我遇到了一个问题，我有多个异步请求从 Facebook API 和我的 Firebase 数据库中获取图像和信息。我想执行我所有的异步请求，然后将我从 Facebook API/Firebase 数据库中获取的所有数据存储到一个我可以快速加载的完整对象中。我已经为每个异步请求设置了完成处理程序，我认为这会强制程序“等待”直到请求完成，然后让程序继续，但这似乎对我不起作用。以下是我的尝试:
func setupEvents(completion: (result: Bool, Event: Event) -> Void){
// Get a reference to Events
eventsReference = Firebase(url:"<DB Name>")
eventAttendeesRef = Firebase(url:"<DB Name>")
//Read the data at our posts reference
println("Event References: \(eventsReference)")
eventsReference.observeEventType(FEventType.ChildAdded, withBlock: { (snapshot) -> Void in
let eventName = snapshot.value["eventName"] as? String
let eventLocation = snapshot.value["eventLocation"] as? String
let eventCreator = snapshot.value["eventCreator"] as? String
var attendees: NSMutableDictionary = [:]
var attendeesImages = [UIImage]()
let attendee: NSMutableDictionary = [:]
let group = dispatch_group_create()
//Get attendees first
dispatch_group_enter(group)
self.getAttendees(snapshot.key as String, completion:{ (result, name, objectID) -> Void in
if(result == true){
println("Finished grabbing \(name!) \(objectID!)")
attendees.addEntriesFromDictionary(attendee as [NSObject : AnyObject])
}
else {
println("False")
}
dispatch_group_leave(group)
})
//Get attendees photos
dispatch_group_enter(group)
self.getAttendeesPictures(attendee, completion: { (result, image) -> Void in
if result == true {
println("Finished getting attendee photos. Now to store into Event object.")
attendeesImages.append(image!)
}
else{
println("false")
}
dispatch_group_leave(group)
})
dispatch_group_notify(group, dispatch_get_main_queue()) {
println("both requests done")
//Maintain array snapshot keys
self.eventIDs.append(snapshot.key)
if snapshot != nil {
let event = Event(eventName: eventName, eventLocation:eventLocation, eventPhoto:eventPhoto, fromDate:fromDate, fromTime:fromTime, toDate:toDate, toTime:toTime, attendees: attendees, attendeesImages:attendeesImages, attendeesImagesTest: attendeesImagesTest, privacy:privacy, eventCreator: eventCreator, eventCreatorID: eventCreatorID)
println("Event: \(event)")
completion(result: true, Event: event)
}
}
}) { (error) -> Void in
println(error.description)
}
}
我知道我的完成处理程序设置正确，因为我已经在我的程序中进行了测试。但是，我想要的是只有在 getAttendees 和 getAttendeesPictures 函数都完成后，我才想存储我抓取的所有信息 snapshot 、getAttendees 和 getAttendeesPictures 函数并将它们存储到 event 对象中。关于如何实现这一目标的任何想法？我试图查看 dispatch_groups 以帮助我通过以下链接处理此问题:Checking for multiple asynchronous responses from Alamofire and Swift但是我的程序似乎只执行getAttendees 函数而不执行getAttendeesPictures 函数。下面还有 getAttendees 和 getAttendeesPictures 函数:
func getAttendees(child: String, completion: (result: Bool, name: String?, objectID: String?) -> Void){
//Get event attendees of particular event
var attendeesReference = self.eventAttendeesRef.childByAppendingPath(child)
println("Loading event attendees")
//Get all event attendees
attendeesReference.observeEventType(FEventType.ChildAdded, withBlock: { (snapshot) -> Void in
let name = snapshot.value.objectForKey("name") as? String
let objectID = snapshot.value.objectForKey("objectID") as? String
println("Name: \(name) Object ID: \(objectID)")
completion(result: true, name: name, objectID: objectID)
}) { (error) -> Void in
println(error.description)
}
func getAttendeesPictures(attendees: NSMutableDictionary, completion: (result: Bool, image: UIImage?)-> Void){
println("Attendees Count: \(attendees.count)")
for (key, value) in attendees{
let url = NSURL(string: "https://graph.facebook.com/\(key)/picture?type=large")
println("URL: \(url)")
let urlRequest = NSURLRequest(URL: url!)
