从包含闭包的Swift函数返回一个值
我写了一个函数,该函数应该返回一个值,但值来自闭包。问题是,如果我试图从闭包中返回一个值,它将它视为来自完成处理程序的返回值。从包含闭包的Swift函数返回一个值
private func loadData() throws -> [Item] {
var items = [Item]()
let jsonUrl = "http://api.openweathermap.org/data/2.5/forecast/daily?units=metric&cnt=7&q=coventry,uk"
print(jsonUrl)
let session = NSURLSession.sharedSession()
guard let shotsUrl = NSURL(string: jsonUrl) else {
throw JSONError.InvalidURL(jsonUrl)
}
session.dataTaskWithURL(shotsUrl, completionHandler: {(data, response, error) -> Void in
do {
let json = try NSJSONSerialization.JSONObjectWithData(data!, options: [])
print(json)
guard let days:[AnyObject] = (json["list"] as! [AnyObject]) else {
throw JSONError.InvalidArray
}
for day in days {
guard let timestamp:Double = day["dt"] as? Double else {
throw JSONError.InvalidKey("dt")
}
print(timestamp)
let date = NSDate(timeIntervalSince1970: NSTimeInterval(timestamp))
guard let weather:[AnyObject] = day["weather"] as? [AnyObject] else {
throw JSONError.InvalidArray
}
guard let desc:String = weather[0]["description"] as? String else {
throw JSONError.InvalidKey("description")
}
guard let icon:String = weather[0]["icon"] as? String else {
throw JSONError.InvalidKey("icon")
}
guard let url = NSURL(string: "http://openweathermap.org/img/w/\(icon).png") else {
throw JSONError.InvalidURL("http://openweathermap.org/img/w/\(icon).png")
}
guard let data = NSData(contentsOfURL: url) else {
throw JSONError.InvalidData
}
guard let image = UIImage(data: data) else {
throw JSONError.InvalidImage
}
guard let temp:AnyObject = day["temp"] else {
throw JSONError.InvalidKey("temp")
}
guard let max:Float = temp["max"] as? Float else {
throw JSONError.InvalidKey("max")
}
let newDay = Item(date: date, description: desc, maxTemp: max, icon: image)
print(newDay)
items.append(newDay)
}
return items // this line fails because I'm in the closure. I want this to be the value returned by the loadData() function.
} catch {
print("Fetch failed: \((error as NSError).localizedDescription)")
}
})
}
添加完成处理程序(在我的例子命名dataHandler
)到您的loadData
功能:
private func loadData(dataHandler: ([Item])->()) throws {
var items = [Item]()
let jsonUrl = "http://api.openweathermap.org/data/2.5/forecast/daily?units=metric&cnt=7&q=coventry,uk"
print(jsonUrl)
let session = NSURLSession.sharedSession()
guard let shotsUrl = NSURL(string: jsonUrl) else {
throw JSONError.InvalidURL(jsonUrl)
}
session.dataTaskWithURL(shotsUrl, completionHandler: {(data, response, error) -> Void in
do {
let json = try NSJSONSerialization.JSONObjectWithData(data!, options: [])
print(json)
guard let days:[AnyObject] = (json["list"] as! [AnyObject]) else {
throw JSONError.InvalidArray
}
for day in days {
guard let timestamp:Double = day["dt"] as? Double else {
throw JSONError.InvalidKey("dt")
}
print(timestamp)
let date = NSDate(timeIntervalSince1970: NSTimeInterval(timestamp))
guard let weather:[AnyObject] = day["weather"] as? [AnyObject] else {
throw JSONError.InvalidArray
}
guard let desc:String = weather[0]["description"] as? String else {
throw JSONError.InvalidKey("description")
}
guard let icon:String = weather[0]["icon"] as? String else {
throw JSONError.InvalidKey("icon")
}
guard let url = NSURL(string: "http://openweathermap.org/img/w/\(icon).png") else {
throw JSONError.InvalidURL("http://openweathermap.org/img/w/\(icon).png")
}
guard let data = NSData(contentsOfURL: url) else {
throw JSONError.InvalidData
}
guard let image = UIImage(data: data) else {
throw JSONError.InvalidImage
}
guard let temp:AnyObject = day["temp"] else {
throw JSONError.InvalidKey("temp")
}
guard let max:Float = temp["max"] as? Float else {
throw JSONError.InvalidKey("max")
}
let newDay = Item(date: date, description: desc, maxTemp: max, icon: image)
print(newDay)
items.append(newDay)
}
dataHandler(items)
} catch {
print("Fetch failed: \((error as NSError).localizedDescription)")
}
}).resume()
}
do {
try loadData { itemsArray in
print(itemsArray)
}
} catch {
print(error)
}
我在操场测试,它的工作原理没有错误:
答案应该可以工作,但它会在操场上引发运行时错误(即使在包含结构和错误枚举时也是如此)。然后我尝试在课堂上公开该方法,并从我的视图控制器调用它。没有错误,但session.dataTaskWithURL回调无法运行。 –
我已经添加了我的Playground运行我的答案没有错误的屏幕截图。请看一看。 //不要忘记,为了在Playground中运行异步代码,您需要启用'XCPSetExecutionShouldContinueIndefinitely'! :) – Moritz
道歉,忘了添加XCPSetExecutionShouldContinueIndefinitely(true) –
您可以传递闭包作为回调或完成处理程序,您可以在要返回项目的位置调用该闭包程序。你需要写出你愿意如何处理你传递给loadData()函数的闭包中的数据。 – iamyogish