I have the following code using Akavache in a Xamarin app and it's not behaving the way I would think it should. Probably my misunderstanding of how it should be but it's driving me crazy.
So in my viewmodel I'm making the call to FetchNewsCategories and specifying a cache of 5 minutes for the item. What I'd expect to happen is that if the cache item is not there, it would make a call to the fetchFunc (ie. FetchNewsCategoriesAsync) but if I call the service any number of times inside the cache timeout of 5 minutes, it should just give me the cached item and not do the server call. In all cases that I've tried, it keeps doing the rest call and never gives me the cached item. I've also tried this with GetAndFetchLatest and if there is a cached item, it doesn't make the rest call but it also doesn't make the call in the subscribe event in the viewmodel.
Any ideas what I'm doing wrong here?
EDIT: I tested this same code on Android (Nexus 5 KitKat API19) and it's working flawlessly. I'm going to reset my IOS emulator and see if something was just out of whack.
NewsService.cs
public static async Task<ServiceResponse<List<ArticleCategoryInfo>>> FetchNewsCategoriesAsync(BlogSourceType blogSource)
{
return await ServiceClient.POST<List<ArticleCategoryInfo>>(Config.ApiUrl + "news/categories", new
{
ModuleId = Int32.Parse(Config.Values[blogSource == BlogSourceType.News ? ConfigKeys.KEY_NEWS_MODULE_ID : ConfigKeys.KEY_BLOG_MODULE_ID])
});
}
public static IObservable<ServiceResponse<List<ArticleCategoryInfo>>> FetchNewsCategories(BlogSourceType blogSource)
{
var cache = BlobCache.LocalMachine;
var cachedCategories = cache.GetOrFetchObject("categories" + blogSource,
async () => await FetchNewsCategoriesAsync(blogSource),
DateTime.Now.AddMinutes(5));
return cachedCategories;
}
NewsViewModel.cs
public async Task LoadCategories()
{
var cachedCategories = NewsService.FetchNewsCategories(blogSource);
cachedCategories.Subscribe((obj) => { Device.BeginInvokeOnMainThread(() => DisplayCategories(obj.Result,"Subscribe"));});
return;
}
private void DisplayCategories(IList<ArticleCategoryInfo> categories, string source)
{
Categories.Clear();
System.Diagnostics.Debug.WriteLine("Redisplaying categories from " + source);
foreach (var item in categories)
{
Categories.Add(item);
}
}
Just wanted to add my resolution to the issue I experienced above for reference to others with this problem.
The ServiceResponse object that I was trying to cache had an HttpResponseMessage in it which I suspect was causing a serialization error, probably a cyclical reference, so it never did get cached and ended up calling the endpoint every time. I ended up putting an [IgnoreDataMemberAttribute] on that property so it wasn't serialized and the problems went away.
I ended up handling the subscribe in the following manner to handle errors and to make sure the activity indicator bound to the IsBusy property was updated properly.
public async Task LoadActivities(bool refresh)
{
IsBusy = true;
if (refresh) OlderThanJournalId = int.MaxValue;
var cached = ActivityService.FetchJournalItems(GroupId, OlderThanJournalId, refresh);
cached.Subscribe((result) => { Device.BeginInvokeOnMainThread(() => {
DisplayActivities(result);
}); }, (err) => HandleError(err), () => IsBusy = false);
}
public void HandleError(Exception ex) {
IsBusy = false;
DialogService.ShowErrorToast(AppResources.ErrorMessage, "Unable to load activity stream.");
System.Diagnostics.Debug.WriteLine(ex.Message);
}
private void DisplayActivities(ServiceResponse<List<JournalItem>> response)
{
if (!response.IsConnected) {
DialogService.ShowInfoToast(AppResources.ErrorMessage, AppResources.NotConnected);
return;
}
if (!response.Authorized) {
App.LoginManager.Logout();
}
Activities.Clear();
foreach (var item in response.Result)
{
Activities.Add(item);
}
}
BeginInvokeOnMainThread is used to make sure that the updates to the ObservableCollection in DisplayActivities are seen in the UI.
Related
I'm trying to figure out how to refresh the client-side component after button click.
Repo Link with example: https://github.com/ovie91/RefreshComponent
Site /test or from nav menu test
So I have OnInitializedAsync method that is retrieving data from API
protected override async Task OnInitializedAsync()
{
result = await (some API Call);
}
Then I have a method connected to the button
private async void ButtonClick()
{
await (some API Call);
result = null;
this.StateHasChanged(); <--- Doesnt work :<
}
I have tried to use this.StateHasChanged(); but there is no reaction.
As a workaround, I can force you to navigate again to the same website but this refresh "Whole" website but not a component.
