Develop Reference

Xamarin iOS app freezes: ViewDidAppear() never gets called

This seems to be a tricky one:
We're working on a Xamarin iOS app that has a UITableView control displaying a list of profiles. Once the user clicks on one of the profiles we'd like to switch to a separate "ProfileViewController" to display more detailed information. This works just fine for 99.9% of the time. In the remaining ~0.1% the item in the list gets highlighted (which happens OnClick) but the app just seemingly freezes and never switches to the other ViewController, leaving the user with an unresponsive list.
The interesting thing to note here is that the app doesn't really freeze, as the simple "swipe back" gesture brings the user back to the UITableView (to be clear the switch-to-profile-viewcontroller animation is never played so it basically switches from the unresponsive list back to the same list, but now it is responsive again).
The tricky thing is that we can't reliably reproduce this bug. It just seems to happen at random, sometimes while stuff is running in the background, other times while the app was previously in an idle state. We are pretty sure that it isn't related to multithreading (we triple-checked everything, using locking and semaphores where necessary), it rather seems to be some rendering issue (as you can still swipe back to the previous screen or at least it isn't your common dead lock).
Using a bunch of Console.WriteLine() tracers and hours of trial and error reproducing this bug we could isolate the problem and discovered the following:
Upon clicking on the list code in ViewDidLoad() and ViewWillAppear() is successfully executed. However ViewDidAppear() never gets invoked. There is nothing else happening in our code or on different threads between ViewWillAppear() and ViewDidAppear() and we don't have any funky / unusual code that gets executed previously (just the usual UI initialization like this in ViewDidLoad()):
...
LblProfileName.TextAlignment = UITextAlignment.Center;
LblProfileName.Text = string.Empty;
LblProfileName.Font = FontAgent.ForSize(30);
LblProfileSlogan.TextAlignment = UITextAlignment.Center;
LblProfileSlogan.Text = string.Empty;
LblProfileSlogan.Font = FontAgent.ForSize(20);
LblProfileRelationInfo.TextAlignment = UITextAlignment.Center;
LblProfileRelationInfo.Text = string.Empty;
LblProfileRelationInfo.Font = FontAgent.ForSize(15);
...
At this point we are kinda out of ideas as to what could be going wrong here and we have independently reviewed any of our code that could remotely be involved in this bug but we found nothing.
We didn't find anything related online but maybe someone else has encountered a similar issue like this in Xamarin / Xamarin iOS before?
Are there any steps we could take that we don't know of? When breaking the app in the Visual Studio for Mac debugger during the freezes the call stacks only contain native code and are not of much use.
Any help or ideas as to what else we could try are hugely appreciated :)
Edit: Adding some code
This is our BaseViewController defining some initialization methods to be used by it's children (again in 99.9% of all cases this works just fine):
public abstract class BaseViewController : UIViewController
{
public BaseViewController(string nibName, NSBundle nSBundle) : base(nibName, nSBundle)
{
ControllerDidInitialize();
}
public ClubmappViewController(IntPtr handle) : base(handle)
{
ControllerDidInitialize();
}
public override void ViewDidLoad()
{
base.ViewDidLoad();
CustomOrientationService.ScreenRotationPortrait();
InitializeStaticContent();
Console.WriteLine("ViewDidLoad() exits just fine...");
}
public override void ViewDidAppear(bool animated)
{
base.ViewDidAppear(animated);
Console.WriteLine("When the bug occurs this never gets executed :C");
RefreshUI();
OnViewDidAppear?.Invoke(this, new EventArgs());
LoadData();
}
private protected abstract void InitializeStaticContent();
private protected abstract void RefreshUI();
private protected virtual void LoadData()
{
}
private protected virtual void ControllerDidInitialize()
{
}
}
This is the ProfileViewController that inherits from BaseViewController and that just stops being rendered or inititialized or whatever the actual problem might be (it just never shows up as described above) but most of the time it works just fine:
public partial class ProfileViewController : BaseViewController
{
public const string STORYBOARD_ID = "ProfileViewController";
public ProfileViewController(IntPtr handle) : base(handle)
{
}
private protected override void ControllerDidInitialize()
{
// do initialization things like initializing variables, etc
// no real logic here
}
private protected override void InitializeStaticContent()
{
// layouting loading
LblProfileTitle.TextAlignment = UITextAlignment.Center;
LblProfileTitle.Text = string.Empty;
LblProfileTitle.Font = FontAgent.ForSize(20);
LblProfileTitle.Font = UIFont.BoldSystemFontOfSize(20);
LblProfileName.TextAlignment = UITextAlignment.Center;
LblProfileName.Text = string.Empty;
LblProfileName.Font = FontAgent.ForSize(30);
LblProfileSlogan.TextAlignment = UITextAlignment.Center;
LblProfileSlogan.Text = string.Empty;
LblProfileSlogan.Font = FontAgent.ForSize(20);
// ... and so on ..
}
private protected override void RefreshUI()
{
// ... theme related stuff ...
ViewProfileActive.BackgroundColor = ThemeAgent.CurrentTheme.OnlineIndicatorColor;
LblProfileName.TextColor = ThemeAgent.CurrentTheme.PrimaryTextColor;
LblProfileSlogan.TextColor = ThemeAgent.CurrentTheme.SecondaryTextColor;
// ...
}
private protected async override void LoadData()
{
// load user data ...
ProfileData data = await ...
}
}
And this is the "OnClick" event that is triggered when a profile is clicked on, that's supposed to initialize and show the ProfileViewController (it's unlikely that something's wrong with this but including it nonetheless):
// ...
(sender, e) =>
{
ProfileViewController profileController = (ProfileViewController)UIStoryboard.FromName("Main", null).InstantiateViewController(ProfileViewController.STORYBOARD_ID);
profileController.ModalPresentationStyle = UIModalPresentationStyle.FullScreen;
currentViewController.NavigationController.PushViewController(profileController, true);
}
// ...

