I'm currently building a chat app and when I click on a user, it takes me to the chat log controller. Here I call a function fetchChatMessages() in viewdidload() that essentially fetches the conversation from firestore. Problem is, whenever I go to the previous controller and open the chat again, it again fetches the messages.
Not sure if it's fetching from cache or from the server itself. But I did write a print statement under the firestore fetch code that prints every time.
Now I'm new to swift so my question is, in other chat apps, you can see that messages seemed to be fetched just once from the server and after that you add a listener and update the collection view to display the new messages. In my case, it seems like everything is fetched over and over again. Even though I have added a listener and followed a highly acclaimed tutorial.
Also, I added a scroll to bottom code whenever messages are fetched, so every time a new message is fetched, the controller automatically scrolls to the bottom. But this happens every time I open the chat. I was trying to fix this bug where the controller keeps scrolling every time the view appears which made me wonder, am I contacting firebase again and again when the controller is opened?
override func viewDidLoad() {
super.viewDidLoad()
fetchCurrentUser()
fetchMessages()
setupLoadView()
}
//MARK: Fetch Messages
var listener: ListenerRegistration?
fileprivate func fetchMessages(){
print("Fetching Messages")
guard let cUid = Auth.auth().currentUser?.uid else {return}
let query = Firestore.firestore().collection("matches").document(cUid).collection(connect.uid).order(by: "Timestamp")
listener = query.addSnapshotListener { (querySnapshot, error) in
if let error = error{
print("There was an error fetching messages", error.localizedDescription)
return
}
querySnapshot?.documentChanges.forEach({ (change) in
if change.type == .added{
let dictionary = change.document.data()
self.items.append(.init(dictionary: dictionary))
print("FIRESTORE HAS BEEN CONTACTED FETCHING MESSAGES")
}
})
self.collectionView.reloadData()
self.collectionView.scrollToItem(at: [0, self.items.count - 1], at: .bottom, animated: true)
print("Fetched messages")
}
}
//MARK: View Disappears
override func viewWillDisappear(_ animated: Bool) {
super.viewWillDisappear(animated)
if isMovingFromParent{
listener?.remove()
}
}
I want the controller to remember its state. Like if I scroll the messages, exit the controller and enter it again, it stays at the scrolled position like in whatsapp.
The problem here is that your controller gets deallocated every time you leave the screen, because you probably push the controller on to the stack and pop it afterwards, this will erase all of the internal state of the controller. This behavior is indeed intended (viewDidLoad is called once the screen is loaded). You can solve this problem in several ways. An easy one would be to introduce a singleton service (a service which is shared across the app) which holds the state of the controller, so every time the controller is created it will ask the service for its state. Keep in mind this is not a very good solution, but it should be sufficient as starting point. If you need an example I will edit my answer accordingly later on.
I cannot give you a definite answer on this, because it really depends on what features the app and the server support, in the end I would have some kind of database service and chat services (these should not be accessed over the singleton pattern, but rather via dependency injection). The chat service would define some kind of policy when the data should be fetched, which means the controller should not be aware of this. The chat service would then store the messages via the database layer in some persistent store like user defaults, realm or core data for each chat. Every time the user enters an already fetched chat the chat service will check if persisted data is available if not it will fetch it from the server.
My question are as follows:
Will creating a custom enum to handle the coredata errors be the best way to handle errors in this case
If the persistent store container doesn't load or we can't save the context then there is no need to use the app and crashing is the best option correct?
In order to present these error messages to the user, wouldn't I have to adapt UIAlertAction? Which would also mean I would need to register notifications?
By law we have to get permission to send notifications, would it be best to create a whole new file for notification & add the error enum in that file or would it make more sense to have the core data stack class conform to the Notification protocol?
