I have a function that I need to perform to do some checks (e.g: CheckForUpdate, CheckNetworkConnection, CheckUserAuthorization, ...) on every page appearing or somehow before user request completed.
so I made a c# class and called it BasePage.cs:
public static class BasePage
{
public static async void CheckForUpdate()
{
// actual codes to check for updates are not included here
// just a sample alert
await App.Current.MainPage.DisplayAlert("Update", "There is a new version avaiable to update, would you like to download?", "Download", "Skip");
}
}
and used it in my pages like below:
LoginPage.cs
[XamlCompilation(XamlCompilationOptions.Compile)]
public partial class LoginPage : ContentPage
{
public LoginPage()
{
InitializeComponent();
}
protected async override void OnAppearing()
{
await Task.Run(() => BasePage.CheckForUpdate());
}
}
I don't know if this is best practice or not (I guess not) but it's not displaying alert anyhow.
so my question is what is the best way to execute functions on every page and why the code above doesn't work.
Your code doesn't seem to run on UI thread. Just use Device.BeginInvokeOnMainThread, try like below
protected override void OnAppearing()
{
Device.BeginInvokeOnMainThread(() => {
BaseClass.CheckForUpdate();
});
}
Hope it help you.
Related
community.
In advance, I apologize if this really has a simple solution. I have a troublesome problem, considering my skill level with Xamarin, that I need to get resolved.
The problem is it seems like Xamarin doesn't have an event/function/etc to call when a page is FULLY displayed, as in you can see everything the page is supposed to display, and then that function is called. Quick note, what does not work for me is the OnAppearing function because it fires off too early before the screen is visible.
I was attempting to decipher a solution here. In this post, a user answers with the following code.
Here is step 1):
private bool _toggleTemp;
public bool ToggleTemp
{
get => _toggleTemp;
set => SetProperty(ref _toggleTemp, value);
}
Step 2)
LoadingVm.ToggleTemp = true;
Step 3)
<Switch IsToggled="{Binding ToggleTemp}" Toggled="Switch_OnToggled" IsVisible="False" />
Step 4)
private async void Switch_OnToggled(object sender, ToggledEventArgs e)
{
/* Your code goes here... */
}
First concern. I created a ViewModel file, it's inside of a folder called "ViewModels", and the user who posted the code in the link said to create a property in the view model, for some reason I'm getting an error stating "the name set property does not exist in the current context". So is it alright I swap that code out and just put the following instead? I mean it SEEMS like the same thing, right?
public bool Temp
{
get { return _toggleTemp; }
set { _toggleTemp = value; }
}
Second concern. I have no clue what "LoadingVm" is in his code. It doesn't come up for me. Is it because I'm missing a "using" at the top? But regardless, the whole code line was this "LoadingVm.ToggleTemp = true;", so he's just calling the function to set it to be true.
In the end, I'm ASSUMING that this will no doubt help me get the code working where I can do whatever I please AFTER the page is completely loaded, correct? I don't really see too many people disagreeing with the method the guy in the link has given, but if so, feel free to give other suggestions.
Again, please forgive me if this is a simple error. I'm still getting used to Xamarin and C# is still somewhat fresh in my mind from not having used it in a while.
You can use custom renderer of ContentPage to know if the view is fully loaded and then use MessagingCenter to notify your shared project:
In iOS:
[assembly:ExportRenderer (typeof(ContentPage), typeof(CameraPageRenderer))]
namespace App362.iOS
{
public class CameraPageRenderer : PageRenderer
{
public override void ViewDidAppear(bool animated)
{
base.ViewDidAppear(animated);
Console.WriteLine("ViewDidAppear");
MessagingCenter.Send<object>(new object(), "ViewLoaded");
}
}
}
In Android:
[assembly: ExportRenderer(typeof(ContentPage), typeof(CameraPageRenderer))]
namespace App362.Droid
{
[Obsolete]
public class CameraPageRenderer : PageRenderer{
protected override void OnAttachedToWindow()
{
base.OnAttachedToWindow();
Console.WriteLine("OnAttachedToWindow");
MessagingCenter.Send<object>(new object(), "ViewLoaded");
}
}
}
In Shared project:
public partial class MainPage : ContentPage
{
public MainPage()
{
InitializeComponent();
}
protected override void OnAppearing()
{
base.OnAppearing();
Console.WriteLine("OnAppearing");
MessagingCenter.Subscribe<object>(new object(), "ViewLoaded", (sender) =>
{
// Do something whenever the "ViewLoaded" message is received
Console.WriteLine("Do something whenever the ViewLoaded message is received");
});
}
}
I’m working on a Xamarin Forms app in which I want to perform some actions on launch. Part of the method that I want to call is responsible for checking whether some conditions are fulfilled, if not I want to display an alert (using DisplayAlert method) to inform user about the actions that he/she should take in order to allow for correct execution of apps functions. Unfortunately, on Android the alert is not being displayed, it doesn’t happen always, sometimes it gets displayed, but in most of the cases it doesn’t. After doing some research I think that the issue might be connected with threading. I added some breakpoints and I noticed that on Android the line of code responsible for displaying an alert is being executed before the page becomes visible. On iOS everything works fine (alert is being displayed when around half of the UI is shown). I have tried to execute everything from the main thread. On Android it helped partially, alert is being displayed correctly in more cases, although I still didn’t achieve 100% accuracy. Moreover, on iOS mentioned changes caused an app to get stuck on splash screen. Below there are two versions of the code that I’ve tried to use.
