I would like some feedback to see if I'm using SimpleIoc in the correct way.
The code below works, but I'm not sure if it's best practice.
I have an UWP XAML DocumentPage class on which I want to show an IRpcDocument.
I want to use the DocumentPage for both RpcDocumentA and RpcDocumentB. The user can navigate to both types of IRpcDocument. So the application should be able to switch between the two 'on the fly'.
So I wrote my DocumentPageViewModel
public class DocumentPageViewModel : ViewModelBase
{
public IRpcDocument RpcDocument;
public DocumentPageViewModel(IRpcDocument rpcDocument)
{
RpcDocument = rpcDocument;
}
}
And my ViewModelLocator
class ViewModelLocator
{
static ViewModelLocator()
{
ServiceLocator.SetLocatorProvider(() => SimpleIoc.Default);
SimpleIoc.Default.Register<DocumentPageViewModel>();
}
public DocumentPageViewModel SimpleIoc.Default.Register<DocumentPageViewModel>
{
get
{
return ServiceLocator.Current.GetInstance<SimpleIoc.Default.Register<DocumentPageViewModel>>(Guid.NewGuid().ToString());
}
}
}
When I'm navigating to the DocumentPage I call:
SimpleIoc.Default.Register<IRpcDocument , RpcDocumentA>();
await NavigationService.NavigateAsync(typeof(DocumentPage), DocumentIdParameter);
The app then navigates to the DocumentPage, constructs the RpcDocumentA, makes the necessary RPC calls to fetch the data and shows the document.
The first line tells the IoC framework it should expect an RpcDocumentA in its constructor, the second one triggers navigation. So in this case, im not registering the interface in the static ViewModelLocator().
So for each time I navigate I call SimpleIoc.Default.Register<IRpcDocument , RpcDocumentA> or SimpleIoc.Default.Register<IRpcDocument , RpcDocumentB>
This works, but is this the right way to do this? I suspect it's not.
Related
My goal
I want to create a new IdentityUser and show all the users already created through the same Blazor page. This page has:
a form through you will create an IdentityUser
a third-party's grid component (DevExpress Blazor DxDataGrid) that shows all users using UserManager.Users property. This component accepts an IQueryable as a data source.
Problem
When I create a new user through the form (1) I will get the following concurrency error:
InvalidOperationException: A second operation started on this context before a previous operation completed. Any instance members are not guaranteed to be thread-safe.
I think the problem is related to the fact that CreateAsync(IdentityUser user) and UserManager.Users are referring the same DbContext
The problem isn't related to the third-party's component because I reproduce the same problem replacing it with a simple list.
Step to reproduce the problem
create a new Blazor server-side project with authentication
change Index.razor with the following code:
#page "/"
<h1>Hello, world!</h1>
number of users: #Users.Count()
<button #onclick="#(async () => await Add())">click me</button>
<ul>
#foreach(var user in Users)
{
<li>#user.UserName</li>
}
</ul>
#code {
[Inject] UserManager<IdentityUser> UserManager { get; set; }
IQueryable<IdentityUser> Users;
protected override void OnInitialized()
{
Users = UserManager.Users;
}
public async Task Add()
{
await UserManager.CreateAsync(new IdentityUser { UserName = $"test_{Guid.NewGuid().ToString()}" });
}
}
What I noticed
If I change Entity Framework provider from SqlServer to Sqlite then the error will never show.
System info
ASP.NET Core 3.1.0 Blazor Server-side
Entity Framework Core 3.1.0 based on SqlServer provider
What I have already seen
Blazor A second operation started on this context before a previous operation completed: the solution proposed doesn't work for me because even if I change my DbContext scope from Scoped to Transient I still using the same instance of UserManager and its contains the same instance of DbContext
other guys on StackOverflow suggests creating a new instance of DbContext per request. I don't like this solution because it is against Dependency Injection principles. Anyway, I can't apply this solution because DbContext is wrapped inside UserManager
Create a generator of DbContext: this solution is pretty like the previous one.
Using Entity Framework Core with Blazor
Why I want to use IQueryable
I want to pass an IQueryable as a data source for my third-party's component because its can apply pagination and filtering directly to the Query. Furthermore IQueryable is sensitive to CUD
operations.
