Blazor Server EF Core Cancelation - entity-framework-core

In my app I have to be able to cancel uploads, I tried with threads cancelation but nothing happens, I think it is because I use DBContextFactory and I create a context for each uploaded file.
So I did this to save files in DB:
private async Task OnFilesDropped(FileUploadModel upload)
{
Uploads.Add(upload);
if (string.IsNullOrEmpty(upload.Error))
{
using var context = DbFactory.CreateDbContext();
upload.Context = context;
context.FileUploads.Add(upload);
upload.UploadCompletion = 45;
await context.SaveChangesAsync();
upload.UploadCompletion = 100;
}
}
and this in case of deleting a uploaded/uploading file:
private async Task DeleteUpload(FileUploadModel upload)
{
Uploads.Remove(upload);
await UploadsChanged.InvokeAsync(Uploads);
if (string.IsNullOrEmpty(upload.Error))
{
if (upload.UploadCompletion != 100)
{
await upload.Context.DisposeAsync();
}
else
{
using var context = DbFactory.CreateDbContext();
context.FileUploads.Remove(upload);
await context.SaveChangesAsync();
}
}
}
This way works because I dispose of the context, but I wonder if there is a better way of doing this? or if this solution could be problematic somehow?
Best Regards.

You should use a CancellationToken.
The SaveChangesAsync method on your context has an overload that can be provided a cancellationToken.
await context.SaveChangesAsync(cancellationToken);
If you already have a CancellationToken higher in the call stack, you can just pass that one down. Otherwise, you can create a CancellationTokenSource and use that to generate a cancellation token and then cancel it when appropriate.

Related

Blazor WASM Load Data before Render Page

I would like to load some Data before I Render my Blazor Application because in depndency to the loaded data I would like to render my app (layout, navbar ...)
Now I want to use the OnInitialised method instead of OnInitialisedAsync and with no async and await keywords.
But now I had a problem to convert the data which I get back from my API.
protected override void OnInitialized()
{
try
{ Console.WriteLine("Test1Mainasync");
LoadCategories();
}
catch (Exception e)
{
jsRuntime.ToastrError(e.Message);
}
}
private void LoadCategories()
{
IEnumerable<CategorieDTO> CategoriesInit1 = new List<CategorieDTO>();
CategoriesInit1 = categorieService.GetAllCategories();
SD.Categories = CategoriesInit1.ToList();
//foreach(var categorie in CategoriesInit){
// SD.Categories.Append(categorie);
//}
Console.WriteLine("Test1Main");
}
Has someone an idea why this converting issues happen?
I think you have this method:
public async Task<IEnumerable<CategorieDTO>> GetAllCategories()
and you should call it this way:
private async Task LoadCategories()
{
IEnumerable<CategorieDTO> CategoriesInit1 = new List<CategorieDTO>();
CategoriesInit1 = await categorieService.GetAllCategories();
and:
protected override async Task OnInitializedAsync()
{
try
{ Console.WriteLine("Test1Mainasync");
await LoadCategories();
}
Has someone an idea why this converting issues happen?
In your code CatagiesInit1 is a Task, it's not a List<CategorieDTO>. You only get the List<CategorieDTO> when the task completes which you have no control over as you don't await the completion of the Task. In all likelyhood, your sync code will run to completion before that happens.
If your CategoryService returns a Task then the code that handles it must be async code. You can't escape from the async world back into the sync world without consequencies. If you want to live in the sync world then all the data pipeline code also needs to be blocking sync code.
If I understand your comments correctly, you want nothing to render until a certain set of conditions are met. If so add some standard Loading... component code to the page if it's page specific or App.razor if it's on initial load, or say MainLayout if it's application wide.
Here's a quick an dirty example:
<Router AppAssembly="#typeof(App).Assembly">
<Found Context="routeData">
#if (Loaded)
{
<RouteView RouteData="#routeData" DefaultLayout="#typeof(MainLayout)" />
<FocusOnNavigate RouteData="#routeData" Selector="h1" />
}
else
{
<div class="m-2 p-5 bg-secondary text-white">
<h3>Loading.....</h3>
</div>
}
</Found>
<NotFound>
<PageTitle>Not found</PageTitle>
<LayoutView Layout="#typeof(MainLayout)">
<p role="alert">Sorry, there's nothing at this address.</p>
</LayoutView>
</NotFound>
</Router>
#code {
private bool Loaded;
protected override async Task OnInitializedAsync()
{
Loaded = false;
// simulate getting the data first
await Task.Delay(5000);
Loaded = true;
}
}
Your call to API endpoint return an awaitable task but not the IEnumerable, So you can not assign awaitable task to IEnumerable so this piece of code wont work
private void LoadCategories()
{
IEnumerable<CategorieDTO> CategoriesInit1 = new List<CategorieDTO>();
CategoriesInit1 = categorieService.GetAllCategories();
}
You should have your LoadCategories function like this
private async Task LoadCategories()
{
IEnumerable<CategorieDTO> CategoriesInit1 = new List<CategorieDTO>();
CategoriesInit1 = await categorieService.GetAllCategories();
}
API calls should be awaitable, else it will stuck your UI
You can use this solution as well
private void LoadCategories()
{
var t = Task.Run(() => categorieService.GetAllCategories()()).GetAwaiter();
t.OnCompleted(() =>
{
CategoriesInit1 = t.GetResult();
// you may need to call statehaschanged as well
StateHasChanged();
});
}

