I am coding in Swift and attempting to use the findObjects() function for the Parse iOS SDK. However, I can't seem to figure out what type of error is being thrown by Parse if this function call fails. I'm a novice in Swift so that may be my issue. I attempt to call the function in a do->catch block and use the try keyword on the function call however I'm not sure what to catch. I can catch the error using the _ but I would like to grab the description from the error. Thanks!
P.S. I don't want to use the findObjectsInBackground() method.
do {
let object = try query.getFirstObject()
// do something with the object
} catch _ {
// this is where I would like to print out the error description
}
In Obj-C, which I assume will be similar, I print out the error.userInfo[#"error"] parameter of the NSError that is returned.
All you need is print(error). An example here:
func getReferenceNumberAsStringSync() -> String? {
let query = PFQuery(className: "PropertyCount")
do {
let object = try query.getFirstObject()
if let referenceNumber = object["count"] as? Int {
return String(referenceNumber)
}
} catch {
print(error)
}
return nil
}
Related
I'm new in iOS development, so maybe I'm thinking in the wrong way. I coded a view model with a function that calls an API, and everything works fine.
class SearchCityViewModel : ViewModelProtocol {
//OBSERVABLES
var cities = PublishSubject<[City]>()
var networkError = PublishSubject<Void>()
var generalError = PublishSubject<Void>()
init(){
print("Init SearchCityViewModel")
reinit()
}
func reinit(){}
func searchCity(stringToSearch: String){
async {
do {
if stringToSearch.count>=2 {
let cities = try await(api.getCities(cityToSearch: stringToSearch)).payload!
self.cities.onNext(cities)
}
else {
self.cities.onNext([])
}
}
catch {
self.generalError.onNext(Void())
}
}
}
Now I want to handle errors. In the catch block I want to distinguish all the errors I want to handle gracefully, and for the other ones I just want to emit a general error. To do that, firstly I need to know which error is thrown when the situation I want to handle occurs. I usually do this with the debugger. For instance, I disable the internet connection, and i create a breakpoint inside the catch block. The idea is to check which error is thrown when the internet connection is disabled, in order to create a catch block for that kind of error.
Image of the debugger
I'm struggling because with the debugger I only see that is an AFError instance, but it's not telling me nothing more that can help me to catch it.
What is wrong with my workflow? Do I really need to read all the docs every time? For each library I use?
Thank you!
Perhaps you can read the articles and then you will know how to do it better, you can use the framework -oslog instead of using print function.
debugging your logging info
I found the way. What I was missing is casting the error as NSError. In this way, with the debugger is possible to see the domain and the code of the error. In the case of Alamofire, the real error is wrapped, and it's accessible through the underlyingError attribute. Once I had the domain and the code of the error, I wrote the following code:
class SearchCityViewModel : ViewModelProtocol {
//OBSERVABLES
var cities = PublishSubject<[City]>()
var networkError = PublishSubject<Void>()
var generalError = PublishSubject<Void>()
init(){
print("Init SearchCityViewModel")
reinit()
}
func reinit(){}
func searchCity(stringToSearch: String){
async {
do {
if stringToSearch.count>=2 {
let cities = try await(api.getCities(cityToSearch: stringToSearch)).payload!
self.cities.onNext(cities)
}
else {
self.cities.onNext([])
}
}
catch {
if let afError = asAFError, let underlyingError = afError.underlyingError as NSError?, underlyingError.domain == NSURLErrorDomain, underlyingError.code == NSURLErrorNotConnectedToInternet || underlyingError.code == NSURLErrorTimedOut {
self.networkError.onNext(Void())
}
else {
self.generalError.onNext(Void())
}
}
}
}
I used this method very much in Swift 1.2: NSURLConnection.sendSynchronousRequest(:_:_:_) but this is apparently deprecated in iOS9. It still works however but now it uses the new Swift 2.0 Error Handling and I don't know how I will get the error message if it fails, ex. if time runs out.
I know I have to put it into a do-catch and then say try before the metho but I dont know how to catch the error message.
do {
let data = try NSURLConnection.sendSynchronousRequest(request, returningResponse: nil)
return data
}
catch _ {
return nil
}
Before I used NSError and then its description property, but now I have no clue.
Use automatic error variable, and you can cast it to NSError if you wish:
catch {
let nsError = error as NSError
print(nsError.localizedDescription)
}
You can now throw any object inheriting ErrorType, and provide custom handling in the catch sentence. You can also cast the error to NSError to access localizedDescription for handling third party errors.
Casting an enum ErrorType will produce a NSError with domain equal to the enum name, code equal to the enum value and an auto-generated localizedDescription with the following format:
The operation couldn’t be completed. (DOMAIN error CODE.)