UPDATE (08/19/2020)
Here you can find the documentation about how to use Blazor and EFCore together
UPDATE (07/22/2020)
EFCore team introduces DbContextFactory inside Entity Framework Core .NET 5 Preview 7
[...] This decoupling is very useful for Blazor applications, where using IDbContextFactory is recommended, but may also be useful in other scenarios.
If you are interested you can read more at Announcing Entity Framework Core EF Core 5.0 Preview 7
UPDATE (07/06/2020)
Microsoft released a new interesting video about Blazor (both models) and Entity Framework Core. Please take a look at 19:20, they are talking about how to manage concurrency problem with EFCore
General solution
I asked Daniel Roth BlazorDeskShow - 2:24:20 about this problem and it seems to be a Blazor Server-Side problem by design.
DbContext default lifetime is set to Scoped. So if you have at least two components in the same page which are trying to execute an async query then we will encounter the exception:
InvalidOperationException: A second operation started on this context before a previous operation completed. Any instance members are not guaranteed to be thread-safe.
There are two workaround about this problem:
(A) set DbContext's lifetime to Transient
services.AddDbContext<ApplicationDbContext>(opt =>
opt.UseSqlServer(Configuration.GetConnectionString("DefaultConnection")), ServiceLifetime.Transient);
(B) as Carl Franklin suggested (after my question): create a singleton service with a static method which returns a new instance of DbContext.
anyway, each solution works because they create a new instance of DbContext.
About my problem
My problem wasn't strictly related to DbContext but with UserManager<TUser> which has a Scoped lifetime. Set DbContext's lifetime to Transient didn't solve my problem because ASP.NET Core creates a new instance of UserManager<TUser> when I open the session for the first time and it lives until I don't close it. This UserManager<TUser> is inside two components on the same page. Then we have the same problem described before:
two components that own the same UserManager<TUser> instance which contains a transient DbContext.
Currently, I solved this problem with another workaround:
I don't use UserManager<TUser> directly instead, I create a new instance of it through IServiceProvider and then it works. I am still looking for a method to change the UserManager's lifetime instead of using IServiceProvider.
tips: pay attention to services' lifetime
This is what I learned. I don't know if it is all correct or not.
I downloaded your sample and was able to reproduce your problem. The problem is caused because Blazor will re-render the component as soon as you await in code called from EventCallback (i.e. your Add method).
public async Task Add()
{
await UserManager.CreateAsync(new IdentityUser { UserName = $"test_{Guid.NewGuid().ToString()}" });
}
If you add a System.Diagnostics.WriteLine to the start of Add and to the end of Add, and then also add one at the top of your Razor page and one at the bottom, you will see the following output when you click your button.
//First render
Start: BuildRenderTree
End: BuildRenderTree
//Button clicked
Start: Add
(This is where the `await` occurs`)
Start: BuildRenderTree
Exception thrown
You can prevent this mid-method rerender like so....
protected override bool ShouldRender() => MayRender;
public async Task Add()
{
MayRender = false;
try
{
await UserManager.CreateAsync(new IdentityUser { UserName = $"test_{Guid.NewGuid().ToString()}" });
}
finally
{
MayRender = true;
}
}
This will prevent re-rendering whilst your method is running. Note that if you define Users as IdentityUser[] Users you will not see this problem because the array is not set until after the await has completed and is not lazy evaluated, so you don't get this reentrancy problem.
I believe you want to use IQueryable<T> because you need to pass it to 3rd party components. The problem is, different components can be rendered on different threads, so if you pass IQueryable<T> to other components then
They might render on different threads and cause the same problem.
They most likely will have an await in the code that consumes the IQueryable<T> and you'll have the same problem again.
Ideally, what you need is for the 3rd party component to have an event that asks you for data, giving you some kind of query definition (page number etc). I know Telerik Grid does this, as do others.
