I have an issue understanding the process in consuming REST api services with SWIFT, seems like i'm missing something simple, but yet important here.
this is the singleton DataManager class, I'm using to consume API with loadNews() method, as you can see it's simple, request method, getter and initializer that will load the data.
for loadNews() I use Alamofire to handle request, and SwiftyJSON to parse the response.
class DataManager{
static let shared = DataManager()
private var data:JSON = JSON()
private init(){
print("testprint1 \(self.data.count)")
loadNews() { response in
self.data = response
print("initprint \(self.data.count)")
print(self.data["response"]["results"].count)
print(self.data["response"]["results"][0]["id"].stringValue)
}
print("testprint2 \(self.data.count)")
}
func getNews() -> JSON {
return data
}
func loadNews(completion: #escaping (JSON) -> ()){
Alamofire.request("...")
.responseJSON{ response in
guard response.result.isSuccess,
let value = response.result.value else {
print("Error: \(String(describing: response.result.error))")
completion([])
return
}
let json = JSON(value)
completion(json)
}
}
}
issue that i'm facing is when i try to call the DataManager() instance in my ViewController, I'm not able to read data in the controller for some reason, here is the controller code (relevant one):
class SecondViewController: UITableViewController {
override func viewDidLoad() {
super.viewDidLoad()
let data1 = DataManager.shared.getNews()
print("qa \(data1.count)")
}
...
}
now what bothers me is - logic behind this should be simple, let data1 = DataManager.shared.getNews() - if i'm not wrong will (should) execute the following flow:
init()->loadNews()->getNews()
initialize method will call loadNews, loadNews will fetch data from API, fill the data array, and getNews is supposed to return the filled data array, but that flow doesn't seem correct then
console output
console output text
testprint1 0
testprint2 0
qa 0
initprint 1
50
commentisfree/2019/dec/07/lost-my-faith-in-tech-evangelism-john-naughton
so it seems like both prints within init() get executed before loadNews() method that is between them, as well as "qa0" print that is printing the size of the array in the ViewController.
now my question is, does anyone see a mistake here, is this happening because of long network query, or am I just missing something, because it seems to me that data is properly loaded and parsed, which is seen in last 2 lines of output, but i can't get it where i need it, like it dissapears. is my logic here wrong? if someone could help I would really appreciate it.
The Alamofire process works asynchronously, but you don't consider it, that's the mistake.
Change the code to
class DataManager{
static let shared = DataManager()
func loadNews(completion: #escaping (JSON) -> ()){
Alamofire.request("...")
.responseJSON{ response in
guard response.result.isSuccess,
let value = response.result.value else {
print("Error:", response.result.error)
completion([])
return
}
let json = JSON(value)
completion(json)
}
}
}
class SecondViewController: UITableViewController {
override func viewDidLoad() {
super.viewDidLoad()
DataManager.shared.loadNews() { response in
print("initprint \(response.count)")
print(response["response"]["results"].count)
print(response["response"]["results"][0]["id"].stringValue)
}
}
...
}
You have to get the data from the completion handler in SecondViewController
I've searched for how to do this but something just isn't making sense for me and I can't do it. All I need to do is get data out of my json session (if thats what you call it). I started programming about 3 weeks ago so I need lay mans terms please. I realize this is probably going to get marked as a duplicate but most of the answers on this / related topics are for other languages and I barely understand swift so they don't help me much.
I've spent hours trying to find the answer and since I'm new I don't know if what I'm searching for is even the right thing to be searching for. I've also tried reading the iOS developer library but either I don't understand what it's telling me or I haven't found the right section because I still can't figure this out. Please try to explain this instead of sending me to read other resources.
here is my function
func parseData() {
let urlString = "http://heroesjson.com/heroes.json"
let session = NSURLSession.sharedSession()
let url = NSURL(string: urlString)!
session.dataTaskWithURL(url) { (data: NSData?, response:NSURLResponse?, error: NSError?) -> Void in
guard let responseData = data else { return }
var json: [[String: AnyObject]]!
do {
json = try NSJSONSerialization.JSONObjectWithData(responseData, options: NSJSONReadingOptions.AllowFragments) as! [[String: AnyObject]]
}
catch {
//handle error
}
var arrayToReturn = [Hero]()
for element in json {
let hero = Hero(fromDictionary: element as! [String: AnyObject])
arrayToReturn.append(hero)
}
}.resume()//Closes Session.dataTaskWithURL
} //Closes parseData()
the goal is to get the json variable in my do statement so I can parse it outside of the function or get my "arrayToReturn" so I can save it to a global variable that I use.
