I have a function in swift to return a string from array, if the array has value return directly, if not I need load it from server and return the value from callback, I don't know how to do it, something like
func getValue(idx: Int)->string {
if arrayValue.count <= 0 {
callbackfunc() { arrayValue in
return arrayValue[idx]
}
} else {
return arrayValue[idx]
}
}
but it is impossible to return from callback, any help? thank you very much
Add a completion handler:
func getValue(idx: Int, completion: @escaping (String) -> Void) {
if arrayValue.count <= 0 {
callbackfunc() { arrayValue in
completion(arrayValue[idx])
}
} else {
completion(arrayValue[idx])
}
}
And call it:
getValue(idx: 2) { result in
print(result)
}
As you have found out you cannot directly return a value from a closure. Therefore you need to handle the returned data within the callback. This can be as complex as needs be, but for a simple example you could just pass the data back to an underlying method.
Depending on what else the app might be doing during the time the callback is running, you may need to be careful how you update this data to make it thread-safe. As a simple precaution it's probably worth performing the update on the main thread:
func getValue(idx: Int)->string {
if arrayValue.count <= 0 {
callbackfunc() { [weak self] arrayValue in
DispatchQueue.main.async {
self.processReturnedData(arrayValue[idx])
}
}
} else {
return arrayValue[idx]
}
}
func processReturnedData(_ idx: IDXType) {
//do something with the data
}