That way you can do the following
Acquire a lock
Run the query with the filter applied
Release the lock
Pass the results to the component
You cannot use lock() in async code, so you'd need to use something like SpinLock to lock a resource.
private SpinLock Lock = new SpinLock();
private async Task<WhatTelerikNeeds> ReadData(SomeFilterFromTelerik filter)
{
bool gotLock = false;
while (!gotLock) Lock.Enter(ref gotLock);
try
{
IUserIdentity result = await ApplyFilter(MyDbContext.Users, filter).ToArrayAsync().ConfigureAwait(false);
return new WhatTelerikNeeds(result);
}
finally
{
Lock.Exit();
}
}
Perhaps not the best approach but rewriting async method as non-async fixes the problem:
public void Add()
{
Task.Run(async () =>
await UserManager.CreateAsync(new IdentityUser { UserName = $"test_{Guid.NewGuid().ToString()}" }))
.Wait();
}
It ensures that UI is updated only after the new user is created.
The whole code for Index.razor
#page "/"
#inherits OwningComponentBase<UserManager<IdentityUser>>
<h1>Hello, world!</h1>
number of users: #Users.Count()
<button #onclick="#Add">click me. I work if you use Sqlite</button>
<ul>
#foreach(var user in Users.ToList())
{
<li>#user.UserName</li>
}
</ul>
#code {
IQueryable<IdentityUser> Users;
protected override void OnInitialized()
{
Users = Service.Users;
}
public void Add()
{
Task.Run(async () => await Service.CreateAsync(new IdentityUser { UserName = $"test_{Guid.NewGuid().ToString()}" })).Wait();
}
}
I found your question looking for answers about the same error message you had.
My concurrency issue appears to have been due to a change that triggered a re-rendering of the visual tree to occur at the same time as (or due to the fact that) I was trying to call DbContext.SaveChangesAsync().
I solved this by overriding my component's ShouldRender() method like this:
protected override bool ShouldRender()
{
if (_updatingDb)
{
return false;
}
else
{
return base.ShouldRender();
}
}
I then wrapped my SaveChangesAsync() call in code that set a private bool field _updatingDb appropriately:
try
{
_updatingDb = true;
await DbContext.SaveChangesAsync();
}
finally
{
_updatingDb = false;
StateHasChanged();
}
The call to StateHasChanged() may or may not be necessary, but I've included it just in case.
This fixed my issue, which was related to selectively rendering a bound input tag or just text depending on if the data field was being edited. Other readers may find that their concurrency issue is also related to something triggering a re-render. If so, this technique may be helpful.
Well, I have a quite similar scenario with this, and I 'solve' mine is to move everything from OnInitializedAsync() to
protected override async Task OnAfterRenderAsync(bool firstRender)
{
if(firstRender)
{
//Your code in OnInitializedAsync()
StateHasChanged();
}
{
It seems solved, but I had no idea to find out the proves. I guess just skip from the initialization to let the component success build, then we can go further.
/******************************Update********************************/
I'm still facing the problem, seems I'm giving a wrong solution to go. When I checked with this Blazor A second operation started on this context before a previous operation completed I got my problem clear. Cause I'm actually dealing with a lot of components initialization with dbContext operations. According to #dani_herrera mention that if you have more than 1 component execute Init at a time, probably the problem appears.
As I took his advise to change my dbContext Service to Transient, and I get away from the problem.
#Leonardo Lurci Had covered conceptually. If you guys are not yet wanting to move to .NET 5.0 preview, i would recommend looking at Nuget package 'EFCore.DbContextFactory', documentation is pretty neat. Essential it emulates AddDbContextFactory. Ofcourse, it creates a context per component.
So far, this is working fine for me so far without any problems...
I ensure single-threaded access by only interacting with my DbContext via a new DbContext.InvokeAsync method, which uses a SemaphoreSlim to ensure only a single operation is performed at a time.
I chose SemaphoreSlim because you can await it.
Instead of this
return Db.Users.FirstOrDefaultAsync(x => x.EmailAddress == emailAddress);
do this
return Db.InvokeAsync(() => ...the query above...);
// Add the following methods to your DbContext
private SemaphoreSlim Semaphore { get; } = new SemaphoreSlim(1);
public TResult Invoke<TResult>(Func<TResult> action)
{
Semaphore.Wait();
try
{
return action();
}
finally
{
Semaphore.Release();
}
}
public async Task<TResult> InvokeAsync<TResult>(Func<Task<TResult>> action)
{
await Semaphore.WaitAsync();
try
{
return await action();
}
finally
{
Semaphore.Release();
}
}
public Task InvokeAsync(Func<Task> action) =>
InvokeAsync<object>(async () =>
{
await action();
return null;
});
public void InvokeAsync(Action action) =>
InvokeAsync(() =>
{
action();
return Task.CompletedTask;
});
#Leonardo Lurci has a great answer with multiple solutions to the problem. I will give my opinion about every solution and which I think it is the best one.