If I understand correctly I can't just assign the value (arrayToReturn) to my global variable (heroes) because this is an asynchronous request so it just returns nil because the command is called before the request is finished. I think I have to use a completion handler or callback function. I don't really understand the difference between them and don't understand how or where to implement them.
Also, I don't understand this code very much either, I just know it's necessary to get what I want.
session.dataTaskWithURL(url) { (data: NSData?, response:NSURLResponse?, error: NSError?) -> Void in
"(data: NSData?, response:NSURLResponse?, error: NSError?)" looks like parameters but they don't seem to be attached to a function so that doesn't make sense to me
"-> Void" Doesn't make sense to me because -> means return whatever follows, but void indicates to me that its returning nothing, so why not just leave it out all together?
"-> Void in" What what is the significance of in here? what does it mean / signal?
Go and read about Swift Closures. To use a value outside of it, you'd need to pass it to another closure.
func parseData(callback: (heroes: [Hero]) -> Void) {
let urlString = "http://heroesjson.com/heroes.json"
let session = NSURLSession.sharedSession()
let url = NSURL(string: urlString)!
session.dataTaskWithURL(url) { (data: NSData?, response:NSURLResponse?, error: NSError?) -> Void in
guard let responseData = data else { return }
var json: [[String: AnyObject]]!
do {
json = try NSJSONSerialization.JSONObjectWithData(responseData, options: NSJSONReadingOptions.AllowFragments) as! [[String: AnyObject]]
}
catch {
//handle error
}
var arrayToReturn = [Hero]()
for element in json {
let hero = Hero(fromDictionary: element as! [String: AnyObject])
arrayToReturn.append(hero)
}
callback(heroes: arrayToReturn)
}.resume()//Closes Session.dataTaskWithURL
} //Closes parseData()
And you'd call it:
parseData { heroes in
// do something with the array
}
I am trying to get learn how to use AlamoFire and I am having trouble.
My method so far is as follows:
func siteInfo()->String?{
var info:NSDictionary!
var str:String!
Alamofire.request(.GET, MY_API_END_POINT).responseJSON {(request, response, JSON, error) in
info = JSON as NSDictionary
str = info["access_key"] as String
//return str
}
return str
}
This returns nil which is a problem. From what I have read here, this is because the request can take a while so the closure doesn't execute till after the return. The suggested solution of moving the return into the closure does not work for me and the compiler just yells (adding ->String after (request,response,JSON,error) which gives "'String' is not a subtype of void"). Same goes for the other solution provided.
Any ideas? Even some source code that is not related to this problem, that uses AlamoFire, would be helpful.
Thanks!
One way to handle this is to pass a closure (I usually call it a completionHandler) to your siteInfo function and call that inside Alamofire.request's closure:
func siteInfo(completionHandler: (String?, NSError?) -> ()) -> () {
Alamofire.request(.GET, MY_API_END_POINT).responseJSON {
(request, response, JSON, error) in
let info = JSON as? NSDictionary // info will be nil if it's not an NSDictionary
let str = info?["access_key"] as? String // str will be nil if info is nil or the value for "access_key" is not a String
completionHandler(str, error)
}
}
Then call it like this (don't forget error handling):
siteInfo { (str, error) in
if str != nil {
// Use str value
} else {
// Handle error / nil value
}
}
In the comments you asked:
So how would you save the info you collect from the get request if you
can only do stuff inside the closure and not effect objects outside of
the closure? Also, how to keep track to know when the request has
finished?
You can save the result of the get request to an instance variable in your class from inside the closure; there's nothing about the closure stopping you from doing that. What you do from there really depends on, well, what you want to do with that data.
