SVProgressHUD代码执行异常

Question

我正在使用NextBus API构建一个公交车预测应用程序，该应用程序将帮助用户获取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，并返回10条公交路线的列表，以最大限度地缩短旅行距离和时间。

这是触发上述功能的@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行似乎只是在WayFinder.shared.findAWay()执行之后才执行任何操作。HUD会显示一小段时间，然后立即被SVProgressHUD.dismiss()取消显示。

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中的代码不能按顺序执行？在调用findAWay之前，hud怎么可能不显示在屏幕上？有什么想法吗？

Answer 1

因此，您需要阅读一些关于"主线程"及其工作方式的内容。也许了解IOS主线程

基本上，你要求系统显示HUD，然后执行，我认为这是一个长时间运行和阻塞的操作，然后在主线程中关闭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周期的一部分(绘制/布局/其他重要内容(。在这种情况下，我会倾向于某种"承诺"API，比如PromiseKit或Hydra，因为它将大大简化线程的期望。

基本意图是-在主线程上，使用后台线程显示HUD，执行查询，完成后，取消HUD，但在主线程中执行。

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！

我还可以考虑使用DispatchSemaphore而不是"狂野运行"的while-loop，所以不要。。

// 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 {}

你可能会用。。。

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，但更有效的