i have this function in my shared project which i referenced to my new project. basically this function job is to fetch contact information from my api.
GetContact() function
i call it when i load my profile page and set the result object (contact) as my profile page bindingcontext.
calling the function
which i binded on my ui.
ui
my problem is, since fetching data on api is always async. my code execute and exit immediately even the result from api is not yet fetch. resulting into a null bindingcontext. is there any way that i can catch the api result and update my bindingcontext so that my be updated ui also?
For the async BindingContext I use the following:
XAML:
<Label Text="{Binding Type}"
TextColor="#142E5F"
Margin="10,0,0,0"
Font="Bold, 15"/>
ViewModel:
public int _type;
public int Type
{
get => _type;
set => SetProperty(ref _type, value);
}
With the SetProperty, you can change the value in execution time. You just need to asign the result.
public override async void OnAppering()
{
base.OnAppering();
Type = await APICall();
}
Related
I have a CollectionView in my .NET MAUI app and I placed it inside a RefreshView. When I call my API to populate this CollectionView, I cache the data so that I don't have to keep making API calls everytime the user hits this page.
In order to streamline my code, I created a private method in my view model that calls my API. The view model looks like this:
public partial MyViewModel : BaseViewModel
{
ObservableCollection<MyModel> MyData { get; } = new();
[RelayCommand]
async Task RefreshData()
{
IsBusy = true;
await GetData(true);
IsBusy = false;
}
private async Task GetData(bool shouldGetFreshData)
{
// Makes API call to get data, then assigns it to MyData collection
}
public async void Init()
{
IsBusy = true;
await GetData(false);
IsBusy = false;
}
}
The XAML for the page looks like this:
<RefreshView
IsRefreshing={Binding IsBusy}
Command={Binding RefreshDataCommand}>
<CollectionView>
...
</CollectionView>
</RefreshView>
I also wired the page to use the MyViewModel as its view model AND OnAppearing(), I call the Init() method of the view model.
Here's what I was expecting which is NOT what's happening:
I thought, the Init() would get called first which then calls the GetData() method with false input parameter. This way, I could use the cached data. And whenever, the user refreshes the CollectionView by pulling it down, the RefreshData() method would be called with true as the input parameter which would force the code to make an API call.
Instead of what I was expecting, here's what's happening:
The Init() method gets called first and as a result, the line with IsBusy = true executes.
This then ends up invoking the RefreshData() method
Then the await GetData(false) in Init() method executes
Then the await GetData(true) in RefreshData() method executes
As a result of all this, the GetData() method gets called twice.
I think, what's triggering this is the IsBusy. I thought IsBusy would only serve as an indicator but not necessarily invoke the RefreshData() method which is bound to the Command of my RefreshView.
Is this normal behavior or am I missing something here?
Apparently, this is "normal" behavior because I'm manually setting IsBusy to true. I decided to leave this question here because this may be a pitfall that affects others.
Here's the actual section in documentation that states this:
And here's the documentation: https://learn.microsoft.com/en-us/dotnet/maui/user-interface/controls/refreshview
So, all I had to do is remove the IsBusy = true in Init() method.
I am trying to create test which be able to check navigation between two viewmodels.
Some code from ResultViewModel
private IMvxCommand homeCommand;
public IMvxCommand HomeCommand => homeCommand ?? (homeCommand = new MvxAsyncCommand(ShowHomePage));
private async Task ShowHomePage()
{
await _navigationService.Navigate<HomeViewModel>();
}
Testing code
private Mock mvxNavigationService = new Mock();
private Mock resultService = new Mock();
[Test]
public void Test1()
{
var viewModel = new ResultViewModel(mvxNavigationService.Object, resultService.Object);
viewModel.HomeCommand.Execute();
mvxNavigationService.Verify(service => service.Navigate<HomeViewModel>());
Assert.Pass();
}
I am getting error
"An expression tree cannot contain a call or invocation that uses optional arguments."
on line
mvxNavigationService.Verify(service => service.Navigate());
Not sure where the issue is.
In Xamarin, a dedicated page navigation method is officially provided.
You can navigate between pages like:
await Navigation.PushAsync (new Page());
You can pop pages from the navigation stack like:
await Navigation.PopAsync ();
For more usage, please refer to: Hierarchical Navigation.
I have a MVC.Net4 Application in which i have Longrunning backround operations which is why i use the System.Threading.Tasks.Task Class.