Any ideas on how to deal with it?
whole code (stripped to minimum):
#page "/test"
#inject HttpClient Http
#if (result == null)
{
<p>Loading...<p>
}
else
{
#result
<button #onclick="(() => ButtonClick())">Click</button>
}
#code {
private APIObject result;
protected override async Task OnInitializedAsync()
{
result = await (some API Call);
}
private async void ButtonClick()
{
await (some API Call);
result = null;
this.StateHasChanged(); <--- Doesnt work :<
}
}
Update
I want to refresh component so OnInitializedAsync would be triggered again and that would mean I don't have to run the same code again after button click. Hope you understand what I mean.
To get the desired output you just have to shuffle the lines a little, from:
private async void ButtonClick()
{
await (some API Call); // UI checks if an update is needed (No)
result = null; // now an update is needed
this.StateHasChanged(); <--- Doesnt work :< // actually: not needed
}
to:
private async Task ButtonClick()
{
result = null; // change the state
//this.StateHasChanged(); // not needed, a request is pending
await (some API Call); // should show '<h3>Loading</h3>' now
}
Note that the UI is updated when an await releases the Thread.
however, from your answer we get
var APICall = await Http.GetAsync("SomeAPI");
Thread.Sleep(2000);
This should work when Http.GetAsync("SomeAPI"); really is an async call and not just some stand-in pseudo code. Because Thread.Sleep(2000); will really freeze things.
If you want to make sure:
private async Task GetData()
{
await Task.Delay(1); // release the thread for rendering
var APICall = await Http.GetAsync("SomeAPI");
Random rnd = new Random();
Thread.Sleep(2000); // Task.Delay() is much preferred
result = "Random Number: " + rnd.Next();
}
Thread.Sleep() is appropriate to simulate some CPU (not I/O) intensive code. So I'm not saying it's wrong but be aware of the difference.
And it is much better to make eventhandlers async Task instead of async void but that is not the direct problem here.
From here:
Blazor uses a synchronization context (SynchronizationContext) to enforce a single logical thread of execution. A component's lifecycle methods and any event callbacks that are raised by Blazor are executed on the synchronization context.
Blazor Server's synchronization context attempts to emulate a single-threaded environment so that it closely matches the WebAssembly model in the browser, which is single threaded. At any given point in time, work is performed on exactly one thread, giving the impression of a single logical thread. No two operations execute concurrently.
So as enet asnwered, you should use async Task signature instead of async void.
I have moved API call to another Method and inside of OnInitializedAsync I called it.
Then when I reset the result variable to see Loading state I'm able to "refresh" component to achieve that you need to add. this.StateHasChanged()
Now I have a responsive component to updates that are happening :)
#page "/test"
#using System.Threading;
#inject HttpClient Http
#if (result == null)
{
<h3>Loading</h3>
}
else
{
#result
<button #onclick="(() => ButtonClick())">Click</button>
}
#code {
private string result;
protected override async Task OnInitializedAsync()
{
await GetData();
}
private async Task GetData()
{
var APICall = await Http.GetAsync("SomeAPI");
Random rnd = new Random();
Thread.Sleep(2000);
result = "Random Number: " + rnd.Next();
}
private async Task ButtonClick()
{
await Http.GetAsync("SomeAPIcall");
result = null; // required to see loading state.
this.StateHasChanged(); // when added model is refreshed and Loading state is visible.
await GetData();
}
}
I have this code:
protected override async void OnAppearing()
{
base.OnAppearing();
Subscribe();
vm.IsBusy = true;
if (Change.firstTimeCardsTab == true)
{
SetButtons(Settings.cc.ShortText());
await Task.Run(() => GetOnAppearingData());
}
}
private void GetOnAppearingData()
{
App.DB.GetData();
AddDetailSection();
SetPageDetails();
Change.firstTimeCardsTab = false;
vm.IsBusy = false;
}
vm.IsBusy = true; shows an activity indicator on the page and vm.IsBusy = false; would I hoped turn it off.
However when I run the code I get a message saying:
UIKit Consistency error: you are calling a UIKit method that can only
be invoked from a UI thread.
Can someone give me advice on how I can set the activity indicator to show, then get the data and then set it to not show? Note that AddDetailSection(); adds some elements to the XAML. Could this be the problem?
UI can be manipulated only from the main thread. There is a method Device.BeginInvokeOnMainThread(Action) that can help to solve your issue, more can be found in the official documentation.
Just wrap all UI related actions by it:
Device.BeginInvokeOnMainThread(() => {
SetPageDetails();
// Any other actions
});
I guess there is not built-in way to achieve that:
I have some cached data, that need to be always up to date (interval of few 10s of minutes). Its generation takes around 1-2 minutes, therefore it leads sometimes to timeout requests.
For performances optimisation, I put it into memory cache, using Cache.GetOrCreateAsync, so I am sure to have fast access to the data during 40 minutes. However it still takes time when the cache expires.