In a swift/firebase project, what causes an array to be readable when .observe is used but not when SingleEvent is used? [duplicate]

Whenever I use addListenerForSingleValueEvent with setPersistenceEnabled(true), I only manage to get a local offline copy of DataSnapshot and NOT the updated DataSnapshot from the server.
However, if I use addValueEventListener with setPersistenceEnabled(true), I can get the latest copy of DataSnapshot from the server.
Is this normal for addListenerForSingleValueEvent as it only searches DataSnapshot locally (offline) and removes its listener after successfully retrieving DataSnapshot ONCE (either offline or online)?
Update (2021): There is a new method call (get on Android and getData on iOS) that implement the behavior you'll like want: it first tries to get the latest value from the server, and only falls back to the cache when it can't reach the server. The recommendation to use persistent listeners still applies, but at least there's a cleaner option for getting data once even when you have local caching enabled.
How persistence works
The Firebase client keeps a copy of all data you're actively listening to in memory. Once the last listener disconnects, the data is flushed from memory.
If you enable disk persistence in a Firebase Android application with:
Firebase.getDefaultConfig().setPersistenceEnabled(true);
The Firebase client will keep a local copy (on disk) of all data that the app has recently listened to.
What happens when you attach a listener
Say you have the following ValueEventListener:
ValueEventListener listener = new ValueEventListener() {
#Override
public void onDataChange(DataSnapshot snapshot) {
System.out.println(snapshot.getValue());
}
#Override
public void onCancelled(FirebaseError firebaseError) {
// No-op
}
};
When you add a ValueEventListener to a location:
ref.addValueEventListener(listener);
// OR
ref.addListenerForSingleValueEvent(listener);
If the value of the location is in the local disk cache, the Firebase client will invoke onDataChange() immediately for that value from the local cache. If will then also initiate a check with the server, to ask for any updates to the value. It may subsequently invoke onDataChange() again if there has been a change of the data on the server since it was last added to the cache.
What happens when you use addListenerForSingleValueEvent
When you add a single value event listener to the same location:
ref.addListenerForSingleValueEvent(listener);
The Firebase client will (like in the previous situation) immediately invoke onDataChange() for the value from the local disk cache. It will not invoke the onDataChange() any more times, even if the value on the server turns out to be different. Do note that updated data still will be requested and returned on subsequent requests.
This was covered previously in How does Firebase sync work, with shared data?
Solution and workaround
The best solution is to use addValueEventListener(), instead of a single-value event listener. A regular value listener will get both the immediate local event and the potential update from the server.
A second solution is to use the new get method (introduced in early 2021), which doesn't have this problematic behavior. Note that this method always tries to first fetch the value from the server, so it will take longer to completely. If your value never changes, it might still be better to use addListenerForSingleValueEvent (but you probably wouldn't have ended up on this page in that case).
As a workaround you can also call keepSynced(true) on the locations where you use a single-value event listener. This ensures that the data is updated whenever it changes, which drastically improves the chance that your single-value event listener will see the current value.
So I have a working solution for this. All you have to do is use ValueEventListener and remove the listener after 0.5 seconds to make sure you've grabbed the updated data by then if needed. Realtime database has very good latency so this is safe. See safe code example below;
public class FirebaseController {
private DatabaseReference mRootRef;
private Handler mHandler = new Handler();
private FirebaseController() {
FirebaseDatabase.getInstance().setPersistenceEnabled(true);
mRootRef = FirebaseDatabase.getInstance().getReference();
}
public static FirebaseController getInstance() {
if (sInstance == null) {
sInstance = new FirebaseController();
}
return sInstance;
}
Then some method you'd have liked to use "addListenerForSingleEvent";
public void getTime(final OnTimeRetrievedListener listener) {
DatabaseReference ref = mRootRef.child("serverTime");
ref.addValueEventListener(new ValueEventListener() {
#Override
public void onDataChange(DataSnapshot dataSnapshot) {
if (listener != null) {
// This can be called twice if data changed on server - SO DEAL WITH IT!
listener.onTimeRetrieved(dataSnapshot.getValue(Long.class));
}
// This can be called twice if data changed on server - SO DEAL WITH IT!
removeListenerAfter2(ref, this);
}
