕获它以使警报消失。

试试看：

typealias DoProcessingCallback = (_ finished: Bool) -> Void

func onButtonClick() {
    self.displaySpinningGear()

    self.doProcessing(callback: { (finished) in

        if finished {
           // Here you DismissViewController
           // Here you DismissAlert
        }

    }) // long running function

}


func doProcessing(callback: DoProcessingCallback) {
    // YOUR LONG CODE....
    // When you know it already finished
    callback(true)
}

希望对你有帮助

Answer 2

我遇到了同样的问题并尝试了很多不同的方法，这就是有效的：

activityView.alpha = 0.8
DispatchQueue.global(qos: .default).async(execute: {
    DispatchQueue.main.async(execute: {
        self.performSegue(withIdentifier: "cropToProcessed", sender: self)
    })
})

基本上，我最初将活动指示器的 alpha 设置为 0.0，当按下按钮时，我将其设置为 0.8，然后在 viewWillDisappear 中将其设置回 0.0，它可以工作