Making DBContext transient - it is a solution but it is not optimized for this cases..
Carl Franklin suggestion - the singleton service will not be able to control the lifetime of the context and will depend on the service requester to dispose the context after use.
Microsoft documentation they talk about injecting DBContext Factory into a component with the IDisposable interface to Dispose the context when the component is destroied. This is not a very good solution, because a lot of problems happen with it, like: performing a context operation and leaving the component before it finishes that operation, will dispose the context and throw exception..
Finally. The best solution so far is to inject the DBContext Factory in the component yes, but whenever you need it, you create a new instance with using statement like bellow:
public async Task GetSomething()
{
using var context = DBFactory.CreateDBContext();
return await context.Something.ToListAsync();
}
Since DbFactory is optimazed when creating new context instances, there is no significante overhead, making it a better choice and better performing than Transient context, it also disposes the context at the end of the method because of "using" statement.
Hope it was useful.
I've created a GitHub repo to better understand the problem here. I have two actions on two different controllers bound to the same route.
http://localhost/sameControllerRoute/{identifier}/values
[Route("sameControllerRoute")]
public class FirstController : Controller
{
public FirstController()
{
// different EF Core DataContext than SecondController and possibly other dependencies than SecondController
}
[HttpGet("{identifier}/values")]
public IActionResult Values(string identifier, DateTime from, DateTime to) // other parameters than SecondController/Values
{
return this.Ok("Was in FirstController");
}
}
[Route("sameControllerRoute")]
public class SecondController : Controller
{
public SecondController()
{
// different EF Core DataContext than FirstController and possibly other dependencies than FirstController
}
[HttpGet("{identifier}/values")]
public IActionResult Values(string identifier, int number, string somethingElse) // other parameters than FirstController/Values
{
return this.Ok("Was in SecondController");
}
}
Since there are two matching routes, the default ActionSelector fails with:
'[...] AmbiguousActionException: Multiple actions matched. [...]'
which is comprehensible.
So I thought I can implement my own ActionSelector. In there I would implement the logic that resolves the issue of multiple routes via same logic depending on the 'identifier' route value (line 27 in code)
If 'identifier' value is a --> then FirstController
If 'identifier' value is b --> then SecondController
and so on...
protected override IReadOnlyList<ActionDescriptor> SelectBestActions(IReadOnlyList<ActionDescriptor> actions)
{
if (actions.HasLessThan(2)) return base.SelectBestActions(actions); // works like base implementation
foreach (var action in actions)
{
if (action.Parameters.Any(p => p.Name == "identifier"))
{
/*** get value of identifier from route (launchSettings this would result in 'someIdentifier') ***/
// call logic that decides whether value of identifier matches the controller
// if yes
return new List<ActionDescriptor>(new[] { action }).AsReadOnly();
// else
// keep going
}
}
return base.SelectBestActions(actions); // fail in all other cases with AmbiguousActionException
}
But I haven't found a good solution to get access to the route values in ActionSelector. Which is comprehensible as well because ModelBinding hasn't kicked in yet since MVC is still trying to figure out the Route.
A dirty solution could be to get hold of IHttpContextAccessor and regex somehow against the path.
But I'm still hoping you could provide a better idea to retrieve the route values even though ModelBinding hasn't happend yet in the request pipeline.