First the base code that I've started with:
public partial class App : Application
{
MainPage mainPage;
public static string DatabaseLocation = string.Empty;
public App(string databaseLocation)
{
//All of the UI code is written in C# in MainPage initializer
mainPage = new MainPage();
MainPage = new NavigationPage(mainPage);
DatabaseLocation = databaseLocation;
}
protected override async void OnStart()
{
base.OnStart();
//in DoThings there is a code containing DisplayAlert
await mainPage.DoThings();
}
}
Below is version of the code in which I've tried to put everything on main thread:
public partial class App : Application
{
MainPage mainPage;
public static string DatabaseLocation = string.Empty;
public App(string databaseLocation)
{
Device.BeginInvokeOnMainThread(() =>
{
mainPage = new MainPage();
MainPage = new NavigationPage(mainPage);
});
DatabaseLocation = databaseLocation;
}
protected override async void OnStart()
{
base.OnStart();
Device.BeginInvokeOnMainThread(async () =>
await mainPage.DoThings());
}
}
I have also tried to run from main thread the line of code specifically responsible for displaying an alert but it also didn't help. DoThings is a method inside MainPage.
public async Task DoThings() {
if (somethingMissing()){
Device.BeginInvokeOnMainThread(async () =>
await DisplayAlert("Error", "Fix errors", "Ok"));
}
else
{
DoStuff();
}
}
In MainPage you can try this code
public async Task DoThings() {
if (somethingMissing()){
Device.BeginInvokeOnMainThread(async () =>
await App.Current.MainPage.DisplayAlert("Error", "Fix errors", "Ok"));
}
else
{
DoStuff();
}
}
So, I've been developing an App for 2 months with prism, and just now I've realized that the method OnNavigatedTo is been calling twice when I select an Item from a MasterDetailPage.
I have no clue why it is happening, I'm sure I'm missing something but I'm about two days trying to solve it.
I'll put some code here, and if u guys need more info I can post more detailed.
Observation: When I'm in the page "A" and I choose the page "A" in the master detail item list, the OnNavigatedTo is called only once, but when I'm in Page "B" and I choose the page "A", The OnNavigatedTo is called twice.
Since now, Thank you guys and sorry about the ignorance.
MasterDetailPage MVVM:
public class PrincipalMasterDetailPageViewModel : ViewModelBase {
public ObservableCollection<PrincipalMasterPageItem> MenuItems { get; set; }
public PrincipalMasterDetailPageViewModel(INavigationService navigationService) : base(navigationService)
{
MenuItems = new ObservableCollection<PrincipalMasterPageItem>();
}
public async override void OnNavigatedTo(NavigationParameters parameters) {
base.OnNavigatedTo(parameters);
.. Here I'm calling an API, thats why I have the async
}
}
Custom Navigation Page MVVM:
public class PrincipalNavigationPageViewModel : ViewModelBase {
public PrincipalNavigationPageViewModel(INavigationService navigationService) : base(navigationService) {
}
}
The Page that I actually show when I select an item in masterdetailpage item list:
public class NewPageTestViewModel : ViewModelBase
{
public NewPageTestViewModel(INavigationService navigationService) : base(navigationService)
{
}
public override void OnNavigatedTo(NavigationParameters parameters)
{
base.OnNavigatedTo(parameters);
Debug.WriteLine("Calling twice HERE!");
}
}
The RegisterTypes of these three examples:
containerRegistry.RegisterForNavigation<PrincipalMasterDetailPage>();
containerRegistry.RegisterForNavigation<PrincipalNavigationPage>();
containerRegistry.RegisterForNavigation<NewPageTest>();
How do I call other pages from PrincipalMasterDetailPageViewModel:
NavigationService.NavigateAsync(string.Format("PrincipalNavigationPage/{0}", item.TargetPageName));
In App.cs I start like the following because I need the login page first:
protected override async void OnInitialized()
{
InitializeComponent();
await NavigationService.NavigateAsync("LoginPage");
}
When the user log in, It navigate like this:
await NavigationService.NavigateAsync("/PrincipalMasterDetailPage/PrincipalNavigationPage/WhateverPageIWantTo");
I don't know if anybody is still interested in this but I ran into the same issue and figured out what is going on.