The end goal is to notify the user that either the persistent store wouldn't load or the moc wouldn't save
Thanks for your time in advance!
enum CoreDataError: Error {
// This is my custom Error handling enum
case persistenStore(description: String )
case saveChanges(description: String )
}
func coreDataErrors(throwError: Bool) throws -> CoreDataError {
// This is my Error Handling function
}
class CoreDataStack {
lazy var managedObjectContext: NSManagedObjectContext = {
let container = self.persistentContainer
return container.viewContext
}()
private lazy var persistentContainer: NSPersistentContainer = {
let container = NSPersistentContainer(name: "SafeHouseCDPhotoVault")
container.loadPersistentStores() { storeDescription, error in
if let error = error as NSError? {
fatalError("Unresolved error: \(error), \(error.userInfo)")
}
}
return container
}()
}
/* In almost every app tutorial available we learn to deal with errors using this fatal error logic to speed through the project
*/
extension NSManagedObjectContext {
func saveChanges() {
if self.hasChanges {
do {
try save()
} catch {
fatalError("Error: \(error.localizedDescription)")
}
}
}
}
// Again the demos have us deal with errors using fatal error
I typically use enums for error cases, but I wouldn't name them according to their source. Name them according to how you will will them for recovery or error-handler behavior. If you can't handle them, there's not much point in generating them.
That gets to the second question; if you can't handle a case at all (such as the MOM missing on launch), crashing is about all you can do. I don't like alerts in that case. What's the user going to do with that information? A crash at least will be sent to Apple and you can see it and do something about it.
If the user can do something, then absolutely provide an error. If there is any hope that the error is transitory (such as a save failure, which may be due to a full disk), then maybe provide them an error/retry. But on iOS this generally is not worth the trouble and the risk of generating bugs. How are you going to test your error/retry system? If you can't test it, how do you know it's better than crashing? (This isn't an idle question; I once built a crash-catching system that had a bug and caused the crash handler to go into a tight loop and drain the battery rapidly. That's worse than crashing.)
If you're a beginner, then you're probably not in a place to handle uncommon Core Data errors and the best and safest thing you're going to do is crash. Handling these things well is quite complex and difficult to test, and I generally do not recommend it on iOS (macOS is a bit different because write-errors are much more often transitory).
That's up to you.
Exit the app properly with an alert and not by crashing it
Why would you need notifications for showing an alert?
This question makes no sense to me.
I am new in swift3.0 I am implementing a custom search box. I wish to know how can i make a search queue such that on text change in searchbox i need to perform search operation with new text and if there is an existing search operation going on cancel that. I also want to include threshold ontextchanged. So that search operation does not get fired very frequently
Your question is somehow general, but let me tell you how I accomplished this in Swift 3 and AFNetworking (this assumes you wish to search for the data on the server).
I hold a reference of the networking manager in the properties of the view controller:
//The network requests manager. Stored here because this view controller extensively uses AFNetworking to perform live search updates when the input box changes.
var manager = AFHTTPRequestOperationManager()
Afterwards, using UISearchController I check to see if there is any text entered in the search box at all and, if it is, I want to make sure there aren't any other ongoing AFNetworking tasks from now by closing any of them which are still running:
//Called when the something is typed in the search bar.
func updateSearchResults (for searchController: UISearchController) {
if !SCString.isStringValid(searchController.searchBar.text) {
searchController.searchResultsController?.view.isHidden = false
tableView.reloadData()
return
}
data.searchText = searchController.searchBar.text!
/**
Highly important racing issue solution. We cancel any current request going on because we don't want to have the list updated after some time, when we already started another request for a new text. Example:
- Request 1 started at 12:00:01
- We clear the containers because Request 2 has to start
- Request 2 started at 12:00:02
- Request 1 finished at 12:00:04. We update the containers because data arrived
- Request 2 finished at 12:00:05. We update the containers because data arrived
- Now we have data from both 1 and 2, something really not desired.
*/
manager.session.getTasksWithCompletionHandler { (dataTasks, uploadTasks, downloadTasks) in
dataTasks.forEach { $0.cancel() }
}
/**
Reloads the list view because we have to remove the last search results.
*/
reloadListView()
}
In the end, I also check in the failure closure if the code of the error is not NSURLErrorCancelled. Because, if that happened, I don't display any error message or toast.
//The operation might be cancelled by us on purpose. In this case, we don't want to interfere with the ongoing logic flow.
if (operation?.error as! NSError).code == NSURLErrorCancelled {
return
}
self.retrieveResultListFailureNetwork()
Hope it helps!