I would like to have a mechanism that auto-refreshes the data before its expiration, so the users are not impacted from this refresh and can still access the "old data" during the refresh.
It would actually be adding a "pre-expiration" process, that would avoid data expiration to arrive at its term.
I feel that is not the functioning of the default IMemoryCache cache, but I might be wrong?
Does it exist? If not, how would you develop this feature?
I am thinking of using PostEvictionCallbacks, with an entry set to be removed after 35 minutes and that would trigger the update method (it involves a DbContext).
This is how I solve it:
The part called by the web request (the "Create" method should be called only the first time).
var allPlaces = await Cache.GetOrCreateAsync(CACHE_KEY_PLACES
, (k) =>
{
k.AbsoluteExpirationRelativeToNow = TimeSpan.FromMinutes(40);
UpdateReset();
return GetAllPlacesFromDb();
});
And then the magic (This could have been implemented through a timer, but didn't want to handle timers there)
// This method adds a trigger to refresh the data from background
private void UpdateReset()
{
var mo = new MemoryCacheEntryOptions();
mo.RegisterPostEvictionCallback(RefreshAllPlacessCache_PostEvictionCallback);
mo.AddExpirationToken(new CancellationChangeToken(new CancellationTokenSource(TimeSpan.FromMinutes(35)).Token));
Cache.Set(CACHE_KEY_PLACES_RESET, DateTime.Now, mo);
}
// Method triggered by the cancellation token that triggers the PostEvictionCallBack
private async void RefreshAllPlacesCache_PostEvictionCallback(object key, object value, EvictionReason reason, object state)
{
// Regenerate a set of updated data
var places = await GetLongGeneratingData();
Cache.Set(CACHE_KEY_PLACES, places, TimeSpan.FromMinutes(40));
// Re-set the cache to be reloaded in 35min
UpdateReset();
}
So the cache gets two entries, the first one with the data, expiring after 40 minutes, the second one expiring after 35min via a cancellation token that triggers the post eviction method.
This callback refreshes the data before it expires.
Keep in mind that this will keep the website awake and using memory even if not used.
** * UPDATE USING TIMERS * **
The following class is registered as a singleton. DbContextOptions is passed instead of DbContext to create a DbContext with the right scope.
public class SearchService
{
const string CACHE_KEY_ALLPLACES = "ALL_PLACES";
protected readonly IMemoryCache Cache;
private readonly DbContextOptions<AppDbContext> AppDbOptions;
public SearchService(
DbContextOptions<AppDbContext> appDbOptions,
IMemoryCache cache)
{
this.AppDbOptions = appDbOptions;
this.Cache = cache;
InitTimer();
}
private void InitTimer()
{
Cache.Set<AllEventsResult>(CACHE_KEY_ALLPLACESS, new AllPlacesResult() { Result = new List<SearchPlacesResultItem>(), IsBusy = true });
Timer = new Timer(TimerTickAsync, null, 1000, RefreshIntervalMinutes * 60 * 1000);
}
public Task LoadingTask = Task.CompletedTask;
public Timer Timer { get; set; }
public long RefreshIntervalMinutes = 10;
public bool LoadingBusy = false;
private async void TimerTickAsync(object state)
{
if (LoadingBusy) return;
try
{
LoadingBusy = true;
LoadingTask = LoadCaches();
await LoadingTask;
}
catch
{
// do not crash the app
}
finally
{
LoadingBusy = false;
}
}
private async Task LoadCaches()
{
try
{
var places = await GetAllPlacesFromDb();
Cache.Set<AllPlacesResult>(CACHE_KEY_ALLPLACES, new AllPlacesResult() { Result = places, IsBusy = false });
}
catch{}
}
private async Task<List<SearchPlacesResultItem>> GetAllPlacesFromDb()
{
// blablabla
}
}
Note:
DbContext options require to be registered as singleton, default options are now Scoped (I believe to allow simpler multi-tenancy configurations)
services.AddDbContext<AppDbContext>(o =>
{
o.UseQueryTrackingBehavior(QueryTrackingBehavior.NoTracking);
o.UseSqlServer(connectionString);
},
contextLifetime: ServiceLifetime.Scoped,
optionsLifetime: ServiceLifetime.Singleton);
private var csv:URLLoader = new URLLoader();
private var array:Array = new Array();
private var urlr:URLRequest = new URLRequest();
public function loadRecipe(path:String):void
{
try
{
csv.dataFormat = URLLoaderDataFormat.TEXT;
urlr = new URLRequest(path);
csv.addEventListener(Event.COMPLETE, finishRecipe);
csv.load(urlr);
}
catch (e:SecurityErrorEvent)
{
trace("1");
}
catch (e:IOErrorEvent)
{
trace("2");
}
}
public function finishRecipe(e:Event):void
{
var csvString:String = csv.data as String;
array = csvString.split(",");
}
My code that I'm working with is above. I can't get the completion event to ever trigger, that is, my array is never populated. Can anyone give me insight as to why?
