I am using ReactiveSwift for a while but suddenly encountered a strange bug or something.
I am using an MVVM architecture and have a simple view controller and a view model for it. The VM has a single property of type Signal<Void, NoError>. In the VM's init I send to its observer an empty value. On the view controller's viewDidLoad I listen for that signal and print some text.
Here is the view model:
class SomeVCModel {
let (someSignal, someSignalObserver) = Signal<Void, NoError>.pipe()
init() {
print (Thread.isMainThread)
DispatchQueue.main.async {
self.someSignalObserver.send(value: ())
}
}
}
And here is the view controller:
class SomeVC {
var model: SomeVCModel!
override func viewDidLoad() {
super.viewDidLoad()
model = SomeVCModel()
model.someSignal.observeValues { (_) in
print ("Some Value")
}
}
}
In this case the print statement doesn't get called. However, when I send a value to an observer by explicitly defining the main thread from DispatchQueue everything works:
DispatchQueue.main.async {
self.someSignalObserver.send(value: ())
}
I can't figure out why is this happening. Init for the view model itself is called on the main thread, so what could cause this behaviour?
Thanks in advance.
Your model sends the value during initialization.
However, your viewDidLoad function sets the observer after it is initialized, so it is not ready to observe it. Subsequent calls will work since the observer is now configured.
You have a few options, but basically you need to change the sequence.
Related
In my view model, I have two properties:
private val databaseDao = QuestionDatabase.getDatabase(context).questionDao()
val allQuestions: LiveData<List<Question>> = databaseDao.getAllQuestions()
I have observers set on "allQuestions" in my fragment and I'm noticing the observer is being called when I rotate the device. Even though the View Model is only being created once (can tell via a log statement in init()), the observer methods are still being called.
Why is this? I would think the point is to have persistency in the View Model. Ideally, I want the database questions to be only loaded once, regardless of rotation.
This happens because LiveData is lifecycle aware.
And When you rotate the screen you UI Controller [Activity/Fragment] goes through various lifecycle states and lifecycle callbacks.
And since LiveData is lifecycle aware, it updates the detail accordingly.
I have tried to explain this with following points:
When the UI Controller is offscreen, Live Data performs no updates.
When the UI Controller is back on screen, it gets current data.
(Because of this property you are getting above behavior)
When UI controller is destroyed, it performs cleanup on its own.
When new UI Controller starts observing live data, it gets current data.
add this check inside observer
if(lifecycle.currentState == Lifecycle.State.RESUMED){
//code
}
I have the same issue, after reading the jetpack guideline doc, I solve it. Just like what #SVK mentioned, after the rotation of the screen, activity/fragment were re-created.
Base on the solution https://stackoverflow.com/a/64062616,
class SingleLiveEvent<T> : MutableLiveData<T>() {
val TAG: String = "SingleLiveEvent"
private val mPending = AtomicBoolean(false)
#MainThread
override fun observe(owner: LifecycleOwner, observer: Observer<in T>) {
if (hasActiveObservers()) {
Log.w(TAG, "Multiple observers registered but only one will be notified of changes.")
}
// Observe the internal MutableLiveData
super.observe(owner, Observer<T> { t ->
if (mPending.compareAndSet(true, false)) {
observer.onChanged(t)
}
})
}
override fun observeForever(observer: Observer<in T>) {
if (hasActiveObservers()) {
Log.w(TAG, "Multiple observers registered but only one will be notified of changes.")
}
// Observe the internal MutableLiveData
super.observeForever { t ->
if (mPending.compareAndSet(true, false)) {
observer.onChanged(t)
}
}
}
#MainThread
override fun setValue(#Nullable t: T?) {
mPending.set(true)
super.setValue(t)
}
/**
* Used for cases where T is Void, to make calls cleaner.
*/
#MainThread
fun call() {
value = null
}
this Main Menu VC will be opened when the app launched for the first time or after the user back to the app (the app become active after enter the background state).
every time this main menu VC is opened, ideally I need to update the time that the date time data comes from the server. in this main menu vc class I call getDateTimeFromServer() after that I updateUI().
but to update the data after the app enter the background and back to the foreground, the getDateTimeFromServer() and updateUI() shall be activated from Appdelegate using function.
func applicationWillEnterForeground(application: UIApplication) {
}
so how do I activate a method that are exist in Main Menu VC from AppDelegate
You don’t need to call the view controller method in app delegate. Observe foreground event in your controller and call your method from there itself.