I'm trying to figure out how I can retry an API call at the ViewController layer when dealing with an RxSwift stream. I understand that I can either wrap my errors in an Element or I can return a stream Error. In either case, it's not at all clear how I can "retry" the operation the discretion of the user (e.g. after putting up an alert).
If I wrap the error, how can/should I communicate back "down" to the API layer to attempt a retry? If I don't wrap the error, I have two questions:
1) How can I make the retry conditional on the user's response to the alert? and
2) How can/should I "re-initialize" the stream?
The only thing I've been able to come up with is passing back a "retry subject" with the error and having the view controller indicate the retry request by emitting something on the associated observable (i.e. assign a value to the subject) which would in term be associated with the API stream. That seems awfully convoluted, however.
More generally, I'd appreciate any references to helpful literature on the subject of application error handling with streams/observables. I feel like I have a pretty good grasp of the RxSwift objects and operators and how to handle the "happy path" situation, but it's just not clear to me how to robustly handle errors in the context of continuously running application.
My solution for this is to wrap them to this type Result<AnyObject,ErrorType>
For example, user login scenario
// where API.login() takes the response data and try to map to JSON
// then try to map into Result<JSON,ErrorType>
let loginStream: Result<JSON,ErrorType> = loginTaps.flatMap { API.login() }
.shareReplay()
let loginResponse = loginStream.filter { $0.value != nil }.map { $0.value }
let loginError = loginStream.filter { $0.error != nil }.map { $0.error }
The stream will never be terminated, so no need to re-subscribe them.
In my application I need to download a JSON file from the web. I have made a ResourceService class that have a download method as seen below. I use this service in "higher level" services of my app. You can see there are multiple of things that may go wrong during the download. The server could be on fire and not be able to successfully respond at the moment, there could be go something wrong during the moving of the temporary file etc.
Now, there is probably not much a user can do with this other than trying later. However, he/she probably want to know that there was something wrong and that the download or the behaviour of the "higher level" methods could not succeed.
Me as a developer is confused as this point because I don't understand how to deal with errors in Swift. I have a completionHandler that takes an error if there was one, but I don't know what kind of error I should pass back to the caller.
Thoughts:
1) If I pass the error objects I get from the NSFileManager API or the NSURLSession API, I would think that I am "leaking" some of the implementation of download method to the callers. And how would the caller know what kind of errors to expect based on the error? It could be both.
2) If I am supposed to catch and wrap those errors that could happen inside the download method, how would that look like?
3) How do I deal with multiple error sources inside a method, and how would the code that calls the method that may throw/return NSError objects look like?
Should you as a caller start intercepting the errors you get back and then write a lot of code that differentiates the messages/action taken based on the error code? I don't get this error handling stuff at all and how it would look like when there are many things that could go wrong in a single method.
func download(destinationUrl: NSURL, completionHandler: ((error: NSError?) -> Void)) {
let request = NSURLRequest(URL: resourceUrl!)
let task = downloadSession.downloadTaskWithRequest(request) {
(url: NSURL?, response: NSURLResponse?, error: NSError?) in
if error == nil {
do {
try self.fileManager.moveItemAtURL(url!, toURL: destinationUrl)
} catch let e {
print(e)
}
} else {
}
}.resume()
}
First of all this is a great question. Error handling is a specific task that applies to a incredible array of situations with who know's what repercussions with your App's state. The key issue is what is meaningful to your user, app and you the developer.
I like to see this conceptually as how the Responder chain is used to handle events. Like an event traversing the responder chain an error has the possibility of bubbling up your App's levels of abstraction. Depending on the error you might want to do a number of things related to the type of the error. Different components of your app may need to know about error, it maybe an error that depending on the state of the app requires no action.
You as the developer ultimately know where errors effect your app and how. So given that how do we choose to implement a technical solution.
I would suggest using Enumerations and Closures as to build my error handling solution.
