我想从我的服务器下载数据以显示在 map 上。因此我使用异步方法来获取数据。目标是显示一组注释对象。
因此我首先下载信息A，然后下载信息B。由于两者都是异步方法，我想我需要等待completionHandler返回true，以便我知道数据已加载。这对于一种方法来说很容易。但是，在 getInformationFromServer 的completionHandler 返回 true 并触发添加注释之前，我该如何处理等待这两个方法呢？
override func viewWillAppear(animated: Bool) {
self.customizeInterface()
self.getInformationFromServer { (completed) -> Void in
if(completed) {
self.mapView.addAnnotations(self.annotationArray)
}
}
}
func getInformationFromServer(completionHandler: (completed: Bool) -> Void) {
getInformationFromServerA { (downloadCompleted) -> Void in
completionHandler(completed: downloadCompleted)
}
// HOW DO I MANAGE TO ONLY RETURN THE COMPLETION HANDLER TRUE WHEN
// BOTH FUNCTIONS RETURNED TRUE?
}
func getInformationFromServerA(completionHandler: (downloadCompleted: Bool) -> Void) {
Server().getJsonInformationFromServer(url: "aeds", completionHandler: { (response) -> Void in
self.parseAEDInformationToAnnotation(response["data"])
completionHandler(downloadCompleted: true)
})
}
func getInformationFromServerB(completionHandler: (downloadCompleted: Bool) -> Void) {
Server().getJsonInformationFromServer(url: "aeds", completionHandler: { (response) -> Void in
self.parseAEDInformationToAnnotation(response["data"])
completionHandler(downloadCompleted: true)
})
}
最佳答案
您可以使用调度组等待两个下载完成。
func getInformationFromServer(completionHandler: (completed: Bool) -> Void) {
let dispatchGroup = dispatch_group_create()
var downloadCompletedA: Bool = false
dispatch_group_enter(dispatchGroup)
getInformationFromServerA { (downloadCompleted) -> Void in
downloadCompletedA = downloadCompleted
dispatch_group_leave(dispatchGroup)
}
var downloadCompletedB: Bool = false
dispatch_group_enter(dispatchGroup)
getInformationFromServerB { (downloadCompleted) -> Void in
downloadCompletedB = downloadCompleted
dispatch_group_leave(dispatchGroup)
}
// wait until both downloads are finished
dispatch_group_wait(dispatchGroup, DISPATCH_TIME_FOREVER)
completionHandler(downloadCompletedA && downloadCompletedB)
}
请参阅 Apple 的 Concurrency Programming Guide :
Dispatch groups are a way to block a thread until one or more tasks finish executing. You can use this behavior in places where you cannot make progress until all of the specified tasks are complete.
关于swift - 从服务器异步下载信息，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/33269748/