Observe for the UIApplicationWillEnterForeground notification in your viewController viewDidLoad:
NotificationCenter.default.addObserver(self, selector: #selector(ViewController.yourMethod), name: NSNotification.Name.UIApplicationWillEnterForeground, object: nil)
Implement this to receive callback when user enters foreground
#objc func yourMethod() {
// Call getDateTimeFromServer()
}
These types of messaging are in most cases done with static context. As it was already mentioned you could alternatively use notification center within the within the view controller to be notified of your application entering foreground. I discourage you creating custom notifications for this though (but is a possible solution as well).
Anyway for your specific case I suggest you have a model that contains your data. Then create a shared instance of it.
class MyDataModel {
static var shared: MyDataModel = {
let model = MyDataModel()
model.reloadData()
return model
}()
var myObjects: [MyObject]?
func reloadData() {
// load data asynchronously
}
}
Now when your view controller needs to reload it simply uses MyDataModel.shared.myObjects as data source.
In app delegate all you do is reload it when app comes back to foreground using MyDataModel.shared.reloadData().
So now a delegate is still missing so we add
protocol MyDataModelDelegate: class {
func myDataModel(_ sender: MyDataModel, updatedObjects objects: [MyObject]?)
}
class MyDataModel {
weak var delegate: MyDataModelDelegate?
static var shared: MyDataModel = {
Now when your view controller appears it needs to assign itself as a delegate MyDataModel.shared.delegate = self. And implement the protocol in which a reload on the view must be made.
A callout to the delegate can simply be done in a model setter:
}()
var myObjects: [MyObject]? {
didSet {
delegate.myDataModel(self, updatedObjects: myObjects)
}
}
func reloadData() {
You can do something like that, using a technique called Key-Value Observation:
class CommonObservableData: NSObject {
// Use #objc and dynamic to ensure enabling Key-Value Observation
#objc dynamic var dateTime: Date?
static let shared = CommonObservableData()
func updateFromWeb() {
// callWebThen is a function you will define that calls your Web API, then
// calls a completion handler you define, passing new value to your handler
callWeb(then: { self.dateTime = $0 })
}
}
Then you observe on it using Swift 4 's new NSKeyValueObservation.
class SomeViewController: UIViewController {
var kvo: NSKeyValueObservation?
func viewDidLoad() {
...
kvo = CommonObservableData.shared.observe(
\CommonObservableData.dateTime, { model, change in
self.label.text = "\(model.dateTime)"
})
}
}
Key-Value Observation is originally an Objective-C technique that is "somewhat revived" by Swift 4, this technique allows you to observe changes on a property (called a Key in Objective-C) of any object.
So, in the previous code snippets, we made a class, and made it a singleton, this singleton has an observable property called dateTime, where we could observe on change of this property, and make any change in this property automatically calls a method where we could update the UI.
Read about KVO here:
Key-Value Observation Apple Programming Guide
Key-Value Observation using Swift 4
Also, if you like Rx and RFP (Reactive Functional Programming), you can use RxSwift and do the observation in a cleaner way using it.
In swift 4 and 5, the notification name is changed the below code working for both.
notifyCenter.addObserver(self, selector: #selector(new), name:UIApplication.willEnterForegroundNotification, object: nil)
#objc func new(){}
I am writing a chess GUI in Swift 3 and use nvzqz/Sage as the chess model/library. Now I face a problem with a Sage closure used for piece promotion.
Sage uses (in its game class) the execute(move: promotion:) method for promotion move execution which has a closure that returns a promotion piece kind. This allows to prompt the user for a promotion piece or perform any other operations before choosing a promotion piece kind, as follows:
try game.execute(move: move) {
...
return .queen
}
I implemented a promotion view controller ("pvc") which is called in this closure so that the player may select the new piece:
// This is in the main View Controller class
/// The piece selected in promotionViewController into which a pawn shall promote
var newPiece: Piece.Kind = ._queen // default value = Queen
try game.execute(move: move) {
boardView.isUserInteractionEnabled = false
// promotionview controller appears to select new promoted piece
let pvc = PromotionViewController(nibName: nil, bundle: nil)
pvc.delegate = self
pvc.modalPresentationStyle = .overCurrentContext
self.present(pvc, animated:true)
return newPiece
}
When the button for the new piece in the pvc is pressed, the pvc dismisses itself and the data of the selected piece (the constant selectedType) is transferred back to the main view controller via delegation:
// This is in the sending PVC class
protocol PromotionViewControllerDelegate {
func processPromotion(selectedType: Piece.Kind)
}
func buttonPressed(sender: UIButton) {
let selectedType = bla bla bla ...