//Asynchronous request to display image
NSURLConnection.sendAsynchronousRequest(urlRequest, queue: NSOperationQueue.mainQueue()) { (response:NSURLResponse!, data:NSData!, error:NSError!) -> Void in
if error != nil{
println("Error: \(error)")
}
// Display the image
let image = UIImage(data: data)
if(image != nil){
completion(result: true, image: image)
}
}
}
}
最佳答案
对于在标题中寻求问题答案的用户，请使用此处概述的 dispatch_group 和 GCD:即将一个组嵌入到另一个 dispatch_group 的通知方法中是有效的。另一种进入更高级别的方法是 NSOperations 和依赖项，这也将提供进一步的控制，例如取消操作。
大纲:
func doStuffonObjectsProcessAndComplete(arrayOfObjectsToProcess: Array) -> Void){
let firstGroup = dispatch_group_create()
for object in arrayOfObjectsToProcess {
dispatch_group_enter(firstGroup)
doStuffToObject(object, completion:{ (success) in
if(success){
// doing stuff success
}
else {
// doing stuff fail
}
// regardless, we leave the group letting GCD know we finished this bit of work
dispatch_group_leave(firstGroup)
})
}
// called once all code blocks entered into group have left
dispatch_group_notify(firstGroup, dispatch_get_main_queue()) {
let processGroup = dispatch_group_create()
for object in arrayOfObjectsToProcess {
dispatch_group_enter(processGroup)
processObject(object, completion:{ (success) in
if(success){
// processing stuff success
}
else {
// processing stuff fail
}
// regardless, we leave the group letting GCD know we finished this bit of work
dispatch_group_leave(processGroup)
})
}
dispatch_group_notify(processGroup, dispatch_get_main_queue()) {
print("All Done and Processed, so load data now")
}
}
}
此答案的其余部分特定于此代码库。
这里似乎有几个问题:
getAttendees 函数接受一个事件子对象并返回一个 objectID 和 Name，它们都是字符串？这个方法不应该返回一组与会者吗？如果不是，那么返回的 objectID 是什么？
返回一组与会者后，您可以对他们进行分组处理以获取照片。
getAttendeesPictures 最终从 Facebook 返回 UIImages。最好将这些缓存到磁盘并传递 path ref - 保留所有这些获取的图像对内存不利，并且根据大小和数量，可能会很快导致问题。
一些例子:
func getAttendees(child: String, completion: (result: Bool, attendees: Array?) -> Void){
let newArrayOfAttendees = []()
// Get event attendees of particular event
// process attendees and package into an Array (or Dictionary)
// completion
completion(true, attendees: newArrayOfAttendees)
}
func getAttendeesPictures(attendees: Array, completion: (result: Bool, attendees: Array)-> Void){
println("Attendees Count: \(attendees.count)")
let picturesGroup = dispatch_group_create()
for attendee in attendees{
// for each attendee enter group
dispatch_group_enter(picturesGroup)
let key = attendee.objectID
let url = NSURL(string: "https://graph.facebook.com/\(key)/picture?type=large")
let urlRequest = NSURLRequest(URL: url!)
//Asynchronous request to display image
NSURLConnection.sendAsynchronousRequest(urlRequest, queue: NSOperationQueue.mainQueue()) { (response:NSURLResponse!, data:NSData!, error:NSError!) -> Void in
if error != nil{
println("Error: \(error)")
}
// Display the image
let image = UIImage(data: data)
if(image != nil){
attendee.image = image
}
dispatch_group_leave(picturesGroup)
}
}
dispatch_group_notify(picturesGroup, dispatch_get_main_queue()) {
completion(true, attendees: attendees)
}
}
func setupEvents(completion: (result: Bool, Event: Event) -> Void){
// get event info and then for each event...
getAttendees(child:snapshot.key, completion: { (result, attendeesReturned) in
if result {
self.getAttendeesPictures(attendees: attendeesReturned, completion: { (result, attendees) in
// do something with completed array and attendees
}
}
else {
}
})
}
以上代码只是一个大纲，但希望能为您指明正确的方向。
关于ios - 在加载数据之前完成所有异步请求？，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/31261500/