All code samples I found register a NavigationPage like this:
containerRegistry.RegisterForNavigation<NavigationPage>("Navigation");
In order to be able to do something like this on app launch:
NavigationService.NavigateAsync($"Main/Navigation/Home");
However, the problem seems to be that when this NavigationPage is instantiated without a specific ViewModel assinged to it, the 'INavigationAware' events are somehow propagated to the MasterDetailPage's ViewModel resulting in the events on that one to be called twice.
I fixed it by registering the NavigationPage for Navigation with a ViewModel like this:
containerRegistry.RegisterForNavigation<NavigationPage, NavigationPageViewModel>("Navigation");
The ViewModel itself is nothing special:
using Prism.Commands;
using Prism.Mvvm;
using Prism.Navigation;
using System;
using System.Collections.Generic;
using System.Linq;
using Unity.Attributes;
namespace SocialRecipe.ViewModels
{
public class NavigationPageViewModel : ViewModelBase
{
public NavigationPageViewModel()
{
}
public override void OnNavigatedFrom(INavigationParameters parameters)
{
}
public override void OnNavigatedTo(INavigationParameters parameters)
{
}
public override void OnNavigatingTo(INavigationParameters parameters)
{
}
}
}
This way the events of the NavigationPage are routed to the NavigationPageViewModel and are no longer propagated to the MasterDetailPage…
I've noticed some inconsistent results with Prism as well from a previous project. If you are only seeing it now and it worked previously I would try and backtrack to see if the version you used was different.
https://github.com/PrismLibrary/Prism/issues
I'm using the Zxing.net.mobile library in my xamarin.forms application, but I've found a big limitation in the using of it.
When the barcode scanner is active and I press the back button, the UI returns to the previous page but there's no way to execute code, because there's no way to trigger an event or passing a callback function to the scanner and the OnAppearing() method doesn't fire. Does anyone know a solution that might work? Thank you!
Edit: example code
public partial class MyPage : ContentPage
public MyPage()
{
InitializeComponent();
}
public async void MyMethod()
{
var scanner = new MobileBarcodeScanner();
scanner.ScanContinuously(MyScanningAction);
}
private async void MyScanningAction(ZXing.Result scanningResult)
{
... do something ....
}
I have the following code:
public class BaseControlClass : System.Web.UI.UserControl
{
protected delegate void AsyncronousAction();
protected virtual void FAsyncEvent() { } //Overidden on derived classes.
protected void Page_Load(object sender, EventArgs e)
{
if (!Page.IsPostBack)
FPerformAsyncronousTasks(FAsyncEvent);
}
protected virtual void FPerformAsyncronousTasks(AsyncronousAction AsyncCallback)
{
new Thread(delegate()
{
AsyncCallback();
}).Start();
}
}
public class DerivedControlClass : BaseClass
{
protected override void FAsyncEvent()
{
//Contact web service, wait for results, add to local database.
}
}
What I was expecting is for the page to load and the code in FAsyncEvent() in my derived class to run behind the scenes however, the page doesn't finish loading until the code in FAsyncEvent() has completed.
Am I doing something wrong with the threading here?
EDIT Strangely, I have noticed if I do a Clean -> Build then run the code, it works fine and the page finishes loading whilst the web service is being contacted. Subsequent runs though mean the page waits until the thread has finished processing.
Your code keeps the reference to the UserControl alive, which might be preventing the request from continuing. Try passing a static method to the new thread, just as a quick check and see if it helps.