delegate?.processPromotion(selectedType: selectedType)
presentingViewController!.dismiss(animated:true)
}
// This is in the receiving main View Controller class
extension GameViewController: PromotionViewControllerDelegate {
func processPromotion(selectedType: Piece.Kind) {
defer {
boardView.isUserInteractionEnabled = true
}
newPiece = selectedType
}
The problem I have is that the closure (in the game.execute method) does not wait until the player made his selection in the pvc (and immediately returns the newPiece variable which is still the default value) so that I never get another promotion piece processed other than the default value.
How do I make the closure wait until the player pressed a button in the pvc?
Of course, I tried to find a solution and read about callbacks, completion handlers or property observers. I do not know which is the best way forward, some thoughts:
Completion handler: the pvc dismisses itself upon button-press event so the completion handler is not in the receiving (main) view controller. How do I deal with this?
Property observer: I could call the try game.execute(move) method only after the promotion piece was set (with didset) but that would make the code difficult to read and not use the nice closure the game.execute method provides.
Callbacks may be related to completion handlers, but am not sure.
So your block in game.execute(move: move) will fully execute which is so designed by the Sage API. You can not pause it as easy but it is doable, still let's try to solve it the other way;
Why do you need to call the presentation of the view controller within this block? By all means try to move that away. The call try game.execute(move: move) { should only be called within processPromotion delegate method. You did not post any code but wherever this try game.execute(move: move) { code is it needs to be replaced by presenting a view controller alone.
Then on delegate you do not even need to preserve the value newPiece = selectedType but rather just call try game.execute(move: move) { return selectedType }.
So about pausing a block:
It is not possible to directly "pause" a block because it is a part of execution which means the whole operation needs to pause which in the end means you need to pause your whole thread. That means you need to move the call to a separate thread and pause that one. Still this will only work if the API supports the multithreading, if the callback is called on the same tread as its execute call... So there are many tools and ways on how to lock a thread so let me just use the most primitive one which is making the thread sleep:
var executionLocked: Bool = false
func foo() {
DispatchQueue(label: "confiramtion queue").async {
self.executionLocked = true
game.execute(move: move) {
// Assuming this is still on the "confiramtion queue" queue
DispatchQueue.main.async {
// UI code needs to be executed on main thread
let pvc = PromotionViewController(nibName: nil, bundle: nil)
pvc.delegate = self
pvc.modalPresentationStyle = .overCurrentContext
self.present(pvc, animated:true)
}
while self.executionLocked {
Thread.sleep(forTimeInterval: 1.0/5.0) // Check 5 times per second if unlocked
}
return self.newPiece // Or whatever the code is
}
}
}
Now in your delegate you need:
func processPromotion(selectedType: Piece.Kind) {
defer {
boardView.isUserInteractionEnabled = true
}
newPiece = selectedType
self.executionLocked = false
}
So what happens here is we start a new thread. Then lock the execution and start execution on game instance. In the block we now execute our code on main thread and then create an "endless" loop in which a thread sleeps a bit every time (the sleep is not really needed but it prevents the loop to take too much CPU power). Now all the stuff is happening on main thread which is that a new controller is presented and user may do stuff with it... Then once done a delegate will unlock the execution lock which will make the "endless" loop exit (on another thread) and return a value to your game instance.
I do not expect you to implement this but if you will then ensure you make all precautions to correctly release the loop if needed. Like if view controller is dismissed it should unlock it, if a delegate has a "cancel" version it should exit...
I have an app that use http calls to stream video from external storage.
When the user's device isn't connected to a network service, I need the app to go back to the previous controller.
the flow is the following: the user access a list of elements (table view cell), select one, then the app goes to the player controller. On this controller, the call is made to stream the file.
I use an api call handler in a class outside of the controller and I don't know how to proceed to make it go back to the previous controller from here (the element list).
Connectivity issues errors are all catched within the api class.
I don't show any code as Im not sure it would be relevant. If you need to see anything, let me know and I will update the question. What should be the way to do that? (of course I use a navigation controller)
Thanks
If you want go back to the previous view controller you should use:
navigationController?.popViewControllerAnimated(true)
If you need to use this function not in the view-controller but in another class you can use NSNotificationCenter for notify the view-controller when it's needed to show the previous controller, just like this:
YourViewController
override func viewDidLoad()
{
...