Here's a contrived example of an ENUM. As you can see it is represents the core of the error handling solution.
public enum MyAppErrorCode {
case NotStartedCode(Int, String)
case ResponseOkCode
case ServiceInProgressCode(Int, String)
case ServiceCancelledCode(Int, String, NSError)
func handleCode(errorCode: MyAppErrorCode) {
switch(errorCode) {
case NotStartedCode(let code, let message):
print("code: \(code)")
print("message: \(message)")
case ResponseOkCode:
break
case ServiceInProgressCode(let code, let message):
print("code: \(code)")
print("message: \(message)")
case ServiceCancelledCode(let code, let message, let error):
print("code: \(code)")
print("message: \(message)")
print("error: \(error.localizedDescription)")
}
}
}
Next we want to define our completionHandler which will replace ((error: NSError?) -> Void) the closure you have in your download method.
((errorCode: MyAppErrorCode) -> Void)
New Download Function
func download(destinationUrl: NSURL, completionHandler: ((errorCode: MyAppErrorCode) -> Void)) {
let request = NSURLRequest(URL: resourceUrl!)
let task = downloadSession.downloadTaskWithRequest(request) {
(url: NSURL?, response: NSURLResponse?, error: NSError?) in
if error == nil {
do {
try self.fileManager.moveItemAtURL(url!, toURL: destinationUrl)
completionHandler(errorCode: MyAppErrorCode.ResponseOkCode)
} catch let e {
print(e)
completionHandler(errorCode: MyAppErrorCode.MoveItemFailedCode(170, "Text you would like to display to the user..", e))
}
} else {
completionHandler(errorCode: MyAppErrorCode.DownloadFailedCode(404, "Text you would like to display to the user.."))
}
}.resume()
}
In the closure you pass in you could call handleCode(errorCode: MyAppErrorCode) or any other function you have defined on the ENUM.
You have now the components to define your own error handling solution that is easy to tailor to your app and which you can use to map http codes and any other third party error/response codes to something meaningful in your app. You can also choose if it is useful to let the NSError bubble up.
EDIT
Back to our contrivances.
How do we deal with interacting with our view controllers? We can choose to have a centralized mechanism as we have now or we could handle it in the view controller and keep the scope local. For that we would move the logic from the ENUM to the view controller and target the very specific requirements of our view controller's task (downloading in this case), you could also move the ENUM to the view controller's scope. We achieve encapsulation, but will most lightly end up repeating our code elsewhere in the project. Either way your view controller is going to have to do something with the error/result code
An approach I prefer would be to give the view controller a chance to handle specific behavior in the completion handler, or/then pass it to our ENUM for more general behavior such as sending out a notification that the download had finished, updating app state or just throwing up a AlertViewController with a single action for 'OK'.
We do this by adding methods to our view controller that can be passed the MyAppErrorCode ENUM and any related variables (URL, Request...) and add any instance variables to keep track of our task, i.e. a different URL, or the number of attempts before we give up on trying to do the download.
Here is a possible method for handling the download at the view controller:
func didCompleteDownloadWithResult(resultCode: MyAppErrorCode, request: NSURLRequest, url: NSURL) {
switch(resultCode) {
case .ResponseOkCode:
// Made up method as an example
resultCode.postSuccessfulDownloadNotification(url, dictionary: ["request" : request])
case .FailedDownloadCode(let code, let message, let error):
if numberOfAttempts = maximumAttempts {
// Made up method as an example
finishedAttemptingDownload()
} else {
// Made up method as an example
AttemptDownload(numberOfAttempts)
}
default:
break
}
}
Long story short: yes
... and then write a lot of code that differentiates the
messages/action taken based on the error code?
Most code examples leave the programmer alone about how to do any error handling at all, but in order to do it right, your error handling code might be more than the code for successful responses. Especially when it comes to networking and json parsing.
