I'm trying to call an async function but when I try and do it it has the red line underneath it. I want the function to be called when the page is opened, thanks in advance.
public partial class Home : ContentPage
{
public class GoogleProfile
{
public string Id { get; set;}
}
public Home()
{
InitializeComponent();
}
protected override async void OnAppearing()
{
await Check(/*What do i put in here*/);
}
public async Task Check(GoogleProfile googleprofile)
{
if (String.IsNullOrEmpty(googleprofile.Id))
{
}
else {
await Navigation.PushAsync(new LoginPage());
}
}
}
how would i call this? Sorry im new to C# and xamarin
You should make your async calls in ContentPage.OnAppearing(). The OnAppearing() event will be called, as the name suggest, right when your page is being displayed. This is the expected behavior by the user. Also note that I changed your Check() method to return Task because, if you are able to edit the method signature, always try to change the return type of async methods from void to Task:
public partial class LoginPage : ContentPage {
public LoginPage() {
InitializeComponent();
}
protected override async void OnAppearing() {
await Check(/* Add code here to get your GoogleProfile object */);
}
public async Task Check(GoogleProfile googleprofile) {
var ID = googleprofile.Id;
if (string.IsNullOrEmpty(ID)) {
return;
} else {
await Navigation.PushAsync(new Home());
}
}
}
Technically, if you are dead set to not use OnAppearing() you could do the Check() before pushing your LoginPage, though without seeing more code, that would seem like it would defeat the purpose of the LoginPage.
Related
I'm currently working on a .net 5 Blazor application.
I use events to pass data from one component to another.
Unfortunately my current logic is synchronous - but I would rather use an asynchronous event handler.
Thus, I need to use the following code to handle my event:
Task.Run(async () => await ChangeNumbers());
Is there a possibility to handle events asynchronously without Task.Run?
My State service looks like this:
public class MyComponentState
{
public int MyNumber { get; set; }
// Is there a way to declare this event Action async??
public event Action OnChange;
public void DoStuff(int myNumber)
{
MyNumber = myNumber;
NotifyStateChanged();
}
private void NotifyStateChanged() => OnChange?.Invoke();
}
The component to handle the state looks like this:
public class MyOtherComponentDisplay : ComponentBase, IDisposable
{
[Inject]
public MyComponentState MyComponentState { get; set; }
protected override void OnInitialized()
{
// this should all be handled async... i could use OnInitializedAsync
MyComponentState.OnChange += OnChangeHandler;
}
private void OnChangeHandler()
{
// Is there a way to handle/declare this without Task.Run(async ...) - but async right away??
Task.Run(async () => await ChangeNumbers());
}
private async Task ChangeNumbers()
{
// Some operations with MyComponentState.MyNumber async needed!!!
StateHasChanged();
}
public void Dispose()
{
MyComponentState.OnChange -= OnChangeHandler;
}
}
Is there a way to declare and handle events async?
Do you know how to solve this problem?
The basic adoptation would be an async void handler:
private async void OnChangeHandler()
{
// Is there a way to handle/declare this without Task.Run(async ...)
// - but async right away??
// Task.Run(async () => await ChangeNumbers());
await ChangeNumbers();
await InvokeAsync(StateHasChanged); // probably needed
}
The way you're doing things looks strange to me. That's not how I do events in Blazor. (Maybe you're coming from Web Forms?)
Generally, a custom event is defined like:
MyControl.razor
[Parameter]
public EventCallback<SomeType> EventName{ get; set; }
#code {
someMethod (){
EventName.InvokeAsync(SomeType data);
}
}
And the handler in the consuming control can be async if you want:
MyPage.razor
<MyControl EventName=OnChangeHandler />
#code {
private async Task OnChangeHandler()
{
await ChangeNumbers();
}
}
I am lost in this one, i want my Viewmodel to use a event delegate so i can subscribe to it, open some dialog and wait for the dialog result. Later the ViewModel should do whatever it wants with the dialog result.
Here is how i implemented it (resumed code):
public class MyViewModel()
{
public delegate TributaryDocument SearchDocumentEventHandler();
public event SearchDocumentEventHandler SearchDocument;
//Command for the search button
public CommandRelay SearchDocumentCommand { get; set; }
//Document that i found in the dialog.
public TributaryDocument Document { get; set; }
public MyViewModel()
{
SearchDocumentCommand = new CommandRelay(DoSearchDocument);
}
//The command execution
public void DoSearchDocument()
{
//Event used here !