How about an example?
Since it looks like you're getting an access key form that get request, maybe you need that for future requests made in other functions.
In that case, you can do something like this:
Note: Asynchronous programming is a huge topic; way too much to cover here. This is just one example of how you might handle the data you get back from your asynchronous request.
public class Site {
private var _accessKey: String?
private func getAccessKey(completionHandler: (String?, NSError?) -> ()) -> () {
// If we already have an access key, call the completion handler with it immediately
if let accessKey = self._accessKey {
completionHandler(accessKey, nil)
} else { // Otherwise request one
Alamofire.request(.GET, MY_API_END_POINT).responseJSON {
(request, response, JSON, error) in
let info = JSON as? NSDictionary // info will be nil if it's not an NSDictionary
let accessKey = info?["access_key"] as? String // accessKey will be nil if info is nil or the value for "access_key" is not a String
self._accessKey = accessKey
completionHandler(accessKey, error)
}
}
}
public func somethingNeedingAccessKey() {
getAccessKey { (accessKey, error) in
if accessKey != nil {
// Use accessKey however you'd like here
println(accessKey)
} else {
// Handle error / nil accessKey here
}
}
}
}
With that setup, calling somethingNeedingAccessKey() the first time will trigger a request to get the access key. Any calls to somethingNeedingAccessKey() after that will use the value already stored in self._accessKey. If you do the rest of somethingNeedingAccessKey's work inside the closure being passed to getAccessKey, you can be sure that your accessKey will always be valid. If you need another function that needs accessKey, just write it the same way somethingNeedingAccessKey is written.
public func somethingElse() {
getAccessKey { (accessKey, error) in
if accessKey != nil {
// Do something else with accessKey
} else {
// Handle nil accessKey / error here
}
}
}
I am trying to make a POST request in Alamofire to return a JSON object. This code worked in Swift 1, but in swift 2 I get this invalid parameter issue:
Tuple types '(NSURLRequest?, NSHTTPURLResponse?, Result<AnyObject>)' (aka '(Optional<NSURLRequest>, Optional<NSHTTPURLResponse>, Result<AnyObject>)') and '(_, _, _, _)' have a different number of elements (3 vs. 4)
It seems like the error parameter was removed, but I am using the error parameter inside the function to check for errors, so how would I do that without an error param?
Here's my code for the POST request:
let response = Alamofire.request(.POST, urlPath, parameters: parameters, encoding: .URL)
.responseJSON { (request, response, data, error) in
if let anError = error
{
// got an error in getting the data, need to handle it
print("error calling POST on /posts")
print(error)
}
else if let data: AnyObject = data
{
// handle the results as JSON, without a bunch of nested if loops
let post = JSON(data)
// to make sure it posted, print the results
print("The post is: " + post.description)
}
}
If you see the documentation in the branch Swift2.0 you can see that the responseJSON function has changed as the error says, it have now three parameters but you can catch the error too, lets take a look:
public func responseJSON(
options options: NSJSONReadingOptions = .AllowFragments,
completionHandler: (NSURLRequest?, NSHTTPURLResponse?, Result<AnyObject>) -> Void)
-> Self
{
return response(
responseSerializer: Request.JSONResponseSerializer(options: options),
completionHandler: completionHandler
)
}
Now it returns an enum Result<AnyObject> and according to the doc :
Used to represent whether a request was successful or encountered an error.
- Success: The request and all post processing operations were successful resulting in the serialization of the
provided associated value.
- Failure: The request encountered an error resulting in a failure. The associated values are the original data
provided by the server as well as the error that caused the failure.
And it have inside an property entitled error, with the following doc:
/// Returns the associated error value if the result is a failure, `nil` otherwise.
public var error: ErrorType? {
switch self {
case .Success:
return nil
case .Failure(_, let error):
return error
}
}
Then if you follow this test case inside Alamofire you can see how to get the error properly:
func testThatResponseJSONReturnsSuccessResultWithValidJSON() {
// Given
let URLString = "https://httpbin.org/get"
let expectation = expectationWithDescription("request should succeed")
var request: NSURLRequest?
var response: NSHTTPURLResponse?
var result: Result<AnyObject>!