NSNotificationCenter.defaultCenter().addObserver(
self,
selector: "goBack:",
name: "goBackNotification",
object: nil)
...
}
func goBack(notification: NSNotification)
{
navigationController?.popViewControllerAnimated(true)
}
AnotherClass
NSNotificationCenter.defaultCenter().postNotificationName("goBackNotification", object: nil)
Don't forget to remove the observer in your YourViewController:
deinit
{
NSNotificationCenter.defaultCenter().removeObserver(self)
}
EDIT 1: you can use obviously a delegate instead of a NSNotification method. If you don't know the differences between NSNotification and delegate I recommend to you this answer.
A common approach besides NSNotificationCenter is to utilize closures or delegates to inform your ViewController that the streaming attempt failed. Using closures, the API of the class responsible for the streaming could be extended to take a completion closure as parameter, and call it with an NSError, if one occurred, or nil if it didn't.
func streamFile(completion: NSError? -> Void) {
// Try to start the streaming and call the closure with an error or nil
completion(errorOrNil)
}
When the call to the API in the ViewController is made you can then pass a closure to the method and check for errors. In case something went wrong an error should be present and the ViewController should be dismissed
func startStream() {
StreamingAPIClient.streamFile(completion: { [weak self] error in
if error != nil {
// Handle error
self.dismissViewControllerAnimated(true, completion: nil)
} else {
// Proceed with the streaming
}
})
}
When I go to a viewController I call within my viewDidAppear Method a function:
override func viewDidAppear(animated: Bool) {
getLessons()
}
This methods loads from parse.com a list of data I want to use in a pickerView.
The function itself:
func getLessons(){
var query = PFQuery(className:"Lesson")
query.orderByAscending("name")
query.findObjectsInBackgroundWithBlock {
(objects: [AnyObject]!, error: NSError!) -> Void in
if error == nil {
for object in objects {
var name = object["name"] as String
self.languagePickerKeys.append(object.objectId)
self.languagePickerValues.append(name)
self.selectedLanguage.text = self.languagePickerValues.first // set the first lessons name into the text field
self.selectedLessonObjectId = self.languagePickerKeys.first // set the first objectId for the lesson
self.languagePicker?.reloadAllComponents()
}
} else {
// Log details of the failure
println("\(error.userInfo)")
}
}
println("getLessons done")
}
The thing is, that the textfield is empty, as the getLesson() gets the data async and the data is not available to the textfield.
I also tried to put the getLesson into the viewDidAppear method, but this doesn't help me, the textfield is empty anyway.
What can I do, to have the data from the getLessons() method ready and loaded its first value into my textfield when the view is shown to the user?
You certainly have to get the data from asyncTask before setting it to pickerView.
Here's the ViewController lifecycle after instantiation:
Preparation if being segued to.
Outlet setting
Appearing and Disappearing.
So, you have two options:
Load the data in previous ViewController and then perform the segue. You need to follow these steps for it.
a. Create a segue from previous ViewController to your ViewController.
b. Call the function when you want to go next ViewController which fetches the data, and the end (after getting the data) call performSegueWithIdentifier which will lead to your ViewController.
c. Set the data in prepareForSegue
let navigationController = segue.destinationViewController as UINavigationController
navigationController.data = yourData //you got from async call
Now when you reach your ViewController, you are sure that your data is present, and you can set it to your pickerView.
If you want to do it in the same ViewController: here's is the lifeCycle of ViewController:so you need to call your function in viewDidLoad, and always set your pickerView after completion of the async network call.
Make sure that you initiate all changes to the UI from the main thread e.g. like so:
dispatch_async(dispatch_get_main_queue(), {
selectedLanguage.text = languagePickerValues.first
self.languagePicker?.reloadAllComponents()
})
The problem is that findObjectsInBackgroundWithBlock is an asynchronous method, so even if you fire it in the ViewDidLoad you will never know when you will receive the response data and you can't be sure that the data will be ready by the time you view appear.
I think you have just 2 possibility:
The first one is to load the data in the previous view controller and then just pass the data that got ready to you view controller.
The second is to use a synchronous method (the findobject method maybe?) and put the call in a method that is fired BEFORE the view appear (like the viewWillAppear: method). But your view will stuck for a moment (I think) while the data is retreiving... However this second solution probably resolve your problem but using synchronous method to retrieve data from a slower data source is usually bad design solution.
D.