Document = SearchDocument?.Invoke();
}
}
public class MyUIControl : UserControl
{
public MainWindow MainWindow { get; }
public MyUIControl()
{
MainWindow = Application.Current.Windows[0] as MainWindow;
DataContextChanged += MyUIControl_DataContextChanged;
}
private void MyUIControl_DataContextChanged(object sender, DependencyPropertyChangedEventArgs e)
{
var modelView = (MyViewModel)DataContext;
modelView.SearchDocument += MyUIControl_SearchDocument;
}
private TributaryDocument MyUIControl_SearchDocument()
{
//Dont know what to do here... i am lost on this part.
return await MainWindow.ShowDialog(new MyDocumentSearcherDialog());
}
}
//The signature for MainWindow.ShowDialog
public async Task<object> ShowDialog(object dialog)
{
return await DialogHost.Show(dialog, "MainDialog");
}
MyDocumentSearcherDialog is just a dialog where i search and return a TributaryDocument object.
The problem to my understanding comes from this part (since i cant compile it):
private TributaryDocument MyUIControl_SearchDocument()
{
return await MainWindow.ShowDialog(new MyDocumentSearcherDialog());
}
I cant use await without changing the method signature to async. If i change it to async then i must return a Task<TributaryDocument> and change the event delegate:
public delegate Task<TributaryDocument> SearchDocumentEventHandler();
//On MyUIControl
private Task<TributaryDocument> MyUIControl_SearchDocument()
{
return await MainWindow.ShowDialog(new MyDocumentSearcherDialog());
}
//On MyViewModel
public async void DoSearchDocument()
{
//Event used here !
Document = await Task.Run(async () => await SearchDocument?.Invoke());
}
If i do this i get the following exception:
Additional information: The calling thread must be STA, because many
UI components require this.
It seems like all you need to do is to remove the Task.Run (there is no need to Offload to another thread in this situation). The Task.Run will definitely give you a STA Thread Exception if you are doing UI work from within.
However, in short the Async and Await Pattern will create a continuation with the current SynchronisationContext, so there is no need to worry about it.
public async void DoSearchDocument()
{
await SearchDocument?.Invoke();
}
Note : Since this is an event, it's about the only place it's OK to use async void.
Assume that i'm in the Page_1 while click the button have to navigate to Page_2.In Page_2 Api call has to done.
MyIssue is when i'm clicking the Button it doesn't navigate to Page_2 immediately it waits for the API response.
How to Navigate Immediately to Page_2 without waiting for the APi response.
Code:
Page_1.cs
public partial class Page_1 : ContentPage
{
public Page_1()
{
InitializeComponent();
}
private void Btn_click(object sender, EventArgs e)
{
Navigation.PushAsync(new Page_2());
}
}
Page_2:
public Page_2()
{
InitializeComponent();
}
protected override void OnAppearing()
{
HttpClient httpClient = new HttpClient();
var obj = httpClient.GetAsync("//Api//").Result;
if (obj.IsSuccessStatusCode)
{
}
}
Same code works good in iOS as expected
You could load your data in an other Task to prevent blocking the UI.
protected override void OnAppearing()
{
Task.Run( () => LoadData());
base.OnAppearing();
}
private async void LoadData()
{
HttpClient httpClient = new HttpClient();
var obj = await httpClient.GetAsync("//Api//");
if (obj.IsSuccessStatusCode)
{
// If you need to set properties on the view be sure to use MainThread
// otherwise you won't see it on the view.
Device.BeginInvokeOnMainThread(() => Name = "your text";);
}
}
As per your question you are calling the API on Page constructor that's why it's taking time to load web API then navigating on page2. If you want to navigate on page2 before a load the api. Check below code
public partial class Page2 : ContentPage
{
bool IsLoading{ get; set; }
public Page2()
{
InitializeComponent();
IsLoading = false;
}
protected async override void OnAppearing()
{
base.OnAppearing();
if (!IsLoading)
{
IsLoading=true
**Call the Web API Method Here**
}
IsLoading=false
}
}
I am trying to consume an interface but I am having some difficulty here.
I am trying to set it to a xamrin list view in behind a content page
public class xxxApiClient : IApi
{
readonly string url = "http://localhost:81/ ";
readonly IHttpService httpService;
public xxxApiClient(IHttpService httpService)
{
this.httpService = httpService;
}
public Task<List<JobsList>> GetJobs() => httpService.Get<List<JobsList>>($"{url}JobsLists");
}
How ever I am not to sure how I cosume getjobs correclty I am trying the following
public partial class JobsPage : ContentPage ,xxxWC.Interface.IApi
{
public xxxWC.Interface.IApi api = new ful;
public JobsPage ()
{
InitializeComponent ();
}
private Task SetItemSource()
. {
. JobListing.ItemsSource = FuelAp
}
How do I use the get jobs correctly above in the method setItemSource?.