// When
Alamofire.request(.GET, URLString, parameters: ["foo": "bar"])
.responseJSON { responseRequest, responseResponse, responseResult in
request = responseRequest
response = responseResponse
result = responseResult
expectation.fulfill()
}
waitForExpectationsWithTimeout(defaultTimeout, handler: nil)
// Then
XCTAssertNotNil(request, "request should not be nil")
XCTAssertNotNil(response, "response should not be nil")
XCTAssertTrue(result.isSuccess, "result should be success")
XCTAssertNotNil(result.value, "result value should not be nil")
XCTAssertNil(result.data, "result data should be nil")
XCTAssertTrue(result.error == nil, "result error should be nil")
}
UPDATE:
Alamofire 3.0.0 introduces a Response struct. All response serializers (with the exception of response) return a generic Response struct.
public struct Response<Value, Error: ErrorType> {
/// The URL request sent to the server.
public let request: NSURLRequest?
/// The server's response to the URL request.
public let response: NSHTTPURLResponse?
/// The data returned by the server.
public let data: NSData?
/// The result of response serialization.
public let result: Result<Value, Error>
}
So you can call it like the following way:
Alamofire.request(.GET, "http://httpbin.org/get")
.responseJSON { response in
debugPrint(response)
}
You can read more about the migration process in the Alamofire 3.0 Migration Guide.
I hope this help you.
Have you tried using three parameters in the .responseJSON and inserting a try catch block around the areas you want error logging, check out this link if you need help with swift 2 try catchs
I haven't read too much into Swift but one thing I noticed is that there are no exceptions.
So how do they do error handling in Swift? Has anyone found anything related to error-handling?
Swift 2 & 3
Things have changed a bit in Swift 2, as there is a new error-handling mechanism, that is somewhat more similar to exceptions but different in detail.
1. Indicating error possibility
If function/method wants to indicate that it may throw an error, it should contain throws keyword like this
func summonDefaultDragon() throws -> Dragon
Note: there is no specification for type of error the function actually can throw. This declaration simply states that the function can throw an instance of any type implementing ErrorType or is not throwing at all.
2. Invoking function that may throw errors
In order to invoke function you need to use try keyword, like this
try summonDefaultDragon()
this line should normally be present do-catch block like this
do {
let dragon = try summonDefaultDragon()
} catch DragonError.dragonIsMissing {
// Some specific-case error-handling
} catch DragonError.notEnoughMana(let manaRequired) {
// Other specific-case error-handlng
} catch {
// Catch all error-handling
}
Note: catch clause use all the powerful features of Swift pattern matching so you are very flexible here.
You may decided to propagate the error, if your are calling a throwing function from a function that is itself marked with throws keyword:
func fulfill(quest: Quest) throws {
let dragon = try summonDefaultDragon()
quest.ride(dragon)
}
Alternatively, you can call throwing function using try?:
let dragonOrNil = try? summonDefaultDragon()
This way you either get the return value or nil, if any error occurred. Using this way you do not get the error object.
Which means that you can also combine try? with useful statements like:
if let dragon = try? summonDefaultDragon()
or
guard let dragon = try? summonDefaultDragon() else { ... }
Finally, you can decide that you know that error will not actually occur (e.g. because you have already checked are prerequisites) and use try! keyword:
let dragon = try! summonDefaultDragon()
If the function actually throws an error, then you'll get a runtime error in your application and the application will terminate.
3. Throwing an error
In order to throw an error you use throw keyword like this
throw DragonError.dragonIsMissing
You can throw anything that conforms to ErrorType protocol. For starters NSError conforms to this protocol but you probably would like to go with enum-based ErrorType which enables you to group multiple related errors, potentially with additional pieces of data, like this
enum DragonError: ErrorType {
case dragonIsMissing
case notEnoughMana(requiredMana: Int)
...
}
Main differences between new Swift 2 & 3 error mechanism and Java/C#/C++ style exceptions are follows:
Syntax is a bit different: do-catch + try + defer vs traditional try-catch-finally syntax.