#Override
public void onCancelled(DatabaseError databaseError) {
removeListenerAfter2(ref, this);
}
});
}
// ValueEventListener version workaround for addListenerForSingleEvent not working.
private void removeListenerAfter2(DatabaseReference ref, ValueEventListener listener) {
mHandler.postDelayed(new Runnable() {
#Override
public void run() {
HelperUtil.logE("removing listener", FirebaseController.class);
ref.removeEventListener(listener);
}
}, 500);
}
// ChildEventListener version workaround for addListenerForSingleEvent not working.
private void removeListenerAfter2(DatabaseReference ref, ChildEventListener listener) {
mHandler.postDelayed(new Runnable() {
#Override
public void run() {
HelperUtil.logE("removing listener", FirebaseController.class);
ref.removeEventListener(listener);
}
}, 500);
}
Even if they close the app before the handler is executed, it will be removed anyways.
Edit: this can be abstracted to keep track of added and removed listeners in a HashMap using reference path as key and datasnapshot as value. You can even wrap a fetchData method that has a boolean flag for "once" if this is true it would do this workaround to get data once, else it would continue as normal.
You're Welcome!
You can create transaction and abort it, then onComplete will be called when online (nline data) or offline (cached data)
I previously created function which worked only if database got connection lomng enough to do synch. I fixed issue by adding timeout. I will work on this and test if this works. Maybe in the future, when I get free time, I will create android lib and publish it, but by then it is the code in kotlin:
/**
* #param databaseReference reference to parent database node
* #param callback callback with mutable list which returns list of objects and boolean if data is from cache
* #param timeOutInMillis if not set it will wait all the time to get data online. If set - when timeout occurs it will send data from cache if exists
*/
fun readChildrenOnlineElseLocal(databaseReference: DatabaseReference, callback: ((mutableList: MutableList<#kotlin.UnsafeVariance T>, isDataFromCache: Boolean) -> Unit), timeOutInMillis: Long? = null) {
var countDownTimer: CountDownTimer? = null
val transactionHandlerAbort = object : Transaction.Handler { //for cache load
override fun onComplete(p0: DatabaseError?, p1: Boolean, data: DataSnapshot?) {
val listOfObjects = ArrayList<T>()
data?.let {
data.children.forEach {
val child = it.getValue(aClass)
child?.let {
listOfObjects.add(child)
}
}
}
callback.invoke(listOfObjects, true)
}
override fun doTransaction(p0: MutableData?): Transaction.Result {
return Transaction.abort()
}
}
val transactionHandlerSuccess = object : Transaction.Handler { //for online load
override fun onComplete(p0: DatabaseError?, p1: Boolean, data: DataSnapshot?) {
countDownTimer?.cancel()
val listOfObjects = ArrayList<T>()
data?.let {
data.children.forEach {
val child = it.getValue(aClass)
child?.let {
listOfObjects.add(child)
}
}
}
callback.invoke(listOfObjects, false)
}
override fun doTransaction(p0: MutableData?): Transaction.Result {
return Transaction.success(p0)
}
}
In the code if time out is set then I set up timer which will call transaction with abort. This transaction will be called even when offline and will provide online or cached data (in this function there is really high chance that this data is cached one).
Then I call transaction with success. OnComplete will be called ONLY if we got response from firebase database. We can now cancel timer (if not null) and send data to callback.
This implementation makes dev 99% sure that data is from cache or is online one.
If you want to make it faster for offline (to don't wait stupidly with timeout when obviously database is not connected) then check if database is connected before using function above:
DatabaseReference connectedRef = FirebaseDatabase.getInstance().getReference(".info/connected");
connectedRef.addValueEventListener(new ValueEventListener() {
#Override
public void onDataChange(DataSnapshot snapshot) {
boolean connected = snapshot.getValue(Boolean.class);
if (connected) {
System.out.println("connected");
} else {
System.out.println("not connected");
}
}
#Override
public void onCancelled(DatabaseError error) {
System.err.println("Listener was cancelled");
}
});
When workinkg with persistence enabled, I counted the times the listener received a call to onDataChange() and stoped to listen at 2 times. Worked for me, maybe helps:
private int timesRead;
private ValueEventListener listener;
private DatabaseReference ref;
private void readFB() {
timesRead = 0;
if (ref == null) {
ref = mFBDatabase.child("URL");
}
if (listener == null) {
listener = new ValueEventListener() {
#Override
public void onDataChange(DataSnapshot dataSnapshot) {
//process dataSnapshot
timesRead++;
if (timesRead == 2) {
ref.removeEventListener(listener);
}
}
#Override
public void onCancelled(DatabaseError databaseError) {
}
};
}
ref.removeEventListener(listener);
ref.addValueEventListener(listener);
}

