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Event when Xamarin page is FULLY loaded?

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");
});
}
}

Android DataBinding - How to Bind A ViewGroup's Properties

I am developing an Android app in Xamarin.Android(C#). However, i do feel that this question can also be answered by any Java devs as well.
I am new in android development. Anyways, i created a fragment with just a LinearLayout and a TextView inside it. When i create the background class for it, i don't inherit(on in JAVA's word, extend) it from the Fragment class but rather from the LinearLayout class.
So, the MyFragment.cs file starts like this :
public class MyFragment : LinearLayout
The JAVA equivalent would be
public class MyFragment extends LinearLayout
(P.S. I have limited knowledge of JAVA and it's sytaxes).
Anyways, all works fine. I have an Initialize method(In JAVA, it should be the Init method) which inflates the view of the fragment. From the view, it tries to find the TextView with the given Id.
So, the codes looks like this :
public class MyFragment : LinearLayout
{
Context mContext;
private void Initialize(Context ctx)
{
//Inflating the layout
mContext = ctx;
var inflatorService = (LayoutInflater)ctx.GetSystemService(Context.LayoutInflaterService);
View v = inflatorService.Inflate(Resource.Layout.MyFragmentView, this, false);
this.AddView(v);
GoalHeader = v.FindViewById<TextView>(Resource.Id.GoalHeader);
}
All works pretty well this far. I then go on implementing the MVVM pattern, using MVVMLight library. I create a ViewModel as follows :
public class Vm_MyFragment : ViewModelBase
{
private string _goaltitle = "";
public string GoalTitle
{
get { return _goaltitle; }
set { Set(ref _goaltitle, value); }
}
public void SetTest()
{
DispatcherHelper.CheckBeginInvokeOnUI(() =>
{
GoalTitle = "Test";
});
}
}
Still, everything's good. The problem starts when i try to bind the TextView's text property to the ViewModel's GoalTitle property, as follows :
private readonly List<Binding> _bindings = new List<Binding>();
private void Initialize(Context ctx)
{
//Inflating the layout
mContext = ctx;
var inflatorService = (LayoutInflater)ctx.GetSystemService(Context.LayoutInflaterService);
View v = inflatorService.Inflate(Resource.Layout.MyFragmentView, this, false);
this.AddView(v);
Vm_MyFragmentView viewmodel = new Vm_MyFragmentView();
GoalHeader = v.FindViewById<TextView>(Resource.Id.GoalHeader);
_bindings.Add(
this.SetBinding(
() => mainViewModel.GoalTitle,
() => GoalHeader.Text));
}
Note : Binding is from the GalaSoft.MvvmLight.Helpers namespace.
I add the fragment in my main view(I mean, MainActivity's view) and debug the app. Upon execution, i get the following error :
Could not activate JNI Handle 0xfff02a68 (key_handle 0x339790a) of Java type 'md55bfae9a06327fa0fdf207b4f768604b1/MyFragment' as managed type 'TestApp.MyFragment'.
Searching google, i realized that i am trying to bind the property before the view is even created(correct me if i'm wrong). The suggestions i found on other SO answers were either to put the code in the OnCreateView method or somehow delay the execution of the binding part's code.
The first solution didn't work for me as LinearLayout aka a View doesn't have such a method OnCreateView which i can override.
So, how am i supposed to bind the TextView to the ViewModel then? And also, am i on the right track on treating the fragment as a LinearLayout as i am inheriting from it?

Im not familiar with MVVMLIght extension but if you are using a fragment as it is supposed to (ie. in a tablayout) you should inherit from a fragment like this (This is a v4 support fragment):
public class CategoryFragment : SupportFragment {
RecyclerView _recyclerView;
private View _view;
public override View OnCreateView (LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
_view = inflater.Inflate (Resource.Layout._CategoryLayout, container, false);
// Get our RecyclerView layout:
_recyclerView = _view.FindViewById<RecyclerView> (Resource.Id.categoryRecyclerView);
// Instantiate the layout manager
var linearLayoutManager = new LinearLayoutManager (Context, LinearLayoutManager.Vertical, false);
_recyclerView.SetLayoutManager (linearLayoutManager);
// Instantiate the adapter and pass in its data source:
_adapter = new CategoryAdapter (_categories);
//Register the item click handler with the adapter:
_adapter.ItemClick += OnItemClick;
// Plug the adapter into the RecyclerView:
_recyclerView.SetAdapter (_adapter);
return _view;
}
}

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

Change language for the whole application inside a fragment?

Let's say this is the call stack:
Fragment2 (current position)
Fragment1
HostingActivity
The user has just navigated to fragment2 (which is the settings screen). He chooses a second language, and then navigates back to Fragment1. I want the app to show content from the second language as soon as he enters fragment1.
I've read that the best approach would be to let all the fragments derive from a base-fragment which configures the locale inside the OnResume() method.
public class BaseFragment : Fragment
{
public override void OnResume()
{
string langCode = prefs.GetString("settings_language", "en_US");
Resources res = Application.Context.Resources;
// Change locale settings in the app.
DisplayMetrics dm = res.DisplayMetrics;
Configuration conf = res.Configuration;
conf.SetLocale(new Locale(langCode.ToLower()));
res.UpdateConfiguration(conf, dm);
OnConfigurationChanged(conf);
base.OnResume();
}
}
I've attempted this, but without any luck. I didn't see any change whatsoever.
I'm open to hear any suggestions.
PS. To ensure that I didn't mess up any of the naming conventions required by Android in the values folder, I tried rebooting the emulator (with adb shell) using the language I wanted and it worked as expected.

You can create a base class that sets the CurrentCulture like this and inherit from it:
internal class MyBaseActivity : Activity
{
protected override void OnResume ()
{
base.OnResume ();
// Get the language from wherever.
var userSelectedCulture = new CultureInfo ("fr-FR");
Thread.CurrentThread.CurrentCulture = userSelectedCulture;
}
}
And here is how to inherit and use it:
public class MainActivity : MyBaseActivity
{
protected override void OnResume()
{
base.OnResume(); // Do not forget to call base.OnResume()
// Rest of Your Code...
}
}

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