Not sure that you need to use ActionSelector at all for your scenario. Accordingly, to provided code, your controllers works with different types of resources (and so they expect different query parameters). As so, it is better to use different routing templates. Something like this for example:
FirstController: /sameControllerRoute/resourceA/{identifier}/values
SecondController: /sameControllerRoute/resourceB/{identifier}/values
In the scope of REST, when we are talking about /sameControllerRoute/{identifier}/values route template, we expect that different identifier means the same resource type, but different resource name. And so, as API consumers, we expect that all of the following requests are supported
/sameControllerRoute/a/values?from=20160101&to=20170202
/sameControllerRoute/b/values?from=20160101&to=20170202
/sameControllerRoute/a/values?number=1&somethingElse=someData
/sameControllerRoute/b/values?number=1&somethingElse=someData
That is not true in your case
I ended up implementing the proposed solution by the ASP.NET team. This was to implement an IActionConstrain as shown here:
// Copyright (c) .NET Foundation. All rights reserved.
// Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.
using System;
using Microsoft.AspNetCore.Mvc.ActionConstraints;
namespace ActionConstraintSample.Web
{
public class CountrySpecificAttribute : Attribute, IActionConstraint
{
private readonly string _countryCode;
public CountrySpecificAttribute(string countryCode)
{
_countryCode = countryCode;
}
public int Order
{
get
{
return 0;
}
}
public bool Accept(ActionConstraintContext context)
{
return string.Equals(
context.RouteContext.RouteData.Values["country"].ToString(),
_countryCode,
StringComparison.OrdinalIgnoreCase);
}
}
}
https://github.com/aspnet/Entropy/blob/dev/samples/Mvc.ActionConstraintSample.Web/CountrySpecificAttribute.cs
I want to create a maintenance Page for my cake website by checking a Database Table for a maintenance flag using a sub-function of my AppController "initilize()" method. If the flag is set, i throw my custom MaintenanceException(Currently containing nothing special):
class MaintenanceException extends Exception{
}
To handle it, I implemented a custom App Exception Renderer:
class AppExceptionRenderer extends ExceptionRenderer {
public function maintenance($error)
{
return "MAINTENANCE";
}
}
I am able to see this maintenance Text on my website if I set my DB flag to true, but I could not find any information in cake's error handling documentation (http://book.cakephp.org/3.0/en/development/errors.html) on how I can actually tell the Exception renderer to render view "maintenance" with Template "infopage".
Can I even us that function using the ExceptionRenderer without a custom error controller? And If not, how should a proper ErrorController implementation look like? I already tried this:
class AppExceptionRenderer extends ExceptionRenderer {
protected function _getController(){
return new ErrorController();
}
public function maintenance($error)
{
return $this->_getController()->maintenanceAction();
}
}
together with:
class ErrorController extends Controller {
public function __construct($request = null, $response = null) {
parent::__construct($request, $response);
if (count(Router::extensions()) &&
!isset($this->RequestHandler)
) {
$this->loadComponent('RequestHandler');
}
$eventManager = $this->eventManager();
if (isset($this->Auth)) {
$eventManager->detach($this->Auth);
}
if (isset($this->Security)) {
$eventManager->detach($this->Security);
}
$this->viewPath = 'Error';
}
public function maintenanceAction(){
return $this->render('maintenance','infopage');
}
}
But this only throws NullPointerExceptions and a fatal error. I am really dissapointed by the cake manual as well, because the code examples there are nowhere close to give me an impression of how anything could be done and what functionality I actually have.
Because I had some more time today, I spent an hour digging into the cake Source and found a solution that works well for me (and is propably the way it should be done, altough the cake documentation does not really give a hint):
Step 1: Override the _template(...)-Method of the ExceptionRenderer in your own class. In my case, I copied the Method of the parent and added the following Code at the beginning of the method:
$isMaintenanceException = $exception instanceof MaintenanceException;
if($isMaintenanceException){
$template = 'maintenance';
return $this->template = $template;
}
This tells our Renderer, that the error Template called "maintentance"(which should be located in Folder: /Error) is the Error Page content it should render.
Step 2: The only thing we have to do now (And its is kinda hacky in my opinion, but proposed by the cake documentation in this exact way) is to set the layout param in our template to the name of the base layout we want to render with. So just add the following code on top of your error template:
$this->layout = "infopage";
The error controller I created is actually not even needed with this approach, and I still don't know how the cake error controller actually works. maybe I will dig into this if I have more time, but for the moment.