Xamarin/Tizen: Executing Navigation.PopAsync() crashes the app

I have a navigation page that sets up three pages. The first page loads, the user has to pick an option from a listview and then it loads the second page with PushAsync(). At this point the user can now navigate between the three pages by turning the clock face. If I call PopToRootAsync() on the second page it works fine. If the rotary clock face is turned clockwise it loads a third page via PushAsync().
The problem is if I call PopAsync() on that third page OR I change the PopToRootAsync() on the second page to a PopAsync(), the app crashes. I have no way to determine what the error is either as I just get segmentation fault and nothing is written to the Tizen log that is seemingly indicative of why it crashed.
Is there some reason that a PopAsync() would cause this? I know I saw some other articles this could occur if the MainPage is not loaded into a NavigationPage but I'm doing that. I've been looking through stuff and writing debug logs for days but have nothing to show for it. Any help would be more than appreciated. Thank you!
App.cs
public App()
{
MainPage = new NavigationPage(new ServerSelectionPage());
}
ServerSelection.cs
private void ServerSelection_OnItemTapped(object sender, ItemTappedEventArgs args)
{
App.SERVER = (Server)args.Item;
Navigation.PushAsync(new ArrowsPage());
}
PageBase.cs
public async void Rotate(RotaryEventArgs args)
{
Page _currentPage = Page.REMOTE_BUTTONS;
if (this.GetType() == typeof(ButtonsPage))
_currentPage = Page.REMOTE_BUTTONS;
else if (this.GetType() == typeof(ArrowsPage))
_currentPage = Page.REMOTE_ARROWS;
else
_currentPage = Page.SERVER_SELECTION;
// When rotating (previous rotation is ongoing, do nothing)
if (_rotating)
{
return;
}
_rotating = true;
if (!(App.SERVER is null))
{
if (_currentPage == Page.SERVER_SELECTION)
{
if (args.IsClockwise)
await Navigation.PushAsync(new ArrowsPage());
}
else if (_currentPage == Page.REMOTE_DIRECTIONAL)
{
if (args.IsClockwise)
await Navigation.PushAsync(new ButtonsPage());
else
await Navigation.PopToRootAsync();
}
else
{
try
{
if (!args.IsClockwise)
await Navigation.PopAsync(); // This will crash the app
}
catch(Exception ex)
{
Log.Debug(ex.Message);
}
}
}
_rotating = false;
}