I start the Tasks after the User clicked a certain Button on the GUI, from that Task im going to use async methods from a intern API which i need to await. This is all working.
public ActionResult DoAsyncAction()
{
//ReturnValue that needs to be further populated by the async action in productive environment
var arv = new AsyncReturnValue
{
ProgressBar = new ProgressBar {Action = "SomeAction", User = "SomeUser"}
};
var t = new Task<AsyncReturnValue>(DoAction, arv);
//Add a Progressbar before Task starts so i can visualize the process on the view
HttpContext.GetSession().ProgressBars.Add(arv.ProgressBar);
//from my understanding this is similar to an event that gets triggered when my DoAction Method finished so i need to remove
//the progressbar there again since the process will be finished in that case
t.ContinueWith(DoActionkComplete);
t.Start();
//Returns the User to the Index Page while the Task is processing
return View("Index");
}
Now what i really want to do is visualizing the operation. I use jQuery Progressbars on the GUI and my Own ProgressBar Object in the Session for this. I have a List of ProgressBars on my Session and a PartialView strongly Typed to a List of those ProgressBars.
ProgressBar Class:
public class ProgressBar
{
public string Action { get; set; }
public string User { get; set; }
}
PartialView:
#using AsyncProj.Models
#model List<AsyncProj.Models.ProgressBar>
#{
ViewBag.Title = "ProgressPartial";
}
#{
var foo = (MySessionObject) HttpContext.Current.Session["__MySessionObject"];
foreach (var pb in foo.ProgressBars)
{
<div style="border: 1px solid black">
<p>#pb.Action</p>
<div id="progressbar">This will be turned into a ProgressBar via jQuery.</div >
</div>
}
}
And then the Object i have in my Session:
public class MySessionObject
{
public List<ProgressBar> ProgressBars { get; set; }
public string User { get; set; }}
Whenever i start a new Task i will add another ProgressBar to that List, which works just fine.
No where i get into Troubles is when i want to Remove the ProgressBars from Session again.
In the DoActionkComplete Method which i set in Task.ContinueWith() i want to Remove the ProgressBar corresponding to the finished action. I have the ProgressBar Ready there, its stored in my AsyncReturnValue Class which i have in the Task.Result at this point:
public class AsyncReturnValue
{
public ProgressBar ProgressBar { get; set; }
}
In this Method i would like to remove the Progressbar from the Session with
HttpContext.GetSession().ProgressBars.Remove(pbToRemove). But the problem with that im still operating on a different Thread so i have no valid HttpContext there and my SessionObject is null on that Thread.
This is what my DoActionComplete Method looks right now:
public void DoActionkComplete(Task<AsyncReturnValue> t)
{
//i set the user hardcode because its only a demo
DeleteProgress.Add(new ProgressBarDeleteObject {ProgressBar = t.Result.ProgressBar, User = "Asd123"});
}
I created a Workaround where i have a static List of Progressbars on my Controller. In the DoActionComplete Method i add the ProgressBars i want to delete to that List. I need to use polling (with jQuery $.get() and setinterval) in order to delete them.
I have a custom Class for the DeleteList on which i can set a Username so i know who is the Owner of that ProgressBar and only show it to him, else everyone would see it because its Static.
public class ProgressBarDeleteObject
{
public ProgressBar ProgressBar { get; set; }
public string User { get; set; }
}
Dont get me wrong, my workaround works just fine but i want to know the clean way. From what i know static Lists on Controllers could technically grow very big and slow the site down. Such Lists also lose its entries when the ApplicationPool restarts the Application.
So my Actual Question would be how can i access a HttpContext SessionObject from a different Thread like i'm using? And if its not possible, what would be the proper Way to achieve what i want?
So my Actual Question would be how can i access a HttpContext SessionObject from a different Thread like i'm using?
That's not possible.
And if its not possible, what would be the proper Way to achieve what i want?
First, let's back up to the original scenario. The problem is here:
I have a MVC.Net4 Application in which i have Longrunning backround operations which is why i use the System.Threading.Tasks.Task Class.
That's the wrong solution for that scenario. The proper solution is to have a reliable queue (Azure queue / MSMQ) with an independent background process (Azure webjob / Win32 service) doing the processing. This is a more reliable solution because any work you toss onto the ASP.NET thread pool may be lost (especially if you don't inform ASP.NET about that work).
Once you have this architecture set up, then you can use SignalR to communicate from your web server to your clients. SignalR will use polling if it has to, but it can also use more efficient methods (such as websockets).
