问题
I understand how completion handlers work, but im a bit confused on the syntax. Below is a function that, given a username, calls a parse query to find out the corresponding userId. The query ends after the function is returned (so it returns nil), which is why we need the completion handler. How do i implement it?
func getUserIdFromUsername(username: String) -> String {
var returnValue = String()
let query = PFQuery(className: "_User")
query.whereKey("username", equalTo: username)
query.findObjectsInBackgroundWithBlock { (objects, error) -> Void in
if let objects = objects {
for object in objects {
returnValue = object.objectId!
}
}
}
return returnValue
}
NOTE: I know examples similar to this exist, but they are either not swift, or extremely lengthy. This is a short and concise version that contains Parse.
回答1:
Here's how to implement it:
func getUserIdFromUsername(username: String, completionHandler: String -> Void) {
let query = PFQuery(className: "_User")
query.whereKey("username", equalTo: username)
query.findObjectsInBackgroundWithBlock { (objects, error) -> Void in
if let objects = objects {
for object in objects {
completionHandler(object.objectId!)
}
}
}
}
And here's how to use it:
getUserIdFromUsername("myUser") { id in
doSomethingWithId(id)
}
来源：https://stackoverflow.com/questions/35152508/how-to-use-completion-handler-correctly