I've written an Xtext-based plugin for some language. I'm now interested in creating a new independent view (as a separate plugin, though it requires my first plugin), which will interact with the currently-active DSL document - and specifically, interact with the model Xtext parsed (I think it's called the Ecore model?). How do I approach this?
I saw I can get an instance of XtextEditor if I do something like this when initializing my view:
getSite().getPage().addPartListener(new MyListener());
And then, in MyListener, override partActivated and partInputChanged to get an IWorkbenchPartReference, which is a reference to the XtextEditor. But what do I do from here? Is this even the right approach to this problem? Should I instead use some notification functionality from the Xtext side?
Found it out! First, you need an actual document:
IXtextDocument doc = editor.getDocument();
Then, if you want to access the model:
doc.modify(new IUnitOfWork.Void<XtextResource>() { // Can also use just IUnitOfWork
#Override public void process(XtextResource state) throws Exception {
...
}
});
And if you want to get live updates whenever it changes:
doc.addModelListener(new IXtextModelListener() {
#Override public void modelChanged(XtextResource resource) {
for (EObject model : resource.getContent()) {
...
}
}
});
I am building a simple application with 4-5 views in Silverlight. I came across MVVM Light toolkit and I think it suits my need.
Background
Application will have views with typical list and details display
Manufacturer
Product
and so on with left navigation, header and footer (User controls).
I am thinking of having a main page with user controls created at design time.
Problem
On selection of links from left navigation control, the central panel should be updated with a different view (like Manufacturer, product and so on)
I understand that Messenger is an option to communicate between different VMs in light toolkit.
Question
How can I design my app with MVVM light toolkit. Central pane need to be loaded with a different view at runtime.
I am particularly looking at help in implementing the navigation portion of the application.
Thank you.
I had to implement basic nagivigtion in an NON mvvm way. I have a message listener sitting on the constructor of my main view that listens for a page navigation message(custom message learn it, love it,use it)then it sets the content source of the nav frame to the url that is sent in the message. I have the URLs for all my page and subpage navigation setup using string constants.
public MainPage()
{
InitializeComponent();
Loaded += OnLoaded;
WebContext.Current.Authentication.LoggedOut +=
new EventHandler<System.ServiceModel.DomainServices.Client.ApplicationServices.AuthenticationEventArgs>(Authentication_LoggedOut);
Messenger.Default.Register<msgs.NavigationRequest<PageURI>>(this, (uri => ContentFrame.Navigate(uri.Content)));
Messenger.Default.Register<WavelengthIS.Core.Messaging.ExceptionMessage>(this, ex => ShowExceptionMessage(ex));
Messenger.Default.Register<WavelengthIS.Core.Messaging.StringMessage>(this, str => ShowMessageForUser(str));
}
public class PageURI : Uri
{
public PageURI(string uriString, UriKind uriKind)
: base(uriString, uriKind)
{
}
}
public class PageLinks
{
public const string SEARCHBYDAYCOUNTVIEW = "/Views/PatientSearchHeaders/SearchByDayCountView.xaml";
public const string SEARCHBYPATIENTCRITERIAVIEW = "/Views/PatientSearchHeaders/SearchByPatientCriteriaView.xaml";
public const string QUESTIONAIRRESHELL = "/Views/QuestionairreViews/QuestionairreShell.xaml";
public const string HOME = "/Views/PrimarySearchView.xaml";
public const string REPORTS = "/Views/ReportsPage.xaml";
public const string LOGINPAGE = "/Views/LoginPageView.xaml";
}
Actual Calling in VM:
private void OnSurveyCommandExecute()
{
Wait.Begin("Loading Patient List...");
_messenger.Send<ReadmitPatientListViewModel>(this);
_messenger.Send<Messages.NavigationRequest<SubClasses.URI.PageURI>>(GetNavRequest_QUESTIONAIRRESHELL());
}
private static Messages.NavigationRequest<SubClasses.URI.PageURI> GetNavRequest_QUESTIONAIRRESHELL()
{
Messages.NavigationRequest<SubClasses.URI.PageURI> navRequest =
new Messages.NavigationRequest<SubClasses.URI.PageURI>(
new SubClasses.URI.PageURI(Helpers.PageLinks.QUESTIONAIRRESHELL, System.UriKind.Relative));
return navRequest;
}