After reading the comment by #vin about checking if the Navigation object is null I suddenly had the thought that it may not be the Navigation page but the RotaryFocusObject. So I changed
if (!args.IsClockwise)
await Navigation.PopAsync();
to
if (!args.IsClockwise)
{
RotaryFocusObject = null;
await Task.Delay(300);
await Navigation.PopAsync();
}
and it no longer crashes. The delay is necessary as if you call PopAsync right after setting the object to null it can still sometimes crash.
So apparently if you pop the current page it causes an error as the focus of the rotary dial is still set to the current navigation page. Why this error does not seemingly occur if you use the Navigation.PopToRootAsync() makes no sense to me.

Navigation.PopToRootAsync() and Navigation.PopToRootAsync() are causing destroy of Page renderer, and the Rotate handler is belong to Page Renderer,
If Page renderer was deleted before completion of Rotate Native Callback that is belong to Page renderer, it will be crashed.
await Task.Delay() let to returning native callback.

Could not create a new view because the main window has not yet been created

I'm having a problem with my app during the start up. I'm getting at exception that says
A method was called at an unexpected time. Could not create a
new view because the main window has not yet been created
First I display a splash screen so I can get some data from the internet in the background. My splash screen works fine and I implemented it correctly as indicated in the documentation.
In App.xaml.ca I have some standard code for splash screen
protected override void OnLaunched(LaunchActivatedEventArgs e)
{
...
if (e.PreviousExecutionState != ApplicationExecutionState.Running)
{
bool loadState = (e.PreviousExecutionState == ApplicationExecutionState.Terminated);
ExtendedSplash extendedSplash = new ExtendedSplash(e.SplashScreen, loadState);
Window.Current.Content = extendedSplash;
}
...
Window.Current.Activate();
}
Then in my App constructor I have this
public static Notifications notifications;
public App()
{
Microsoft.ApplicationInsights.WindowsAppInitializer.InitializeAsync(
Microsoft.ApplicationInsights.WindowsCollectors.Metadata |
Microsoft.ApplicationInsights.WindowsCollectors.Session);
this.InitializeComponent();
this.Suspending += OnSuspending;
SomeClass.RunTasks(); //acquire data from a REST service
//initializing the object for subscribing to push notification, not sure if this is the best place to put this.
App.notifications = new Notifications("hubname", "myEndpoint");
}
The exception occurs inside my RunTasks() method which looks like this
public class SomeClass
{
GetHTTPResponse _aggregateData = new GetHTTPResponse("http://someRestService");
public async void RunTasks()
{
try
{
HttpResponseMessage aggregateData = await _aggregateData.AcquireResponse();
await Windows.ApplicationModel.Core.CoreApplication.MainView.CoreWindow.Dispatcher.RunAsync(CoreDispatcherPriority.Normal,
() =>
{
//do a bunch of stuff with the data
//NOTE: I am making updates to my ViewModel here with the data I acquired
//for example App.ViewModel.Time = somevalue
//when finished dismiss the splash screen
ExtendedSplash.Instance.DismissExtendedSplash();
}
);
}
catch (System.Exception ex)
{
}
}
}
Any ideas how I can improve this and eliminate the error?
Could it have something to do with me updating my ViewModel items (which are data bound to UI components)?
EDIT when I remove the creation of my notifications object from App.cs constructor (and move it into the RunTasks() method, the error goes away.
App.notifications = new Notifications("hubname", "myEndpoint");

The reason you get the exception is because Windows.ApplicationModel.Core.CoreApplication.MainView is not valid in your application's constructor as the main view has not been created yet.
You can access it once you have received the Application.OnLaunched/OnActivated event.
Thanks!
Stefan Wick -
Windows Developer Platform