Exception handling usually incurs much higher execution time in exception path than in success path. This is not the case with Swift 2.0 errors, where success path and error path cost roughly the same.
All error throwing code must be declared, while exceptions might have been thrown from anywhere. All errors are "checked exceptions" in Java nomenclature. However, in contrast to Java, you do not specify potentially thrown errors.
Swift exceptions are not compatible with ObjC exceptions. Your do-catch block will not catch any NSException, and vice versa, for that you must use ObjC.
Swift exceptions are compatible with Cocoa NSError method conventions of returning either false (for Bool returning functions) or nil (for AnyObject returning functions) and passing NSErrorPointer with error details.
As an extra syntatic-sugar to ease error handling, there are two more concepts
deferred actions (using defer keyword) which let you achieve the same effect as finally blocks in Java/C#/etc
guard statement (using guard keyword) which let you write little less if/else code than in normal error checking/signaling code.
Swift 1
Runtime errors:
As Leandros suggests for handling runtime errors (like network connectivity problems, parsing data, opening file, etc) you should use NSError like you did in ObjC, because the Foundation, AppKit, UIKit, etc report their errors in this way. So it's more framework thing than language thing.
Another frequent pattern that is being used are separator success/failure blocks like in AFNetworking:
var sessionManager = AFHTTPSessionManager(baseURL: NSURL(string: "yavin4.yavin.planets"))
sessionManager.HEAD("/api/destoryDeathStar", parameters: xwingSquad,
success: { (NSURLSessionDataTask) -> Void in
println("Success")
},
failure:{ (NSURLSessionDataTask, NSError) -> Void in
println("Failure")
})
Still the failure block frequently received NSError instance, describing the error.
Programmer errors:
For programmer errors (like out of bounds access of array element, invalid arguments passed to a function call, etc) you used exceptions in ObjC. Swift language does not seem to have any language support for exceptions (like throw, catch, etc keyword). However, as documentation suggests it is running on the same runtime as ObjC, and therefore you are still able to throw NSExceptions like this:
NSException(name: "SomeName", reason: "SomeReason", userInfo: nil).raise()
You just cannot catch them in pure Swift, although you may opt for catching exceptions in ObjC code.
The questions is whether you should throw exceptions for programmer errors, or rather use assertions as Apple suggests in the language guide.
Update June 9th 2015 - Very important
Swift 2.0 comes with try, throw, and catch keywords and the most exciting is:
Swift automatically translates Objective-C methods that produce errors into methods that throw an error according to Swift's native error handling functionality.
Note: Methods that consume errors, such as delegate methods or methods
that take a completion handler with an NSError object argument, do not
become methods that throw when imported by Swift.
Excerpt From: Apple Inc. “Using Swift with Cocoa and Objective-C (Swift 2 Prerelease).” iBooks.
Example: (from the book)
NSFileManager *fileManager = [NSFileManager defaultManager];
NSURL *URL = [NSURL fileURLWithPath:#"/path/to/file"];
NSError *error = nil;
BOOL success = [fileManager removeItemAtURL:URL error:&error];
if (!success && error){
NSLog(#"Error: %#", error.domain);
}
The equivalent in swift will be:
let fileManager = NSFileManager.defaultManager()
let URL = NSURL.fileURLWithPath("path/to/file")
do {
try fileManager.removeItemAtURL(URL)
} catch let error as NSError {
print ("Error: \(error.domain)")
}
Throwing an Error:
*errorPtr = [NSError errorWithDomain:NSURLErrorDomain code:NSURLErrorCannotOpenFile userInfo: nil]
Will be automatically propagated to the caller:
throw NSError(domain: NSURLErrorDomain, code: NSURLErrorCannotOpenFile, userInfo: nil)
From Apple books, The Swift Programming Language it's seems errors should be handle using enum.