In one of my last projects (a lot of stateful json server communication) I have implemented the following approach: I have asked myself: How should the app possibly react to the user in case of an error (and translate it to be more user friendly)?
ignore it
show a message/ an alert (possibly only one)
retry by itself (how often?)
force the user to start over
assume (i.e. a previously cached response)
To achieve this, I have create a central ErrorHandler class, which does have several enums for the different types of errors (i.e. enum NetworkResponseCode, ServerReturnCode, LocationStatusCode) and one enum for the different ErrorDomains:
enum MyErrorDomain : String {
// if request data has errors (i.e. json not valid)
case NetworkRequestDomain = "NetworkRequest"
// if network response has error (i.e. offline or http status code != 200)
case NetworkResponseDomain = "NetworkResponse"
// server return code in json: value of JSONxxx_JSON_PARAM_xxx_RETURN_CODE
case ServerReturnDomain = "ServerReturnCode"
// server return code in json: value of JSONxxxStatus_xxx_JSON_PARAM_xxx_STATUS_CODE
case ServerStatusDomain = "ServerStatus"
// if CLAuthorizationStatus
case LocationStatusDomain = "LocationStatus"
....
}
Furthermore there exists some helper functions named createError. These methods do some checking of the error condition (i.e. network errors are different if you are offline or if the server response !=200). They are shorter than you would expect.
And to put it all together there is a function which handles the error.
func handleError(error: NSError, msgType: String, shouldSuppressAlert: Bool = false){
...
}
This method started with on switch statement (and needs some refactoring now, so I won't show it as it still is one). In this statement all possible reactions are implemented. You might need a different return type to keep your state correctly in the app.
Lessons learned:
Although I thought that I have started big (different enums, central user alerting), the architecture could have been better (i.e. multiple classes, inheritance, ...).
I needed to keep track of previous errors (as some are follow ups) in order to only show one error message to the user -> state.
There are good reasons to hide errors.
Within the errorObj.userInfo map, it exits a user friendly error message and a technicalErrorMessage (which is send to a tracking provider).
We have introduced numeric error codes (the error domain is prefixed with a letter) which are consistent between client and server. They are also shown to the user. This has really helped to track bugs.
I have implemented a handleSoftwareBug function (which is almost the same as the handleError but much less cases). It is used in a lot of else-blocks which you normally do not bother to write (as you think that this state can never be reached). Surprisingly it can.
ErrorHandler.sharedInstance.handleSoftwareBug("SW bug? Unknown received error code string was code: \(code)")
How does it look like in code: There are a lot of similar backend network requests where a lot of code looks something like the following:
func postAllXXX(completionHandler:(JSON!, NSError!) -> Void) -> RegisteringSessionTask {
log.function()
return postRegistered(jsonDict: self.jsonFactory.allXXX(),
outgoingMsgType: JSONClientMessageToServerAllXXX,
expectedIncomingUserDataType: JSONServerResponseAllXXX,
completionHandler: {(json, error) in
if error != nil {
log.error("error: \(error.localizedDescription)")
ErrorHandler.sharedInstance.handleError(error,
msgType: JSONServerResponseAllXXX, shouldSuppressAlert: true)
dispatch_async(dispatch_get_main_queue(), {
completionHandler(json, error)
})
return
}
// handle request payload
var returnList:[XXX] = []
let xxxList = json[JSONServerResponse_PARAM_XXX][JSONServerResponse_PARAM_YYY].arrayValue
.....
dispatch_async(dispatch_get_main_queue(), {
completionHandler(json, error)
})
})
}
Within the above code you see that I call a completionHandler and give this caller the chance to customize error handling, too. Most of the time, this caller only handles success.
Whenever I have had the need for retries and other and not so common handling, I have also done it on the caller side, i.e.
private func postXXXMessageInternal(completionHandler:(JSON!, NSError!) -> Void) -> NSURLSessionDataTask {
log.function()
return self.networkquery.postServerJsonEphemeral(url, jsonDict: self.jsonFactory.xxxMessage(),
outgoingMsgType: JSONClientMessageToServerXXXMessage,
expectedIncomingUserDataType: JSONServerResponseXXXMessage,
completionHandler: {(json, error) in
if error != nil {
self.xxxMessageErrorWaitingCounter++
log.error("error(\(self.xxxMessageErrorWaitingCounter)): \(error.localizedDescription)")
if (something || somethingelse) &&
self.xxxMessageErrorWaitingCounter >= MAX_ERROR_XXX_MESSAGE_WAITING {
// reset app because of too many errors
xxx.currentState = AppState.yyy
ErrorHandler.sharedInstance.genericError(MAX_ERROR_XXX_MESSAGE_WAITING, shouldSuppressAlert: false)
dispatch_async(dispatch_get_main_queue(), {
completionHandler(json, nil)
})
self.xxxMessageErrorWaitingCounter = 0
return
}
// handle request payload
if let msg = json[JSONServerResponse_PARAM_XXX][JSONServerResponse_PARAM_ZZZ].stringValue {
.....