Caliburn.Micro Go Back called in On Activate not working in WinRT

This question is specific to Windows Phone 8.1 (WinRT); it may also be applicable Windows 8.1. I am using Caliburn.Micro 2.0.1
In my ViewModel's OnActivate I check whether an item is a database, if it isn't, I want to navigate back to the previous page.
The simplist solution will be just to call GoBack in the OnActivate method (this works in Windows Phone 8.0):
INavigationService _navigationService;
protected override void OnActivate()
{
_item = GetItemFromDB();
if(_item == null)
{
_navigationService.GoBack()
}
}
To navigate to the view model I call:
_navigationService.NavigateToViewModel<MyViewModel>(_param);
But it does not work, it ignores the GoBack call and stays on the page which I do not want to view.
When stepping through the code you can see that the GoBack code is called inside the NavigateToViewModel method; I expect this is the reason why it does not work (something to do with a queuing issue maybe?).
I have a very "hacky" solution that involves a timer (that works), but I really despise it since it is prone to threading issues and has the possibility of being called during the NavigateToViewModel call (if it takes long to finish), which will then again not work:
protected override void OnActivate()
{
_item = GetItemFromDB();
if(_item == null)
{
DispatcherTimer navigateBackTimer = new DispatcherTimer();
navigateBackTimer.Interval = TimeSpan.FromMilliseconds(300);
navigateBackTimer.Tick += GoBackAfterNavigation;
navigateBackTimer.Start();
}
}
public void GoBackAfterNavigation(object sender, object e)
{
_navigationService.GoBack();
(sender as DispatcherTimer).Stop();
}
Is there a better way to navigate back? Why doesn't the GoBack work in OnActivate? Is there a way to get it to work in OnActivate?

You can use
Execute.OnUIThreadAsync(() => /* navigationCode */);
instead of a timer to queue the action immediately after the processing of the current stack has finished.

WP8: Fast app resume + secondary tile + MainPage = 2 instances

I'm having the same problem posed here:
http://social.msdn.microsoft.com/Forums/wpapps/en-us/af8615e7-8e90-4069-aa4d-3c4a84a6a3d0/windows-phone-8-fast-app-resume-with-deeplinks?forum=wpdevelop
I'm no C# or WP expert, so please bear with me.
I have secondary tiles which link to "/MainPage.xaml?id=XX".
I have fast app resume enabled. (ActivationPolicy="Resume" in the app manifest)
I only have one page in my app: MainPage.xaml.
Problem: When I resume the app using a secondary tile ("/MainPage.xaml?id=XX"), I get a brief view of the previous instance (that would have resumed) and then the MainPage initializes again, creating a new instance. In effect, the app is loading from scratch after giving me a peek of what was previously open.
That is obviously undesired behavior. I want to use the existing instance to perform my task.
Attempt 1:
Use e.Cancel = true; to cancel the navigation to the MainPage.xaml:
(using the App.xaml.cs code from the official Fast App Resume sample to identify how the app was launched)
...
else if (e.NavigationMode == NavigationMode.New && wasRelaunched)
{
// This block will run if the previous navigation was a relaunch
wasRelaunched = false;
if (e.Uri.ToString().Contains("="))
{
// This block will run if the launch Uri contains "=" (ex: "id=XX") which
// was specified when the secondary tile was created in MainPage.xaml.cs
sessionType = SessionType.DeepLink;
e.Cancel = true; // <======================== Here
// The app was relaunched via a Deep Link.
// The page stack will be cleared.
}
}
...
Problem: In doing so, my OnNavigatedTo event handlers never fire, so my query string is never parsed.
protected override void OnNavigatedTo(System.Windows.Navigation.NavigationEventArgs e)
{
String navId;
if (e.NavigationMode != System.Windows.Navigation.NavigationMode.Back)
{
if (NavigationContext.QueryString.TryGetValue("id", out navId))
{
MessageBox.Show(navId.ToString()); // Not reached
}
}
...
Attempt 2:
Use e.Cancel = true; to cancel the navigation to the MainPage.xaml, AND pass the Uri to a method in MainPage:
// App.xaml.cs
...
else if (e.NavigationMode == NavigationMode.New && wasRelaunched)
{
// This block will run if the previous navigation was a relaunch
wasRelaunched = false;
if (e.Uri.ToString().Contains("="))
{
// This block will run if the launch Uri contains "=" (ex: "id=XX") which
// was specified when the secondary tile was created in MainPage.xaml.cs
sessionType = SessionType.DeepLink;
e.Cancel = true;
MainPage.GoToDeepLink(e.Uri); // <======================== Here
// The app was relaunched via a Deep Link.
// The page stack will be cleared.
}
}
...
// MainPage.xaml.cs
public static void GoToDeepLink(Uri uri) // <======================== Here
{
// Convert the uri into a list and navigate to it.
string path = uri.ToString();
string id = path.Substring(path.LastIndexOf('=') + 1);
MyList list = App.ViewModel.ListFromId(Convert.ToInt32(id));
pivotLists.SelectedItem = list;
}
Problem: I get an error that pivotLists is non-static and thus requires an object reference. I think that in order to get this to work I'd need to create a new instance of MainPage (MainPage newMainPage = new MainPage();) and call newMainPage.pivotLists.SelectedItem = list; -- BUT I don't know how to use newMainPage instead of the existing one/replace it... or if that's something I want/won't cause further problems/complications.
I don't know what the solution is to this problem, and I may be going in the completely wrong direction. Please keep all suggestions in simple terms with code examples if you can, I'm still learning.
Thanks for any help.