Here is an example from the book.
enum ServerResponse {
case Result(String, String)
case Error(String)
}
let success = ServerResponse.Result("6:00 am", "8:09 pm")
let failure = ServerResponse.Error("Out of cheese.")
switch success {
case let .Result(sunrise, sunset):
let serverResponse = "Sunrise is at \(sunrise) and sunset is at \(sunset)."
case let .Error(error):
let serverResponse = "Failure... \(error)"
}
From: Apple Inc. “The Swift Programming Language.” iBooks. https://itun.es/br/jEUH0.l
Update
From Apple news books, "Using Swift with Cocoa and Objective-C". Runtime exceptions not occur using swift languages, so that's why you don't have try-catch. Instead you use Optional Chaining.
Here is a stretch from the book:
For example, in the code listing below, the first and second lines are
not executed because the length property and the characterAtIndex:
method do not exist on an NSDate object. The myLength constant is
inferred to be an optional Int, and is set to nil. You can also use an
if–let statement to conditionally unwrap the result of a method that
the object may not respond to, as shown on line three
let myLength = myObject.length?
let myChar = myObject.characterAtIndex?(5)
if let fifthCharacter = myObject.characterAtIndex(5) {
println("Found \(fifthCharacter) at index 5")
}
Excerpt From: Apple Inc. “Using Swift with Cocoa and Objective-C.” iBooks. https://itun.es/br/1u3-0.l
And the books also encourage you to use cocoa error pattern from Objective-C (NSError Object)
Error reporting in Swift follows the same pattern it does in
Objective-C, with the added benefit of offering optional return
values. In the simplest case, you return a Bool value from the
function to indicate whether or not it succeeded. When you need to
report the reason for the error, you can add to the function an
NSError out parameter of type NSErrorPointer. This type is roughly
equivalent to Objective-C’s NSError **, with additional memory safety
and optional typing. You can use the prefix & operator to pass in a
reference to an optional NSError type as an NSErrorPointer object, as
shown in the code listing below.
var writeError : NSError?
let written = myString.writeToFile(path, atomically: false,
encoding: NSUTF8StringEncoding,
error: &writeError)
if !written {
if let error = writeError {
println("write failure: \(error.localizedDescription)")
}
}
Excerpt From: Apple Inc. “Using Swift with Cocoa and Objective-C.” iBooks. https://itun.es/br/1u3-0.l
There are no Exceptions in Swift, similar to Objective-C's approach.
In development, you can use assert to catch any errors which might appear, and need to be fixed before going to production.
The classic NSError approach isn't altered, you send an NSErrorPointer, which gets populated.
Brief example:
var error: NSError?
var contents = NSFileManager.defaultManager().contentsOfDirectoryAtPath("/Users/leandros", error: &error)
if let error = error {
println("An error occurred \(error)")
} else {
println("Contents: \(contents)")
}
The recommended 'Swift Way' is:
func write(path: String)(#error: NSErrorPointer) -> Bool { // Useful to curry error parameter for retrying (see below)!
return "Hello!".writeToFile(path, atomically: false, encoding: NSUTF8StringEncoding, error: error)
}
var writeError: NSError?
let written = write("~/Error1")(error: &writeError)
if !written {
println("write failure 1: \(writeError!.localizedDescription)")
// assert(false) // Terminate program
}
However I prefer try/catch as I find it easier to follow because it moves the error handling to a separate block at the end, this arrangement is sometimes called "Golden Path". Lucky you can do this with closures:
TryBool {
write("~/Error2")(error: $0) // The code to try
}.catch {
println("write failure 2: \($0!.localizedDescription)") // Report failure
// assert(false) // Terminate program
}
Also it is easy to add a retry facility:
TryBool {
write("~/Error3")(error: $0) // The code to try
}.retry {
println("write failure 3 on try \($1 + 1): \($0!.localizedDescription)")
return write("~/Error3r") // The code to retry
}.catch {
println("write failure 3 catch: \($0!.localizedDescription)") // Report failure
// assert(false) // Terminate program
}
The listing for TryBool is:
class TryBool {
typealias Tryee = NSErrorPointer -> Bool
typealias Catchee = NSError? -> ()
typealias Retryee = (NSError?, UInt) -> Tryee
private var tryee: Tryee
private var retries: UInt = 0
private var retryee: Retryee?