EDIT:
I changed to get rid of all the weak references and check for errors. I don't get any errors.
I've run into this bug frequently over the years. When certain browsers (FireFox, Chrome) retrieve the file from cache instead of network, the loader will dispatch a PROGRESS event but never COMPLETE.
Try clearing your browser cache and see whether the file loads correctly next time. If so, you can do one of two things as a workaround:
Break the cache by adding a random string to the end of your request URL.
urlr = new URLRequest(path + "?cachebust=" + Math.floor(100000+900000*Math.random()));
This is simple to code, but has obvious disadvantages, forcing unnecessary reloads.
Listen for both COMPLETE and PROGRESS events. In the PROGRESS handler, check to see if bytesLoaded matches bytesTotal. If it does, remove all handlers and continue as if it were a COMPLETE event.
... somewhere in your code ...
loader.addEventListener(Event.COMPLETE, handleComplete);
loader.addEventListener(ProgressEvent.PROGRESS, handleProgress);
... somewhere else
private function handleProgress(evt:ProgressEvent):void
{
checkLoadComplete();
}
private function handleComplete(evt:Event):void
{
checkLoadComplete();
}
private function checkLoadComplete():void
{
if(loader.bytesTotal > 0 && loader.bytesLoaded == loader.bytesTotal) {
loader.removeEventListener(Event.COMPLETE, handleComplete);
loader.removeEventListener(ProgressEvent.PROGRESS, handleProgress);
... your code here
}
}
Ummm just looking at the code you provided it seems like you actually try to catch the errors using try/catch. In order to find errors, you have to start listening to them on the actual loader. Like this:
public function Foobar() {
var loader:URLLoader;
loader.addEventListener(IOErrorEvent.IO_ERROR, onIOError);
loader.addEventListener(SecurityErrorEvent.SECURITY_ERROR, onSecurityError);
loader.addEventListener(ProgressEvent.PROGRESS, onProgress);
loader.addEventListener(Event.COMPLETE, onComplete);
}
private function onComplete(e:Event):void {}
private function onProgress(e:ProgressEvent):void {}
private function onSecurityError(e:SecurityErrorEvent):void {}
private function onIOError(e:IOErrorEvent):void {}
I have a list of addresses that i want to visit using httpWebRequest.
All i need is the statuscode returned by the server.
I have tried to foreach through them and begin a httpWebRequest on each of them, but then i only receive the callback from the last one.
It seems like only one webrequest is allowed at a time.
I'm having quite a hard time understanding how to do this without the GetResponse, which is not allowed in silverlight.
The code is running in a backgroundworker.
And i am using Mango - WP7.1
How do i solve that?
foreach (var current in Addresses)
{
var request = HttpWebRequest.Create(current);
request.BeginGetResponse(r =>
{
try
{
var response = (HttpWebResponse)request.EndGetResponse(r);
Deployment.Current.Dispatcher.BeginInvoke(() =>
{
//BOOM RECEIVED
});
}
catch (Exception)
{
Debug.WriteLine("Error in EndGetResponse");
}
}, null);
}
Thanks in advance =)
Your problem of a single response is most likely being caused by your use of anonymous methods and the the way scoping works when you put these inside loops. You are throwing away the earlier request references on each step through the loop.
See my blogpost on the topic here http://csainty.blogspot.com/2010/10/windows-phone-7asynchronous-programming.html
The simplest way to illustrate this is to rewrite your code with full methods, this forces you to consider the scope instead of just blindly referening external variables in your delegates.
foreach (var current in Addresses)
{
var request = HttpWebRequest.Create(current);
request.BeginGetResponse(EndGetResponse, new RequestState { Request = request, Address = current });
}
private void EndGetResponse(IAsyncResult result) {
try {
var state = (RequestState)result.AsyncState;
var response = (HttpWebResponse)state.Request.EndGetResponse(result);
Deployment.Current.Dispatcher.BeginInvoke(GotResponse, state.Address, response.StatusCode);
} catch (Exception) {
Debug.WriteLine("Error in EndGetResponse");
}
}
private void GotResponse(Address address, HttpStatusCode code) {
//BOOM RECEIVED
}
public class RequestState {
HttpWebRequest Request { get; set; }
Address Address { get; set; }
}
Once you solve the scoping issues you can rewrite back into anonymos methods for stylistic reasons if you like.
This will only solve your first problem of getting all the responses back however, I assume you also need to run some code when all the requests are complete to check the status of the whole batch?
That is a different problem altogether.
You can not use WaitOne() or anything like that, it will lock your thread and stop the requests from actually running at all. You will probably want to call off to another method in you BOOM code that stores away the result and checks if all the results are in yet.