You can specify the SynchroniztionContext that the ContinueWith task continues on and then you should be able to access the progress bars. Try changing your t.ContinueWith(DoActionkComplete); call to
t.ContinueWith(DoActionkComplete, TaskScheduler.FromCurrentSynchronizationContext());
If you are using .NET 4.5 you can rewrite your method with async\await
public async Task<ActionResult> DoAsyncAction()
{
//ReturnValue that needs to be further populated by the async action in productive environment
var arv = new AsyncReturnValue
{
ProgressBar = new ProgressBar {Action = "SomeAction", User = "SomeUser"}
};
//Add a Progressbar before Task starts so i can visualize the process on the view
HttpContext.GetSession().ProgressBars.Add(arv.ProgressBar);
var result = await Task.Run(DoAction());
//i set the user hardcode because its only a demo
DeleteProgress.Add(new ProgressBarDeleteObject {ProgressBar = result.ProgressBar, User = "Asd123"});
//Returns the User to the Index Page while the Task is processing
return View("Index");
}
And if you make the DoAction method async as well, you can remove the Task.Run part as that uses up a thread from the thread pool.
I have a list of items that goes to another page, That page is hooked up to a view model. In the constructor of this view model I have code that grabs data from the server for that particular item.
What I found is that when I hit the back button and choose another item fromt hat list and it goes to the other page the constructor does not get hit.
I think it is because the VM is now created and thinks it does not need a new one. I am wondering how do I force a cleanup so that a fresh one is always grabbed when I select from my list?
I faced the same issue, that's how i solved it.
Have a BaseView class, override OnNavigatedTo
protected override void OnNavigatedTo(NavigationEventArgs e)
{
base.OnNavigatedTo(e);
if (NavigatedToCommand != null && NavigatedToCommand.CanExecute(null))
NavigatedToCommand.Execute(null);
}
add DependencyProperty.
public static readonly DependencyProperty NavigatedToCommandProperty =
DependencyProperty.Register("NavigatedToCommand", typeof(ICommand), typeof(BaseView), null);
public ICommand NavigatedToCommand
{
get { return (ICommand)GetValue(NavigatedToCommandProperty); }
set { SetValue(NavigatedToCommandProperty, value); }
}
On the necessary pages, add to xaml (and, of course, inherit BaseView )
NavigatedToCommand="{Binding OnNavigatedToCommand}"
In the ViewModel, make command itself
public RelayCommand OnNavigatedToCommand
{ get { return new RelayCommand(OnNavigatedTo); } }
and implement method you want to call to update list
public async void OnNavigatedTo()
{
var result = await myDataService.UpdateMyList();
if (result.Status == OK)
MyList = result.List;
}
So, now, every time you navigate to page with list, inside of overriden OnNavigatedTo(), a NavigatedToCommand would be executed, which would execute OnNavigatedToCommand (which you set in xaml), which would call OnNavigatedTo, which would update your list.
A bit messy, but MVVM :)
EDIT: What about cleanings, they can be done in OnNavigatedFrom(), which works the same. Or OnNavigatingFrom(), which also can be useful in some cases.
I'm loading pivot items based on a call a webservice call. Given that all I have is asynchronous calls available, how do I go about catching when it's finished?
My main reason is that I'd like to keep a loading dialog up while it's waiting for the callback. However, I'm loading in a viewmodel class, and obviously the loading bar is in the page class.
Honestly, if I could just know when one pivot item was loaded, that would be fine, however setting an event handler on loadedpivotitem never seems to trigger.
I assume you are databinding your View to your ViewModel. In that case all you need to do is create a bool property and set it to true while loading/awaiting the async call. You could do something like this:
private bool isSyncing;
public bool IsSynchronizing
{
get { return this.isSyncing; }
set
{
this.isSyncing = value;
this.RaisePropertyChanged(() => this.IsSynchronizing); //Use appropriate RaisePropertyChanged method for your MVVM implementation
}
}
Before starting the async call you would set IsSynchronizing = true. At the end of the eventhandler set IsSynchronizing = false;
From your view you can bind to this bool. For the loadingbar it could be like this:
<ProgressBar Visibility="{Binding IsSynchronizing, Converter={StaticResource booleanToVisibilityConverter}}" IsIndeterminate="{Binding IsSynchronizing}" Style="{StaticResource PerformanceProgressBar}" />
In your scenario you can use an inverted BooleanToVisibilityConverter to hide the pivot while it is still loading.
Hope this helps, let me know if you need more info on using the BooleanToVisibilityConverters
You would need to hook up an event handler similar to as shown in this block of code:
public void LoadData()
{
SampleDataServiceClient client = new SampleDataServiceClient();
client.GetDataCompleted += new EventHandler<GetDataCompletedEventArgs>(client_GetDataCompleted);
client.GetDataAsync();
}
void client_GetDataCompleted(object sender, GetDataCompletedEventArgs e)
{
this.DataContext = e.Result;
}