The bit I am having hard time to understand is here.
How do I call the base GetJobs method I have already created in API Client.
Task<List<JobsList>> IApi.GetJobs()
{
throw new NotImplementedException();
}
private Task SetItemSource()
{
JobListings.ItemsSource =await GetJobs();
}
}
Edit 2
Ok based on suggestions below I updated My Code as such
IHttpService httpService;
xxxApiClient _api = newxxxApiClient(httpService);
public JobsPage ()
{
InitializeComponent ();
}
private Task SetItemSource()
{
JobListings.ItemsSource =await GetJobs();
}
But i get the below error
Severity Code Description Project File Line Suppression State
Error CS0236 A field initializer cannot reference the non-static
field, method, or property
'JobsPage.httpService' xxxCallManagmentAppMobile C:\Work\xxxCallAppDev\XamForms\xxxCallManagmentApp\xxxCallManagmentAppMobile\FuelCallManagmentAppMobile\Views\JobsPage.xaml.cs 17 Active
Can someone explain why
Edit 3
Ok i got a bit further but still having some issues. as the main method is not awaited how do I call set SetItemSource.
xxxApiClient _api ;
public JobsPage ()
{
InitializeComponent ()
SetItemSource();
}
private async Task SetItemSource()
{
JobListings.ItemsSource = await client.GetJobs();
}
Assuming that IApi has been mapped to xxxApiClient implementation
Try resolving the service using the DependencyService so that it is available to be used in the view
public partial class JobsPage : ContentPage {
public readonly IApi client;
public JobsPage () {
InitializeComponent ();
client = DependencyService.Get<IApi>();
}
private async Task SetItemSource() {
JobListing.ItemsSource = await client.GetJobs();
//...
}
}
As for calling the SetItemSource, it is async so should be awaited. That can't be done in the constructor.
Consider creating a event that can be raised and its handler used to await the desired behavior.
private event EventHandler loadingData = delegate { };
private async void onLoadingData(object sender, Eventargs args) {
JobListing.ItemsSource = await client.GetJobs();
}
Full code
public partial class JobsPage : ContentPage {
public readonly IApi client;
public JobsPage () {
InitializeComponent ();
//resolving client
client = DependencyService.Get<IApi>();
//subscribing to event
loadingData += onLoadingData;
//raising event
loadingData(this, EventArgs.Empty);
}
private async Task SetItemSource() {
JobListing.ItemsSource = await client.GetJobs();
//...
}
private event EventHandler loadingData = delegate { };
private async void onLoadingData(object sender, Eventargs args) {
JobListing.ItemsSource = await client.GetJobs();
}
}
Although a custom event was created, you could just as easily used on of the event/eventhandler of the view.
All of that code should actually live inside of a view model and then bound to the view in a binding context.
In a background agent I create (or update) one of application live tiles and this works as expected.
Problem is that when I click this live tile screen flickers but my app is not "restarted" nor "shown".
What's wrong?
I attach small part of the code, but ask for more is you need.
MAIN PAGE
public partial class MainPage : PhoneApplicationPage
{
protected override void OnNavigatedTo(NavigationEventArgs e)
{
base.OnNavigatedTo(e);
}
public MainPage()
{
InitializeComponent();
// Runs background agent: code is simplified
StartAgent();
}
}
BACKGROUND AGENT
public class TileAgent : ScheduledTaskAgent
{
protected override void OnInvoke(ScheduledTask task)
{
Deployment.Current.Dispatcher.BeginInvoke(() =>
{
Vars.UpdateTiles();
});
NotifyComplete();
}
}
STATIC CLASS
public class Vars
{
private static Uri uri = new Uri(
"/MainPage.xaml?tile",
UriKind.RelativeOrAbsolute);
private static RadExtendedTileData ExtendedData
{
get
{
return new RadExtendedTileData()
{
VisualElement = frontTile,
BackVisualElement = backTile,
};
}
}
public static void UpdateTiles()
{
// I perform some task here
// Then I create/update live tile
Telerik.Windows.Controls.LiveTileHelper.CreateOrUpdateTile(
ExtendedData, uri);
}
}
Try /MainPage.xaml?tile=true instead of /MainPage.xaml?tile...
And move NotifyComplete() into the dispatcher call. Otherwise it will be called before the operation has been completed...