init(tryee: Tryee) {
self.tryee = tryee
}
func retry(retries: UInt, retryee: Retryee) -> Self {
self.retries = retries
self.retryee = retryee
return self
}
func retry(retryee: Retryee) -> Self {
return self.retry(1, retryee)
}
func retry(retries: UInt) -> Self {
// For some reason you can't write the body as "return retry(1, nil)", the compiler doesn't like the nil
self.retries = retries
retryee = nil
return self
}
func retry() -> Self {
return retry(1)
}
func catch(catchee: Catchee) {
var error: NSError?
for numRetries in 0...retries { // First try is retry 0
error = nil
let result = tryee(&error)
if result {
return
} else if numRetries != retries {
if let r = retryee {
tryee = r(error, numRetries)
}
}
}
catchee(error)
}
}
You can write a similar class for testing an Optional returned value instead of Bool value:
class TryOptional<T> {
typealias Tryee = NSErrorPointer -> T?
typealias Catchee = NSError? -> T
typealias Retryee = (NSError?, UInt) -> Tryee
private var tryee: Tryee
private var retries: UInt = 0
private var retryee: Retryee?
init(tryee: Tryee) {
self.tryee = tryee
}
func retry(retries: UInt, retryee: Retryee) -> Self {
self.retries = retries
self.retryee = retryee
return self
}
func retry(retryee: Retryee) -> Self {
return retry(1, retryee)
}
func retry(retries: UInt) -> Self {
// For some reason you can't write the body as "return retry(1, nil)", the compiler doesn't like the nil
self.retries = retries
retryee = nil
return self
}
func retry() -> Self {
return retry(1)
}
func catch(catchee: Catchee) -> T {
var error: NSError?
for numRetries in 0...retries {
error = nil
let result = tryee(&error)
if let r = result {
return r
} else if numRetries != retries {
if let r = retryee {
tryee = r(error, numRetries)
}
}
}
return catchee(error)
}
}
The TryOptional version enforces a non-Optional return type that makes subsequent programming easier, e.g. 'Swift Way:
struct FailableInitializer {
init?(_ id: Int, error: NSErrorPointer) {
// Always fails in example
if error != nil {
error.memory = NSError(domain: "", code: id, userInfo: [:])
}
return nil
}
private init() {
// Empty in example
}
static let fallback = FailableInitializer()
}
func failableInitializer(id: Int)(#error: NSErrorPointer) -> FailableInitializer? { // Curry for retry
return FailableInitializer(id, error: error)
}
var failError: NSError?
var failure1Temp = failableInitializer(1)(error: &failError)
if failure1Temp == nil {
println("failableInitializer failure code: \(failError!.code)")
failure1Temp = FailableInitializer.fallback
}
let failure1 = failure1Temp! // Unwrap
Using TryOptional:
let failure2 = TryOptional {
failableInitializer(2)(error: $0)
}.catch {
println("failableInitializer failure code: \($0!.code)")
return FailableInitializer.fallback
}
let failure3 = TryOptional {
failableInitializer(3)(error: $0)
}.retry {
println("failableInitializer failure, on try \($1 + 1), code: \($0!.code)")
return failableInitializer(31)
}.catch {
println("failableInitializer failure code: \($0!.code)")
return FailableInitializer.fallback
}
Note auto-unwrapping.
Edit: Although this answer works, it is little more than Objective-C transliterated into Swift. It has been made obsolete by changes in Swift 2.0. Guilherme Torres Castro's answer above is a very good introduction to the preferred way of handling errors in Swift. VOS
It took a bit of figuring it out but I think I've sussed it. It seems ugly though. Nothing more than a thin skin over the Objective-C version.
Calling a function with an NSError parameter...
var fooError : NSError ? = nil
let someObject = foo(aParam, error:&fooError)
// Check something was returned and look for an error if it wasn't.
if !someObject {
if let error = fooError {
// Handle error
NSLog("This happened: \(error.localizedDescription)")
}
} else {
// Handle success
}`
Writing the function that takes an error parameter...
func foo(param:ParamObject, error: NSErrorPointer) -> SomeObject {
// Do stuff...
if somethingBadHasHappened {
if error {
error.memory = NSError(domain: domain, code: code, userInfo: [:])
}
return nil
}
// Do more stuff...