What causes a cache issue that occurs with observeSingleEvent but not with .observe(DataEventType.value? [duplicate]

Whenever I use addListenerForSingleValueEvent with setPersistenceEnabled(true), I only manage to get a local offline copy of DataSnapshot and NOT the updated DataSnapshot from the server.
However, if I use addValueEventListener with setPersistenceEnabled(true), I can get the latest copy of DataSnapshot from the server.
Is this normal for addListenerForSingleValueEvent as it only searches DataSnapshot locally (offline) and removes its listener after successfully retrieving DataSnapshot ONCE (either offline or online)?
Update (2021): There is a new method call (get on Android and getData on iOS) that implement the behavior you'll like want: it first tries to get the latest value from the server, and only falls back to the cache when it can't reach the server. The recommendation to use persistent listeners still applies, but at least there's a cleaner option for getting data once even when you have local caching enabled.
How persistence works
The Firebase client keeps a copy of all data you're actively listening to in memory. Once the last listener disconnects, the data is flushed from memory.
If you enable disk persistence in a Firebase Android application with:
Firebase.getDefaultConfig().setPersistenceEnabled(true);
The Firebase client will keep a local copy (on disk) of all data that the app has recently listened to.
What happens when you attach a listener
Say you have the following ValueEventListener:
ValueEventListener listener = new ValueEventListener() {
#Override
public void onDataChange(DataSnapshot snapshot) {
System.out.println(snapshot.getValue());
}
#Override
public void onCancelled(FirebaseError firebaseError) {
// No-op
}
};
When you add a ValueEventListener to a location:
ref.addValueEventListener(listener);
// OR
ref.addListenerForSingleValueEvent(listener);
If the value of the location is in the local disk cache, the Firebase client will invoke onDataChange() immediately for that value from the local cache. If will then also initiate a check with the server, to ask for any updates to the value. It may subsequently invoke onDataChange() again if there has been a change of the data on the server since it was last added to the cache.
What happens when you use addListenerForSingleValueEvent
When you add a single value event listener to the same location:
ref.addListenerForSingleValueEvent(listener);
The Firebase client will (like in the previous situation) immediately invoke onDataChange() for the value from the local disk cache. It will not invoke the onDataChange() any more times, even if the value on the server turns out to be different. Do note that updated data still will be requested and returned on subsequent requests.
This was covered previously in How does Firebase sync work, with shared data?
Solution and workaround
The best solution is to use addValueEventListener(), instead of a single-value event listener. A regular value listener will get both the immediate local event and the potential update from the server.
A second solution is to use the new get method (introduced in early 2021), which doesn't have this problematic behavior. Note that this method always tries to first fetch the value from the server, so it will take longer to completely. If your value never changes, it might still be better to use addListenerForSingleValueEvent (but you probably wouldn't have ended up on this page in that case).
As a workaround you can also call keepSynced(true) on the locations where you use a single-value event listener. This ensures that the data is updated whenever it changes, which drastically improves the chance that your single-value event listener will see the current value.
So I have a working solution for this. All you have to do is use ValueEventListener and remove the listener after 0.5 seconds to make sure you've grabbed the updated data by then if needed. Realtime database has very good latency so this is safe. See safe code example below;
public class FirebaseController {
private DatabaseReference mRootRef;
private Handler mHandler = new Handler();
private FirebaseController() {
FirebaseDatabase.getInstance().setPersistenceEnabled(true);
mRootRef = FirebaseDatabase.getInstance().getReference();
}
public static FirebaseController getInstance() {
if (sInstance == null) {
sInstance = new FirebaseController();
}
return sInstance;
}
Then some method you'd have liked to use "addListenerForSingleEvent";
public void getTime(final OnTimeRetrievedListener listener) {
DatabaseReference ref = mRootRef.child("serverTime");
ref.addValueEventListener(new ValueEventListener() {
#Override
public void onDataChange(DataSnapshot dataSnapshot) {
if (listener != null) {
// This can be called twice if data changed on server - SO DEAL WITH IT!
listener.onTimeRetrieved(dataSnapshot.getValue(Long.class));
}
// This can be called twice if data changed on server - SO DEAL WITH IT!
removeListenerAfter2(ref, this);
}
#Override