For example, the following code:
enum AwfulError: ErrorType {
case Bad
case Worse
case Terrible
}
func throwingFunction() throws {
throw AwfulError.Worse
}
do {
try throwingFunction()
}
catch AwfulError.Bad {
print("Bad error")
}
catch let error as NSError {
print(error.localizedDescription)
}
Will print
The operation couldn’t be completed. (AwfulError error 1.)
Despite the question title specifying Swift 2, this answer is for Swift 3.
As #redent84 points out, since Swift 2 an Error object may be a home-made one. Here's a method I wrote to analyze and print the default error object available in a "catch" statement that doesn't specify any specific error type:
// Method to print an unknown Error type object to the system output.
static func printCaughtError(_ unknownError : Error) {
let objectDescription = String(describing: unknownError)
let localizedDescription = unknownError.localizedDescription
if localizedDescription != "" {
if localizedDescription.contains(objectDescription) {
print(localizedDescription)
return
}
if !objectDescription.contains(localizedDescription) {
print(objectDescription + ": " + localizedDescription)
return
}
}
print(objectDescription)
}
Then you can call it like this:
catch {
printCaughtError(error)
}
How to get the error message that is inside userInfo:
let errorMessage = (error as NSError).userInfo["message"] as? String
I'm trying to convert a CNContact array to vCard using the method CNContactVCardSerialization.dataWithContacts(). But it is giving me the following error.
2016-07-25 14:05:00.115 AddressBook-ios9[902:28918] Exception writing contacts to vCard (data): A property was not requested when contact was fetched.
I made sure that I'm passing an valid array of CNContacts, but still it is giving this exception. Can anybody guide to me to what I've done wrong?
I'm attaching the source code below.
func getVcardFromSearchingName(name: String) -> NSData? {
do {
if let contacts = searchMultiContacts(name) {
print(contacts)
let vCard = try CNContactVCardSerialization.dataWithContacts(contacts)
return vCard
} else {
return nil
}
} catch {
return nil
}
}
I found out my mistake. On the keys to fetch contact, I was missing CNContactVCardSerialization.descriptorForRequiredKeys(). After adding it, the code is working flawlessly.
I used this method very much in Swift 1.2: NSURLConnection.sendSynchronousRequest(:_:_:_) but this is apparently deprecated in iOS9. It still works however but now it uses the new Swift 2.0 Error Handling and I don't know how I will get the error message if it fails, ex. if time runs out.
I know I have to put it into a do-catch and then say try before the metho but I dont know how to catch the error message.
do {
let data = try NSURLConnection.sendSynchronousRequest(request, returningResponse: nil)
return data
}
catch _ {
return nil
}
Before I used NSError and then its description property, but now I have no clue.
Use automatic error variable, and you can cast it to NSError if you wish:
catch {
let nsError = error as NSError
print(nsError.localizedDescription)
}
You can now throw any object inheriting ErrorType, and provide custom handling in the catch sentence. You can also cast the error to NSError to access localizedDescription for handling third party errors.
Casting an enum ErrorType will produce a NSError with domain equal to the enum name, code equal to the enum value and an auto-generated localizedDescription with the following format:
The operation couldn’t be completed. (DOMAIN error CODE.)
For example, the following code:
enum AwfulError: ErrorType {
case Bad
case Worse
case Terrible
}
func throwingFunction() throws {
throw AwfulError.Worse
}
do {
try throwingFunction()
}
catch AwfulError.Bad {
print("Bad error")
}
catch let error as NSError {
print(error.localizedDescription)
}
Will print
The operation couldn’t be completed. (AwfulError error 1.)
Despite the question title specifying Swift 2, this answer is for Swift 3.
As #redent84 points out, since Swift 2 an Error object may be a home-made one. Here's a method I wrote to analyze and print the default error object available in a "catch" statement that doesn't specify any specific error type:
// Method to print an unknown Error type object to the system output.
static func printCaughtError(_ unknownError : Error) {
let objectDescription = String(describing: unknownError)
let localizedDescription = unknownError.localizedDescription
if localizedDescription != "" {
if localizedDescription.contains(objectDescription) {
print(localizedDescription)
return
}
if !objectDescription.contains(localizedDescription) {
print(objectDescription + ": " + localizedDescription)
return
}
}
print(objectDescription)
}
Then you can call it like this:
catch {
printCaughtError(error)
}
How to get the error message that is inside userInfo:
let errorMessage = (error as NSError).userInfo["message"] as? String
After I installed Xcode 7 beta and convert my swift code to Swift 2, I got some issue with the code that I can't figure out. I know Swift 2 is new so I search and figure out since there is nothing about it, I should write a question.
