We Keep Coding

Cannot display an alert on app launch Xamarin Forms

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();
}
}

Execute same function on every page in Xamarin.Forms

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.

Android Things with Xamarin Issue with IGpioCallback

I'm just starting with Android Things with Xamarin, and I've already successfully turned on a LED, but I'm having trouble to detect a push button input.
I think the problem is the "RegisterGpioCallback" in the code below, but I'm not sure and really don't know how to fix it. Can somebody help me?? This is the code I'm using:
public class BlinkActivity : Activity
{
private IGpio gpio;
private IGpio button;
private IGpioCallback mButtonCallback;
protected override void OnCreate(Bundle savedInstanceState)
{
this.mButtonCallback = mButtonCallback;
PeripheralManager peripheralManager = PeripheralManager.Instance;
gpio = peripheralManager.OpenGpio("BCM17");
gpio.SetDirection(Gpio.DirectionOutInitiallyLow);
gpio.Value = false;
button = peripheralManager.OpenGpio("BCM4");
button.SetDirection(Gpio.DirectionIn);
button.SetEdgeTriggerType(Gpio.EdgeNone);
button.RegisterGpioCallback(new Handler(), mButtonCallback);
base.OnCreate(savedInstanceState);
Task.Run(() =>
{
if (mButtonCallback.OnGpioEdge(button) == true)
{
gpio.Value = !gpio.Value;
}
});
}
}

You need to actually implement the IGpioCallback interface so the com.google.android.things.pio library can make a "call back" into your application when the value of the GPIO changes.
Assign the RegisterGpioCallback to the actual object instance that has implemented the interface, in the following example, that will be on the Activity.
public class BlinkActivity : Activity, IGpioCallback
{
~~~~
button.RegisterGpioCallback(new Handler(), this);
~~~~
// remove the Task.Run block
public OnGpioEdge(Gpio gpio)
{
Log.Debug("SO", gpio.Value.ToString());
}
~~~~
}

I had some issues following this in Maui. I'd created an IGPIO interface in the shared code, and then a platform-specific GPIO class inside the Android platform code. The code would run, but then crash when it got to the Registration of the callback. The error said I had to pass a Java.Lang.Object or Java.Lang.Throwable as argument 2 to com.google.android.things.pio.impl.GpioImpl.registerGpioCallback(android.os.Handler, com.google.android.things.pio.GpioCallback).
I tried using each of these as the base class for my GPIO class, but then the app wouldn't build. When I'd autogenerated the IGpioCallback interface implementation in the class it had created a dispose method and a Handle property along with the OnGpioEdge callback method. Removing these allowed the app to work properly. so my class definition ended up looking something like this for the registration and event:
public class GPIO : Java.Lang.Throwable, IGPIO, IGpioCallback
{
public event EventHandler OnButtonEdge;
IGpio ButtonPin;
public void registerPinForEdgeDetection(string pinName)
{
using (var peripheralManager = PeripheralManager.Instance)
{
ButtonPin = peripheralManager?.OpenGpio(pinName);
ButtonPin.SetDirection(Gpio.DirectionIn);
ButtonPin.SetEdgeTriggerType(Gpio.EdgeBoth);
ButtonPin.RegisterGpioCallback(new Android.OS.Handler(), this);
}
}
public bool OnGpioEdge(IGpio gpio)
{
OnButtonEdge?.Invoke(ButtonPin, EventArgs.Empty);
return true;
}
}

Xamarin Forms - Prism - OnNavigatedTo Calling Twice

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

Alert Dialog in ViewModel - MVVMCross

In the ViewModel, I have Save method where I check isValid property.
If isValid is false, then I want to display an error message.
Since AlertDialog is platform specific, I wonder how do you handle that situation in the ViewModel?
public void Save()
{
if (isValid)
{
OnExit(this, null);
}
else
{
//issue an alert dialog here
}
}
Update
I have used the following plugin and added the following line of code as follows, but it throws an error.
else
{
Mvx.Resolve<IUserInteraction>().Alert("it is not valid");
}
Update 2
Chance.MvvmCross.Plugins.UserInteraction is a namespace but it is used as a type error.
Update 3
I have added Acr.UserDialogs plugin and called as follows, but I have got the same error.
Mvx.Resolve<IUserDialogs>().Alert("it is not valid");

Using ACR User Dialogs is the simplest approach.
In your App.cs (Core/PCL) you will need to register the interface:
public class App : MvxApplication
{
public override void Initialize()
{
// Example Other registrations
CreatableTypes()
.EndingWith("Service")
.AsInterfaces()
.RegisterAsLazySingleton();
Mvx.RegisterSingleton<IUserDialogs>(() => UserDialogs.Instance);
}
}
Then you can call your alert form your ViewModel.
Mvx.Resolve<IUserDialogs>().Alert("it is not valid");
Note for Android Platform support
Then if you are supporting Android you will need to initialize UserDialog with an instance of the activity context. This will have to be done in each activity that you will be making use of UserDialogs or if you have a shared base activity you can do it there.
[Activity]
public class MainActivity : MvxActivity
{
protected override void OnCreate(Bundle bundle)
{
base.OnCreate(bundle);
SetContentView(Resource.Layout.activity_main);
// Initialize Acr UserDialogs
UserDialogs.Init(this);
}
}
Alternatively
You can follow the Mvvmcross document on using platform specific implementations of an interface if you need a more custom modal implementation.

This is how I handle the Alert messages in the viewmodel. Try this.
await App.Current.MainPage.DisplayAlert("Active subscription required", "You do not have an active subscription for Part 2 exams", "OK");

There is an existing MvvmCross plugin called User Interaction that allows displaying alerts and collecting inputs from ViewModels.
From the author BrianChance:
Really simple, easy, beautiful ways to show a message box or to collect user input from your ViewModels
Check it out here and NuGet Link Here.
To install the plugin, make sure you override LoadPlugins in your SetUp Class on iOS and Android (and windows phone) like so:
public override void LoadPlugins(MvvmCross.Platform.Plugins.IMvxPluginManager pluginManager)
{
base.LoadPlugins(pluginManager);
pluginManager.EnsurePluginLoaded<Chance.MvvmCross.Plugins.UserInteraction>();
}

My approach is that i use an event for this scenario. My base class for my view models has a EventHandler OnUserNotification, where the views can kinda subscribe to. The UserNotificationType is just an enum and i let the view kinda decide how it reacts to the situation.
The property:
public EventHandler<UserNotificationType> OnUserNotification { get; set; }
The call:
if (OnUserNotification != null)
{
OnUserNotification.Invoke(this, UserNotificationType.ENetworkError);
}
In the view:
private void onUserNotification(object sender, UserNotificationType userNotificationType)
{
// Do Something like showing a Snackbar, AlertDialog, etc...
}
Of course you can make the eventtype more complex if needed.
Didnt try the plugin which got suggested by wishmaster though, so that might be a smoother implementation?

Use Acr.UserDialogs. There is a great examples on github
You can grab it on nuget
It works well with dependency injection or a static singleton UserDialogs.Instance