public void onCancelled(DatabaseError databaseError) {
removeListenerAfter2(ref, this);
}
});
}
// ValueEventListener version workaround for addListenerForSingleEvent not working.
private void removeListenerAfter2(DatabaseReference ref, ValueEventListener listener) {
mHandler.postDelayed(new Runnable() {
#Override
public void run() {
HelperUtil.logE("removing listener", FirebaseController.class);
ref.removeEventListener(listener);
}
}, 500);
}
// ChildEventListener version workaround for addListenerForSingleEvent not working.
private void removeListenerAfter2(DatabaseReference ref, ChildEventListener listener) {
mHandler.postDelayed(new Runnable() {
#Override
public void run() {
HelperUtil.logE("removing listener", FirebaseController.class);
ref.removeEventListener(listener);
}
}, 500);
}
Even if they close the app before the handler is executed, it will be removed anyways.
Edit: this can be abstracted to keep track of added and removed listeners in a HashMap using reference path as key and datasnapshot as value. You can even wrap a fetchData method that has a boolean flag for "once" if this is true it would do this workaround to get data once, else it would continue as normal.
You're Welcome!
You can create transaction and abort it, then onComplete will be called when online (nline data) or offline (cached data)
I previously created function which worked only if database got connection lomng enough to do synch. I fixed issue by adding timeout. I will work on this and test if this works. Maybe in the future, when I get free time, I will create android lib and publish it, but by then it is the code in kotlin:
/**
* #param databaseReference reference to parent database node
* #param callback callback with mutable list which returns list of objects and boolean if data is from cache
* #param timeOutInMillis if not set it will wait all the time to get data online. If set - when timeout occurs it will send data from cache if exists
*/
fun readChildrenOnlineElseLocal(databaseReference: DatabaseReference, callback: ((mutableList: MutableList<#kotlin.UnsafeVariance T>, isDataFromCache: Boolean) -> Unit), timeOutInMillis: Long? = null) {
var countDownTimer: CountDownTimer? = null
val transactionHandlerAbort = object : Transaction.Handler { //for cache load
override fun onComplete(p0: DatabaseError?, p1: Boolean, data: DataSnapshot?) {
val listOfObjects = ArrayList<T>()
data?.let {
data.children.forEach {
val child = it.getValue(aClass)
child?.let {
listOfObjects.add(child)
}
}
}
callback.invoke(listOfObjects, true)
}
override fun doTransaction(p0: MutableData?): Transaction.Result {
return Transaction.abort()
}
}
val transactionHandlerSuccess = object : Transaction.Handler { //for online load
override fun onComplete(p0: DatabaseError?, p1: Boolean, data: DataSnapshot?) {
countDownTimer?.cancel()
val listOfObjects = ArrayList<T>()
data?.let {
data.children.forEach {
val child = it.getValue(aClass)
child?.let {
listOfObjects.add(child)
}
}
}
callback.invoke(listOfObjects, false)
}
override fun doTransaction(p0: MutableData?): Transaction.Result {
return Transaction.success(p0)
}
}
In the code if time out is set then I set up timer which will call transaction with abort. This transaction will be called even when offline and will provide online or cached data (in this function there is really high chance that this data is cached one).
Then I call transaction with success. OnComplete will be called ONLY if we got response from firebase database. We can now cancel timer (if not null) and send data to callback.
This implementation makes dev 99% sure that data is from cache or is online one.
If you want to make it faster for offline (to don't wait stupidly with timeout when obviously database is not connected) then check if database is connected before using function above:
DatabaseReference connectedRef = FirebaseDatabase.getInstance().getReference(".info/connected");
connectedRef.addValueEventListener(new ValueEventListener() {
#Override
public void onDataChange(DataSnapshot snapshot) {
boolean connected = snapshot.getValue(Boolean.class);
if (connected) {
System.out.println("connected");
} else {
System.out.println("not connected");
}
}
#Override
public void onCancelled(DatabaseError error) {
System.err.println("Listener was cancelled");
}
});
When workinkg with persistence enabled, I counted the times the listener received a call to onDataChange() and stoped to listen at 2 times. Worked for me, maybe helps:
private int timesRead;
private ValueEventListener listener;
private DatabaseReference ref;
private void readFB() {
timesRead = 0;
if (ref == null) {
ref = mFBDatabase.child("URL");
}
if (listener == null) {
listener = new ValueEventListener() {
#Override
public void onDataChange(DataSnapshot dataSnapshot) {
//process dataSnapshot
timesRead++;
if (timesRead == 2) {
ref.removeEventListener(listener);
}
}
#Override
public void onCancelled(DatabaseError databaseError) {
}
};
}
ref.removeEventListener(listener);
ref.addValueEventListener(listener);
}