Here is the error:
Call can throw, but it is not marked with 'try' and the error is not
handled
Code:
func deleteAccountDetail(){
let entityDescription = NSEntityDescription.entityForName("AccountDetail", inManagedObjectContext: Context!)
let request = NSFetchRequest()
request.entity = entityDescription
//The Line Below is where i expect the error
let fetchedEntities = self.Context!.executeFetchRequest(request) as! [AccountDetail]
for entity in fetchedEntities {
self.Context!.deleteObject(entity)
}
do {
try self.Context!.save()
} catch _ {
}
}
Snapshot:
You have to catch the error just as you're already doing for your save() call and since you're handling multiple errors here, you can try multiple calls sequentially in a single do-catch block, like so:
func deleteAccountDetail() {
let entityDescription = NSEntityDescription.entityForName("AccountDetail", inManagedObjectContext: Context!)
let request = NSFetchRequest()
request.entity = entityDescription
do {
let fetchedEntities = try self.Context!.executeFetchRequest(request) as! [AccountDetail]
for entity in fetchedEntities {
self.Context!.deleteObject(entity)
}
try self.Context!.save()
} catch {
print(error)
}
}
Or as #bames53 pointed out in the comments below, it is often better practice not to catch the error where it was thrown. You can mark the method as throws then try to call the method. For example:
func deleteAccountDetail() throws {
let entityDescription = NSEntityDescription.entityForName("AccountDetail", inManagedObjectContext: Context!)
let request = NSFetchRequest()
request.entity = entityDescription
let fetchedEntities = try Context.executeFetchRequest(request) as! [AccountDetail]
for entity in fetchedEntities {
self.Context!.deleteObject(entity)
}
try self.Context!.save()
}
When calling a function that is declared with throws in Swift, you must annotate the function call site with try or try!. For example, given a throwing function:
func willOnlyThrowIfTrue(value: Bool) throws {
if value { throw someError }
}
this function can be called like:
func foo(value: Bool) throws {
try willOnlyThrowIfTrue(value)
}
Here we annotate the call with try, which calls out to the reader that this function may throw an exception, and any following lines of code might not be executed. We also have to annotate this function with throws, because this function could throw an exception (i.e., when willOnlyThrowIfTrue() throws, then foo will automatically rethrow the exception upwards.
If you want to call a function that is declared as possibly throwing, but which you know will not throw in your case because you're giving it correct input, you can use try!.
func bar() {
try! willOnlyThrowIfTrue(false)
}
This way, when you guarantee that code won't throw, you don't have to put in extra boilerplate code to disable exception propagation.
try! is enforced at runtime: if you use try! and the function does end up throwing, then your program's execution will be terminated with a runtime error.
Most exception handling code should look like the above: either you simply propagate exceptions upward when they occur, or you set up conditions such that otherwise possible exceptions are ruled out. Any clean up of other resources in your code should occur via object destruction (i.e. deinit()), or sometimes via defered code.
func baz(value: Bool) throws {
var filePath = NSBundle.mainBundle().pathForResource("theFile", ofType:"txt")
var data = NSData(contentsOfFile:filePath)
try willOnlyThrowIfTrue(value)
// data and filePath automatically cleaned up, even when an exception occurs.
}
If for whatever reason you have clean up code that needs to run but isn't in a deinit() function, you can use defer.
func qux(value: Bool) throws {
defer {
print("this code runs when the function exits, even when it exits by an exception")
}
try willOnlyThrowIfTrue(value)
}
Most code that deals with exceptions simply has them propagate upward to callers, doing cleanup on the way via deinit() or defer. This is because most code doesn't know what to do with errors; it knows what went wrong, but it doesn't have enough information about what some higher level code is trying to do in order to know what to do about the error. It doesn't know if presenting a dialog to the user is appropriate, or if it should retry, or if something else is appropriate.
Higher level code, however, should know exactly what to do in the event of any error. So exceptions allow specific errors to bubble up from where they initially occur to the where they can be handled.
Handling exceptions is done via catch statements.
func quux(value: Bool) {
do {
try willOnlyThrowIfTrue(value)
} catch {
// handle error
}
}
You can have multiple catch statements, each catching a different kind of exception.
do {
try someFunctionThatThowsDifferentExceptions()
} catch MyErrorType.errorA {
// handle errorA
} catch MyErrorType.errorB {
// handle errorB
} catch {
// handle other errors
}
For more details on best practices with exceptions, see http://exceptionsafecode.com/. It's specifically aimed at C++, but after examining the Swift exception model, I believe the basics apply to Swift as well.
For details on the Swift syntax and error handling model, see the book The Swift Programming Language (Swift 2 Prerelease).