SVProgressHUD代码无序执行

Question

我正在使用NextBus应用程序接口构建一个公交车预测应用程序，它将帮助用户获得预测时间和公交车信息。我已经实现了一个函数，该函数获取用户的当前位置和选定的地址，并返回一个包含10条公交路线的列表，这些路线最大限度地减少了行程和时间。

下面是触发上述函数的@IBAction：

@IBAction func findAWayPressed(_ sender: UIButton) {
    // Hide confirm button.
    confirmButton.isHidden = true

    // Setup loading HUD.
    let blue = UIColor(red: 153/255, green: 186/255, blue: 221/255, alpha: 1.0)
    SVProgressHUD.setBackgroundColor(blue)
    SVProgressHUD.setStatus("Finding a way for you...")
    SVProgressHUD.setBorderColor(UIColor.black)
    SVProgressHUD.show()

    // Finds a list of ten bus routes that minimizes the distance from the user and their destination.
    WayFinder.shared.findAWay(startCoordinate: origin!, endCoordinate: destination!)

    SVProgressHUD.dismiss()
}

问题是，confirmButton.isHidden = true和SVProgressHUD行似乎只有在WayFinder.shared.findAWay()执行之后才能执行任何操作。在被SVProgressHUD.dismiss()立即关闭之前，HUD会显示一小段时间。

下面是findAWay函数：

func findAWay(startCoordinate: CLLocationCoordinate2D, endCoordinate: CLLocationCoordinate2D) {
    // Get list of bus routes from NextBus API.
    getRoutes()

    guard !self.routes.isEmpty else {return}

    // Initialize the the lists of destination and origin stops.
    closestDestinations = DistanceData(shortestDistance: 1000000, stops: [])
    closestOrigins = DistanceData(shortestDistance: 1000000, stops: [])

    // Fetch route info for every route in NextBus API.
    var routeConfigsDownloaded: Int = 0
    for route in routes {
        // Counter is always one whether the request fails
        // or succeeds to prevent app crash.
        getRouteInfo(route: route) { (counter) in
            routeConfigsDownloaded += counter
        }
    }

    while routeConfigsDownloaded != routes.count {}

    // Iterate through every stop and retrieve a list
    // of 10 possible destination stops sorted by distance.
    getClosestDestinations(endCoordinate: endCoordinate)
    // Use destination stop routes to find stops near
    // user's current location that end at destination stops.
    getOriginStops(startCoordinate: startCoordinate)

    // Sort routes by adding their orign distance and destination
    // distance and sorting by total distance.
    getFoundWays()
}

private func getRouteInfo(route: Route, completion: @escaping (Int) -> Void) {
    APIWrapper.routeFetcher.fetchRouteInfo(routeTag: route.tag) { (config) in
        if let config = config {
            self.routeConfigs[route.tag] = config
        } else {
            print("Error retrieving route config for Route \(route.tag).")
        }
        completion(1)
    }
}

为什么@IBAction中的代码不能按顺序执行？为什么在调用findAWay之前，hud不会显示在屏幕上？有什么想法吗？

Answer 1

所以，你需要阅读一些关于“主线程”及其工作原理的文章。也许是UNDERSTANDING THE IOS MAIN THREAD

基本上，您要求系统显示HUD，然后执行，我认为这是一个长时间运行和阻塞的操作，然后在主线程中清除所有HUD。

在方法存在之前，系统不可能显示HUD，因为它将是下一个周期(绘制/布局/其他重要内容)的一部分。在这样的情况下，我会倾向于某种"promise“API，比如PromiseKit或Hydra，因为它将大大简化线程的希望。

基本意图是-当在主线程上时，呈现HUD，使用后台线程，执行查询，当查询完成时，取消HUD，但在主线程上执行。

可能看起来像这样..

SVProgressHUD.show()
DispatchQueue.global(qos: .userInitiated).async {
    WayFinder.shared.findAWay(startCoordinate: origin!, endCoordinate: destination!)
    DispatchQueue.main.async {
        SVProgressHUD.dismiss()
    }
}

现在请记住，永远不要从主线程上下文之外修改UI，如果操作系统检测到这一点，它将使您的应用程序崩溃！

我可能还会考虑使用DispatchSemaphore而不是“狂野奔跑”的while-loop，这样就可以代替..

// Fetch route info for every route in NextBus API.
var routeConfigsDownloaded: Int = 0
for route in routes {
    // Counter is always one whether the request fails
    // or succeeds to prevent app crash.
    getRouteInfo(route: route) { (counter) in
        routeConfigsDownloaded += counter
    }
}

while routeConfigsDownloaded != routes.count {}

你可以用这样的东西..。

let semaphore = DispatchSemaphore(value: routes.count)
// Fetch route info for every route in NextBus API.
var routeConfigsDownloaded: Int = 0
for route in routes {
    // Counter is always one whether the request fails
    // or succeeds to prevent app crash.
    getRouteInfo(route: route) { (counter) in
        semaphore.signal()
    }
}

semaphore.wait()

它将做同样的事情，但效率更高