How to leverage IApplicationLifecycleAware - xamarin.forms

Can someone point me to an example of how to leverage IApplicationLifecycleAware for Prism and Xamarin.Forms? I have a view model that implements it but OnResume and OnSleep never fire (although it fires within my App.xaml.cs).

To use IApplicationLifecycleAware you simply need to implement it on any ViewModel you wish to use it, and add the code to the OnSleep and OnResume methods that you require.
It is very important though that if you implement an override of these methods in your App class that you include the base execution otherwise the PrismApplication cannot execute the IApplicationLifecycleAware methods on the ViewModel of your current page.
public partial class App : PrismApplication
{
protected override void OnSleep()
{
// Handle IApplicationLifecycle
base.OnSleep();
// Handle when your app sleeps
}
protected override void OnResume()
{
// Handle IApplicationLifecycle
base.OnResume();
// Handle when your app resumes
}
}

Related

Proper Page.Loaded event in Xamarin.Forms

For the past 2 month I have been searching tirelessly for a way to implement a proper Page.Loaded event when using Xamarin.Forms but I couldn't implement or find a way to do it.
Most people suggest overriding Page.OnAppearing or adding an event handler for Page.Appearing both of which are not the answers or the proper way to achieve the desired effect and don't event behave as a real Page.Loaded event would.
I would like to know the following:
Why doesn't Xamarin.Forms have a built-in Page.Loaded event?
Is there's a work around?
Can I implement it from the native side?
Edit:
What I mean by "proper Page.Loaded" event is:
It must be called ONCE AND ONLY ONCE the page has loaded all of it's controls, laid them out, initialized them and rendered them for the first time.
It must NOT be called when returning from modal pages.
1.Why not load the data/controls in the constructor of the ContentPage? The constructor method is call only once and it is also called before Page.OnAppearing.
Can I implement it from the native side?
Yes, I think you can.
In iOS, override the ViewDidLoad method in custom renderer:
[assembly:ExportRenderer (typeof(ContentPage), typeof(MyPageRenderer))]
namespace App487.iOS
{
public class MyPageRenderer : PageRenderer
{
protected override void OnElementChanged(VisualElementChangedEventArgs e)
{
base.OnElementChanged(e);
}
public override void ViewDidLoad()
{
base.ViewDidLoad();
//call before ViewWillAppear and only called once
}
public override void ViewWillAppear(bool animated)
{
base.ViewWillAppear(animated);
}
}
}
In Android, try to override the OnAttachedToWindow method:
[assembly: ExportRenderer(typeof(ContentPage), typeof(MyPageRenderer))]
namespace App487.Droid
{
public class MyPageRenderer : PageRenderer
{
public MyPageRenderer(Context context) : base(context)
{
}
protected override void OnElementChanged(ElementChangedEventArgs<Page> e)
{
base.OnElementChanged(e);
}
protected override void OnAttachedToWindow()
{
base.OnAttachedToWindow();
}
}
}
Currently Xamarin.Forms doesn't not provide a proper/complete life cycle events to fulfill all specific requirements, but things are improving, the Dev team is currently working on to address this issue, below mentioned issues and recent pull request on the official GitHub Repos (you may follow, get ideas and maybe implement it yourself before they even ship it), they will for sure provide that in the future, although it is not clear when it will be ready.
Specification: Enhancement Add better life cycle events #2210.
Issue: LifeCycle events for controls #556.
Pull request: Life cycle events for controls
GitHub Branch where currently working on.
MAUI repo (Next evolution of Xamarin) Cross-Platform LifeCycle.
Specification Add Loaded/Unloaded to VisualElement.

How to navigate to page when on resume is called in app.xaml.cs for mvvm in xamarin forms app

I have an issue with navigation. When I click home screen button of device and get back to app I get app homescreen instead of pin page. Ideally it should show pin page and its working fine with back button of device.
OnStart() method has navigationasync but the same is not working with OnResume() method.
Do I have to go to each of the Platform project cs file and add the navigation there like for Android OnRestart()/OnResume() method?
If anyone knows the solution please let me know
Most commonly when writing your Xamarin Application with Prism you will have something like:
protected override void OnInitialized()
{
NavigationService.NavigateAsync("SomePage");
}
OnInitialized is called each time the App's ctor is invoked. This is an important consideration here because this means that any time that the native platform tombstones the app in the background or otherwise refreshes the app by calling OnCreate in your MainActivity or FinishedLaunching in your AppDelegate, then OnInitialized will be invoked resetting your App's Navigation stack to SomePage.
You can however override the OnStart/OnResume in PrismApplication and use whatever business logic you need to determine where to navigate and how you might want to restore your application.
public override void OnStart()
{
NavigationService.NavigateAsync("MainPage");
}
public override void OnResume()
{
if(someCondition)
{
NavigationService.NavigateAsync("SomePage");
}
else
{
NavigationService.NavigateAsync("AnotherPage");
}
}