}
.....
dispatch_async(dispatch_get_main_queue(), {
completionHandler(json, error)
})
})
}
Here is another example where the user is forced to retry
// user did not see a price. should have been fetched earlier (something is wrong), cancel any ongoing requests
ErrorHandler.sharedInstance.handleSoftwareBug("potentially sw bug (or network to slow?): no payment there? user must retry")
if let st = self.sessionTask {
st.cancel()
self.sessionTask = nil
}
// tell user
ErrorHandler.sharedInstance.genericInfo(MESSAGE_XXX_PRICE_REQUIRED)
// send him back
xxx.currentState = AppState.zzz
return
For any request, you get either an error or an http status code. Error means: Your application never managed to talk properly to the server. http status code means: Your application talked to a server. Be aware that if you take your iPhone into the nearest Starbucks, "your application talked to a server" doesn't mean "your application talked to the server it wanted to talk to". It might mean "your application managed to talk to the Starbucks server which asks you to log in and you have no idea how to do that".
I divide the possible errors into categories: "It's a bug in my code". That's where you need to fix your code. "Something went wrong, and the user can do something about it". For example when WiFi is turned off. "Something went wrong, maybe it works later". You can tell the user to try later. "Something went wrong, and the user can't do anything about it". Tough. "I got a reply from the server that I expected. Maybe an error, maybe not, but something that I know how to handle". You handle it.
I also divide calls into categories: Those that should run invisibly in the background, and those that run as a result of a direct user action. Things running invisibly in the background shouldn't give error messages. (Bloody iTunes telling me it cannot connect to the iTunes Store when I had no interest in connecting to the iTunes Store in the first place is an awful example of getting that wrong).
When you show things to the user, remember that the user doesn't care. To the user: Either it worked, or it didn't work. If it didn't work, the user can fix the problem if it is a problem they can fix, they can try again later, or it's just tough luck. In an enterprise app, you might have a message "call your help desk at xxxxxx and tell them yyyyyy".
And when things don't work, don't annoy the user by showing error after error after error. If you send then requests, don't tell the user ten times that the server is on fire.
There are things that you just don't expect to go wrong. If you download a file, and you can't put it where it belongs, well, that's tough. It shouldn't happen. The user can't do anything about it. (Well, maybe they can. If the storage of the device is full then you can tell the user). Apart from that, it's the same category as "Something went wrong, and the user can't do anything about it". You may find out as a developer what the cause is and fix it, but if it happens with an application out in the user's hands, there's nothing reasonable you can do.
Since all such requests should be asynchronous, you will always pass either one or two callback blocks to the call, one for success and one for failure. I have most of the error handling in the download code, so things like asking the user to turn WiFi on happen only once, and calls may even be repeated automatically if such an error condition is fixed by the user. The error callback is mostly used to inform the application that it won't get the data that it wanted; sometimes the fact that there is an error is useful information in itself.
For consistent error handling, I create my own errors representing either errors returned by the session, or html status codes interpreted as errors. Plus two additional errors "user cancelled" and "no user interaction allowed" if either there was a UI involved and the user cancelled the operation, or I wanted to use some user interaction but wasn't allowed to. The last two errors are different - these errors will never be reported to the user.
I would wrap the errors in your own, but pass the underlying error as a property on your error class (ala C#'s InnerException). That way you are giving consumers a consistent interface, but also providing lower level error detail if required. However, the main reason I would do this is for unit testing. It makes it much easier to mock your ResourceService class and test the code paths for the various errors that could occur.
I don't like the thought of passing back an array of errors, as it adds complexity for the consumer. Instead I would provide an array of InnerException instances. If they are instances of your own error class, they would potentially have their own InnerException's with underlying errors. However, this would probably only make sense if you were doing your own validations where multiple errors might make sense. Your download method will probably have to bail out after the first error encountered.