It seems that when you reopen your App from secondary tile, then it's reactivated and new instance of MainPage is created (even if there is one from previous run). If I understood you correctly, I've managed to do such a thing:
In app.xaml.cs:
I've added a variable which indicates if I should return to previous MainPage after Navigating from secondary tile - it needs to be static as I want to have access to it from MainPage
public static bool returnPage = false;
In RootFrame_Navigating I'm setting this variable to true in:
// ...
else if (e.NavigationMode == NavigationMode.New && wasRelaunched)
{
// This block will run if the previous navigation was a relaunch
wasRelaunched = false;
returnPage = true;
// ...
In ClearBackStackAfterReset - prevent from deleting the old Page, when returning:
// ...
if (e.NavigationMode != NavigationMode.New || returnPage)
return;
// ...
In MainPage.cs:
I've changed a little constructor, as I don't want to see a blink of a new Page:
public MainPage()
{
if (!App.returnPage)
InitializeComponent();
}
In MainPage I've also variable which is passed from secondary tile - it's also static, as I need only one instance of it:
private static string navId = "";
And the core of the trick - OnNavigatedTo:
protected override void OnNavigatedTo(NavigationEventArgs e)
{
if (App.returnPage)
{
App.returnPage = false;
NavigationContext.QueryString.TryGetValue("id", out navId);
NavigationService.GoBack();
}
else if (e.NavigationMode != NavigationMode.Reset)
{
// normal navigation
}
}
It works like this:
when you launch normally your App, returnPage is false, everything goes normal
when you activate it from secondary tile few things happen:
1. first goes navigation to your previous page with NavigationMode.Reset - we are not interested in it, so I switched it off - nothing should happen
2. then program tries to create new instance of MainPage, but returnPage is true, and because of the if statement, InitializeComponent won't run. Just after this, in OnNavigatedTo, program saves passed querystring and Navigates Back to previous instance of MainPage - from previous run
3. at last we are navigating to right MainPage with NavigationMode.Back and we have our querystring saved in static variable.
You must be aware of two things: first - probably it can be little rebuild (I'm not sure if wasRelaunched is needed and so on) - you need to debug it and see of what you can get rid off. Second - you will probably need to test your App with Tombstone case.
Hope this helps.