xamarin.forms detect ios app entering foreground from background

I am creating app in Xamarin.Forms shared proeject. I want to detect app when it comes from background
currently, I have implemented OnActivated method in AppDelegate but this method is also called when we open control center in iphone. I want my app to reconnect with my server when app comes to foreground from background and show message accordingly but this message is also shown when I swipe up and open control center.
In native Xamarin.iOS app, there is a method WillEnterForegroundNotification but this method is not available here.
Is there any method that's called only if app comes from background ?
in App.xaml.cs
protected override void OnResume()
In native Xamarin.iOS app, there is a method
WillEnterForegroundNotification but this method is not available here.
You can access WillEnterForegroundNotification method in Xamarin.forms project, just override it in AppDelegate.cs:
public partial class AppDelegate : global::Xamarin.Forms.Platform.iOS.FormsApplicationDelegate
{
//
// This method is invoked when the application has loaded and is ready to run. In this
// method you should instantiate the window, load the UI into it and then make the window
// visible.
//
// You have 17 seconds to return from this method, or iOS will terminate your application.
//
public override bool FinishedLaunching(UIApplication app, NSDictionary options)
{
global::Xamarin.Forms.Forms.Init();
LoadApplication(new App());
return base.FinishedLaunching(app, options);
}
public override void WillEnterForeground(UIApplication uiApplication)
{
//handle your event here
base.WillEnterForeground(uiApplication);
}
}

Is it possible to delay-load PRISM / Xamarin Forms components that aren't immediately needed?

I have the following AppDelegate which takes quite some time to load:
Syncfusion.ListView.XForms.iOS.SfListViewRenderer.Init();
new Syncfusion.SfNumericUpDown.XForms.iOS.SfNumericUpDownRenderer();
Syncfusion.SfCarousel.XForms.iOS.SfCarouselRenderer.Init();
Syncfusion.XForms.iOS.Buttons.SfSegmentedControlRenderer.Init();
Syncfusion.XForms.iOS.Buttons.SfCheckBoxRenderer.Init();
new Syncfusion.XForms.iOS.ComboBox.SfComboBoxRenderer();
//Syncfusion.XForms.iOS.TabView.SfTabViewRenderer.Init();
new Syncfusion.SfRotator.XForms.iOS.SfRotatorRenderer();
new Syncfusion.SfRating.XForms.iOS.SfRatingRenderer();
new Syncfusion.SfBusyIndicator.XForms.iOS.SfBusyIndicatorRenderer();
What options should I consider when I know some of these components aren't needed for the main screen, but for subscreens?
I am using PRISM, and it appears that every tab is pre-loaded immediately before allowing display or interaction with the end user. What can I do to delay the pre-rendering that the Prism TabView does prior to showing the interface?
Should I use Lazy<T>? What is the right approach?
Should I move these components to another initialization section?
There are a number of ways you could ultimately achieve this, and it all depends on what your real goals are.
If your goal is to ensure that you get to a Xamarin.Forms Page as fast as possible so that you have some sort of activity indicator, that in essence says to the user, "it's ok I haven't frozen, we're just doing some stuff to get ready for you", then you might try creating a "SpashScreen" page where you do additional loading. The setup might look something like the following:
public partial class AppDelegate : FormsApplicationDelegate
{
public override bool FinishedLaunching(UIApplication app, NSDictionary options)
{
global::Xamarin.Forms.Forms.Init();
LoadApplication(new App(new iOSInitializer()));
return base.FinishedLaunching(app, options);
}
}
}
public class iOSInitializer : IPlatformInitializer, IPlatformFinalizer
{
public void RegisterTypes(IContainerRegistry containerRegistry)
{
containerRegistry.RegisterInstance<IPlatformFinalizer>(this);
}
public void Finalize()
{
new Syncfusion.SfNumericUpDown.XForms.iOS.SfNumericUpDownRenderer();
Syncfusion.SfCarousel.XForms.iOS.SfCarouselRenderer.Init();
Syncfusion.XForms.iOS.Buttons.SfSegmentedControlRenderer.Init();
Syncfusion.XForms.iOS.Buttons.SfCheckBoxRenderer.Init();
}
}
public class App : PrismApplication
{
protected override async void OnInitialized()
{
await NavigationService.NavigateAsync("SplashScreen");
}
}
public class SplashScreenViewModel : INavigationAware
{
private IPlatformFinalizer _platformFinalizer { get; }
private INavigationService _navigationService { get; }
public SplashScreenViewModel(INavigationService navigationService, IPlatformFinalizer platformFinalizer)
{
_navigationService = navigationService;
_platformFinalizer = platformFinalizer;
}
public async void OnNavigatedTo(INavigationParameters navigationParameters)
{
_platformFinalizer.Finalize();
await _navigationService.NavigateAsync("/MainPage");
}
}
If you're working with Modules you could take a similar approach though any Modules that would initialize at Startup would still be making that call to Init the renderers before you've set a Page to navigate to. That said, working with Modules does give you a number of benefits here as you only ever would have to initialize things that the app actually requires at that point.
All of that said I'd be surprised if you see much in the way of gain as these Init calls are typically empty methods only designed to prevent the Linker from linking them out... if you aren't linking or have a linker file you could simply instruct the Linker to leave your Syncfusion and other libraries alone.