UI Thread issue with view model in MVVMCross

I am using MVVMCross with my cross-platform Windows Phone and Android app. In the core project's main view model, I am doing some background work using TPL and I want to make sure that in the callback, when I make changes to the properties of the view model which will trigger UI change, that the code is run on UI thread, how do I achieve this?
For code, here is how it likes
private MvxGeoLocation _currentLocation;
private Task<MvxGeoLocation> GetCurrentLocation()
{
return Task.Factory.StartNew(() =>
{
while (_currentLocation == null && !LocationRetrievalFailed)
{
}
return _currentLocation;
});
}
var location = await GetCurrentLocation();
if (LocationRetrievalFailed)
{
if (location == null)
{
ReverseGeocodingRequestFailed = true;
return;
}
// Show toast saying that we are using the last known location
}
Address = await GooglePlaceApiClient.ReverseGeocoding(location);
Did you try IMvxMainThreadDispatcher?
var dispatcher = Mvx.Resolve<IMvxMainThreadDispatcher>();
dispatcher.RequestMainThreadAction(()=> { .... });
See more on the implementation:
https://github.com/MvvmCross/MvvmCross/search?q=IMvxMainThreadDispatcher&type=Code
Usually I don't think you need this though.
Since you start the async processing from main thread, the async operations should return back to main thread.
Can you give an example of the async code you are doing?
Update on 24th August 2020:
As #claudio-redi has mentioned, ExecuteOnMainThreadAsync needs to be used. But Mvx.Resolve is now obsolete. So the latest snippet would be:
var mainThreadAsyncDispatcher = Mvx.IoCProvider.Resolve<IMvxMainThreadAsyncDispatcher>();
await mainThreadAsyncDispatcher.ExecuteOnMainThreadAsync( async ()=> { await SomeAsyncTask() });
Method RequestMainThreadAction is now obsolete. Today you have to do
var dispatcher = Mvx.Resolve<IMvxMainThreadAsyncDispatcher>();
await dispatcher.ExecuteOnMainThreadAsync(()=> { .... });

Correct way to call async methods from within a data-bound property setter?