}
Basic wrapper around objective C that gives you the try catch feature.
https://github.com/williamFalcon/SwiftTryCatch
Use like:
SwiftTryCatch.try({ () -> Void in
//try something
}, catch: { (error) -> Void in
//handle error
}, finally: { () -> Void in
//close resources
})
As Guilherme Torres Castro said, in Swift 2.0, try, catch, do can be used in the programming.
For example, In CoreData fetch data method, instead of put &error as a parameter into the managedContext.executeFetchRequest(fetchRequest, error: &error), now we only need to use use managedContext.executeFetchRequest(fetchRequest) and then handle the error with try, catch (Apple Document Link)
do {
let fetchedResults = try managedContext.executeFetchRequest(fetchRequest) as? [NSManagedObject]
if let results = fetchedResults{
people = results
}
} catch {
print("Could not fetch")
}
If you have already download the xcode7 Beta. Try to search throwing errors in Documentations and API Reference and choose the first showing result, it gives a basic idea what can be done for this new syntax. However, fully documentation is not post for many APIs yet.
More fancy Error Handling techniques can be found in
What's New in Swift (2015 Session 106 28m30s)
This is an update answer for swift 2.0. I am looking forward feature rich Error handling model like in java. Finally, they announced the good news. here
Error handling model: The new error handling model in Swift 2.0 will
instantly feel natural, with familiar try, throw, and catch keywords.
Best of all, it was designed to work perfectly with the Apple SDKs and
NSError. In fact, NSError conforms to a Swift’s ErrorType. You’ll
definitely want to watch the WWDC session on What’s New in Swift to
hear more about it.
e.g :
func loadData() throws { }
func test() {
do {
try loadData()
} catch {
print(error)
}}
Starting with Swift 2, as others have already mentioned, error handling is best accomplished through the use of do/try/catch and ErrorType enums. This works quite well for synchronous methods, but a little cleverness is required for asynchronous error handling.
This article has a great approach to this problem:
https://jeremywsherman.com/blog/2015/06/17/using-swift-throws-with-completion-callbacks/
To summarize:
// create a typealias used in completion blocks, for cleaner code
typealias LoadDataResult = () throws -> NSData
// notice the reference to the typealias in the completionHandler
func loadData(someID: String, completionHandler: LoadDataResult -> Void)
{
completionHandler()
}
then, the call to the above method would be as follows:
self.loadData("someString",
completionHandler:
{ result: LoadDataResult in
do
{
let data = try result()
// success - go ahead and work with the data
}
catch
{
// failure - look at the error code and handle accordingly
}
})
This seems a bit cleaner than having a separate errorHandler callback passed to the asynchronous function, which was how this would be handled prior to Swift 2.
Error handling is a new feature of Swift 2.0. It uses the try, throw and catch keywords.
See the Apple Swift 2.0 announcement on the official Apple Swift blog
Nice and simple lib to handle exception:
TryCatchFinally-Swift
Like a few others it wraps around the objective C exception features.
Use it like this:
try {
println(" try")
}.catch { e in
println(" catch")
}.finally {
println(" finally")
}
enum CheckValidAge : Error{
case overrage
case underage
}
func checkValidAgeForGovernmentJob(age:Int)throws -> Bool{
if age < 18{
throw CheckValidAge.underage
}else if age > 25{
throw CheckValidAge.overrage
}else{
return true
}
}
do {
try checkValidAgeForGovernmentJob(age: 26)
print("You are valid for government job ")
}catch CheckValidAge.underage{
print("You are underage for government job ")
}catch CheckValidAge.overrage{
print("You are overrage for government job ")
}
Change age in try checkValidAgeForGovernmentJob(age: 26)
Out Put
You are overrage for government job
What I have seen is that because of the nature of the device you don't want to be throwing a bunch of cryptic error handling messages at the user. That is why most functions return optional values then you just code to ignore the optional. If a function comes back nil meaning it failed you can pop a message or whatever.