SVProgressHUD 代码乱序执行

Question

我正在使用 NextBus API 构建公交车预测应用程序，这将帮助用户获得预测时间和公交车信息。我实现了一个函数，该函数获取用户的当前位置和所选地址以及 returns 10 条公交路线的列表，这些路线可最大限度地减少旅行距离和时间。

这里是触发上述功能的@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 行似乎只在 WayFinder.shared.findAWay() 执行后才做任何事情。 HUD 会显示一小会儿，然后立即被 SVProgressHUD.dismiss() 关闭。

这是 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中的代码没有按顺序执行？在调用 findAWay 之前，hud 怎么会不显示在屏幕上？有什么想法吗？

Answer 1

因此，您需要阅读一些有关 "main thread" 及其工作原理的资料。也许 UNDERSTANDING THE IOS MAIN THREAD

基本上，您要求系统显示 HUD，然后执行，我假设是一个长运行和阻塞操作，然后在主线程中关闭所有 HUD。

在该方法存在之前，系统不可能显示 HUD，因为它将成为下一个循环的一部分（paint/layout/other 重要内容）。在这种情况下，我会倾向于某种 "promise" API，例如 PromiseKit or Hydra，因为它会大大简化希望的线程。

基本意图是 - 在主线程上显示 HUD，使用后台线程执行查询，完成后关闭 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()
    }
}

记住，永远不要在主线程上下文之外修改 UI，如果 OS 检测到这种情况，它会使您的应用程序崩溃！

我可能还会考虑使用 DispatchSemaphore 而不是 "wild running" while-loop，所以不是..

// 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 {}

您可以使用类似...

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()

这将做同样的事情，但效率更高

SVProgressHUD 代码乱序执行

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swift