Now I know properties do not support async/await for good reasons. But sometimes you need to kick off some additional background processing from a property setter - a good example is data binding in a MVVM scenario.
In my case, I have a property that is bound to the SelectedItem of a ListView. Of course I immediately set the new value to the backing field and the main work of the property is done. But the change of the selected item in the UI needs also to trigger a REST service call to get some new data based on the now selected item.
So I need to call an async method. I can't await it, obviously, but I also do not want to fire and forget the call as I could miss exceptions during the async processing.
Now my take is the following:
private Feed selectedFeed;
public Feed SelectedFeed
{
get
{
return this.selectedFeed;
}
set
{
if (this.selectedFeed != value)
{
this.selectedFeed = value;
RaisePropertyChanged();
Task task = GetFeedArticles(value.Id);
task.ContinueWith(t =>
{
if (t.Status != TaskStatus.RanToCompletion)
{
MessengerInstance.Send<string>("Error description", "DisplayErrorNotification");
}
});
}
}
}
Ok so besides the fact I could move out the handling from the setter to a synchronous method, is this the correct way to handle such a scenario? Is there a better, less cluttered solution I do not see?
Would be very interested to see some other takes on this problem. I'm a bit curious that I was not able to find any other discussions on this concrete topic as it seems very common to me in MVVM apps that make heavy use of databinding.
I have a NotifyTaskCompletion type in my AsyncEx library that is essentially an INotifyPropertyChanged wrapper for Task/Task<T>. AFAIK there is very little information currently available on async combined with MVVM, so let me know if you find any other approaches.
Anyway, the NotifyTaskCompletion approach works best if your tasks return their results. I.e., from your current code sample it looks like GetFeedArticles is setting data-bound properties as a side effect instead of returning the articles. If you make this return Task<T> instead, you can end up with code like this:
private Feed selectedFeed;
public Feed SelectedFeed
{
get
{
return this.selectedFeed;
}
set
{
if (this.selectedFeed == value)
return;
this.selectedFeed = value;
RaisePropertyChanged();
Articles = NotifyTaskCompletion.Create(GetFeedArticlesAsync(value.Id));
}
}
private INotifyTaskCompletion<List<Article>> articles;
public INotifyTaskCompletion<List<Article>> Articles
{
get { return this.articles; }
set
{
if (this.articles == value)
return;
this.articles = value;
RaisePropertyChanged();
}
}
private async Task<List<Article>> GetFeedArticlesAsync(int id)
{
...
}
Then your databinding can use Articles.Result to get to the resulting collection (which is null until GetFeedArticlesAsync completes). You can use NotifyTaskCompletion "out of the box" to data-bind to errors as well (e.g., Articles.ErrorMessage) and it has a few boolean convenience properties (IsSuccessfullyCompleted, IsFaulted) to handle visibility toggles.
Note that this will correctly handle operations completing out of order. Since Articles actually represents the asynchronous operation itself (instead of the results directly), it is updated immediately when a new operation is started. So you'll never see out-of-date results.
You don't have to use data binding for your error handling. You can make whatever semantics you want by modifying the GetFeedArticlesAsync; for example, to handle exceptions by passing them to your MessengerInstance:
private async Task<List<Article>> GetFeedArticlesAsync(int id)
{
try
{
...
}
catch (Exception ex)
{
MessengerInstance.Send<string>("Error description", "DisplayErrorNotification");
return null;
}
}
Similarly, there's no notion of automatic cancellation built-in, but again it's easy to add to GetFeedArticlesAsync:
private CancellationTokenSource getFeedArticlesCts;
private async Task<List<Article>> GetFeedArticlesAsync(int id)
{
if (getFeedArticlesCts != null)
getFeedArticlesCts.Cancel();
using (getFeedArticlesCts = new CancellationTokenSource())
{
...
}
}
This is an area of current development, so please do make improvements or API suggestions!
public class AsyncRunner
{
public static void Run(Task task, Action<Task> onError = null)
{
if (onError == null)
{
task.ContinueWith((task1, o) => { }, TaskContinuationOptions.OnlyOnFaulted);
}
else
{
task.ContinueWith(onError, TaskContinuationOptions.OnlyOnFaulted);
}
}
}
Usage within the property
private NavigationMenuItem _selectedMenuItem;
public NavigationMenuItem SelectedMenuItem
{
get { return _selectedMenuItem; }
set
{
_selectedMenuItem = val;
AsyncRunner.Run(NavigateToMenuAsync(_selectedMenuItem));
}
}
private async Task NavigateToMenuAsync(NavigationMenuItem newNavigationMenu)
{
//call async tasks...
}