如何在后台使用 MKMapSnapshotter?

How do I use MKMapSnapshotter in the background?

我有一个应用程序应该在后台获取对象并使用它们的位置数据为它们生成地图快照。当然,我尝试了 MKMapSnapshotter。

事实证明(在对黑色地图快照感到困惑数周之后)这个工具似乎只有在从主线程调用时才有效,如下所示:

dispatch_async(dispatch_get_main_queue(), ^{
   MKMapSnapshotter *snapshotter = [[MKMapSnapshotter alloc] initWithOptions:options];
        [snapshotter startWithQueue:dispatch_get_main_queue() completionHandler:^(MKMapSnapshot * _Nullable snapshot, NSError * _Nullable error) {
       //Use image here. Image would be completely black if not for the first line of code specifying main thread.
   }];
});

这是框架错误吗?

问题:这仅在我的应用程序位于前台时运行。

这对于我正在开发的应用程序来说有点复杂,因为有很多调用为多个缩放级别下载一组地图图块,所以下面的代码可能比您需要的要复杂一些(但是显示队列可用于快照)。例如,我需要一个 dispatchSemaphore 来避免排队成百上千个并发快照——这将它们限制为在线程上捕获大约 25 个并发快照。

此外,我在 Swift 3 中执行此操作,因此 GCD 可能会发生变化,让我在向您展示问题时执行此操作。

这里的逻辑是让所有请求在 processQueue 中启动,同时主队列保持畅通,以便 UI 保持活动状态。然后,在任何时候,最多有 25 个请求通过信号量门,它们通过 snapshotter.start 调用进入快照队列。当一个快照完成时,将启动另一个快照,直到 processQueue 为空。

unowned let myself = self   // Avoid captures in closure

let processQueue = DispatchQueue(label: "processQueue", qos: .userInitiated)
let snapshotQueue = DispatchQueue(label: "snapshotQueue")
var getSnapshotter = DispatchSemaphore(value: 25)

processQueue.async
        {
            var centerpoint = CLLocationCoordinate2D()
            centerpoint.latitude = (topRight.latitude + bottomLeft.latitude) / 2.0
            centerpoint.longitude = (topRight.longitude + bottomLeft.longitude) / 2.0
            let latitudeDelta = abs(topRight.latitude - bottomLeft.latitude)
            let longitudeDelta = abs(topRight.longitude - bottomLeft.longitude)
            let mapSpan = MKCoordinateSpanMake(latitudeDelta, longitudeDelta)

            var mapRegion = MKCoordinateRegion()
            mapRegion.center = centerpoint
            mapRegion.span = mapSpan

            let options = MKMapSnapshotOptions()
            options.region = mapRegion
            options.mapType = .standard               
            options.scale = 1.0
            options.size = CGSize(width: 256, height: 256)

            myself.getSnapshotter.wait()       // Limit the number of concurrent snapshotters since we could invoke very many

            let snapshotter = MKMapSnapshotter(options: options)

            snapshotter.start(with: myself.snapshotQueue, completionHandler: {snapshot, error in
                if error == nil
                {
                    self.saveTile(path: path, tile: snapshot!.image, z: z, x: x, y: y)
                    // saveTile writes the image out to a file in the mapOverlay file scheme
                } else {
                    print("Error Creating Map Tile: ", error!)
                }
                if myself.getSnapshotter.signal() == 0
                {  
                    // show status as completed (though could be up to 20 snapshots finishing, won't take long at this point 
                }
            })
    }

这对我来说很有用,可以获取多达 5K 的快照来构建 7 缩放级别的离线地图图像集,而不会阻塞 UI,所以我对代码非常满意。