MVVMLight Messenger.Unregister - should I unregister and how to do it?

I've looked at 'Messenger and references' discussion, but I'm writing a new topic, because my issue is not technical, and I don't want to offtop there.
I've encountered a doubt - Have I to code cleanup()/RequestCleanup() method implementation to unregister previously registered Messenger in my viewmodel class? I'm afraid of memory leaks in the future.
I think I've found the documentation not to be enough bright for me.
Description of Messenger.Register is: '... Registering a recipient does not create a hard reference to it, so if this recipient is deleted, no memory leak is caused.'
1) Is this mean that I don't have to take care of it and implement-develop following solutions?
On the other hand, we can find in the code of GalaSoft.MvvmLight.ViewModelBase abstract class the short info about the Cleanup() method:
//
// Summary:
// Unregisters this instance from the Messenger class.
// To cleanup additional resources, override this method, clean up and then
// call base.Cleanup().
public virtual void Cleanup();
so 2) Is only invoking a Cleanup enough to unregister class-instance out of the Messenger?
3) Or maybe I have to invoke Messenger.Default.Unregister(this); in the body of a Cleanup method?
4) In the Unregister(Object) doc we read 'Unregisters a messager recipient completely' - what does the 'completely' mean?
I'm very sorry if my post seems to have out of the context quotes, I wanted to point out what I'm more interested in.
EDIT 1:
Hello Joel, thanks for reply. I've got several questions:
1) I have used your code. There's defined override void Cleanup() in CustomerMasterViewModel. Where to call it? Should I declare destructor in this case or maybe the ViewModelBase has an automatic mechanism for invoking the Cleanup()?
2) I have in my project another base class from a different toolkit, so my VMs cannot derive from both at the same time. How to organise your code to get the same effect by implementing only ICleanup interface?
public class CustomerMasterViewModel : SomeBaseClass, ICleanup
{
public CustomerMasterViewModel()
{
Messenger.Default.Register<Message>(this, this.MessageReceived);
}
#region messages
private void MessageReceived(Message obj)
{
//do something
}
#endregion
#region helper methods
public override void Cleanup()
{
//base.Cleanup();//there's no implementaction in an interface
ViewModelLocator.Cleanup();
}
#endregion
}
You have to invoke the Cleanup() method in GalaSoft.MvvmLight.ViewModelBase on each of you view models you wan't to dispose don't need any longer.
Example:
Let say your application has a tab control with different tabs. Each of your tabs displays a UserControl which has a dedicated ViewModel. The user has the ability to close a tabs which causes the underlining ViewModel to become obsolete.
Want you want to do now is to clean up the ViewModel calling the Cleanup() method in GalaSoft.MvvmLight.ViewModelBase. This will unregister ALL registered messages. The GarbageCollector will take care of you viewmodel if there are no other references.
Assuming you use the ViewModelLocator which also comes with the MVVM Light Framework you're not done yet because at least the ViewModelLocator itself has a reference to your viewmodel! Therefore the Garbage Collector can't finalize your viewmodel.
But it also has another side effect. When the user reopens the tab (Lets say the user is able to do so) the UserControl is loaded again and the ViewModelLocator will give you the same ViewModel instance. The only difference is that there are not registered messages because you cleaned them by calling the CleanUp() method.
What you need is a new instance of your ViewModel. To achieve this you have to clean up your ViewModelLocator as well!
You have to unregister them (Unregister<CustomerMasterViewModel>()) one by one or simply call Reset() which will unregister all viewmodels.
Then there should be no other reference to you viewmodel and the GarbageCollector can finally take care about it.
Here is an example to do so:
ViewModelLocator:
public class ViewModelLocator
{
public ViewModelLocator()
{
ServiceLocator.SetLocatorProvider(() => SimpleIoc.Default);
SimpleIoc.Default.Register<CustomerMasterViewModel>();
}
public CustomerMasterViewModel CustomerMasterViewModel
{
get
{
return ServiceLocator.Current.GetInstance<CustomerMasterViewModel>();
}
}
public static void Cleanup()
{
SimpleIoc.Default.Reset();
//Don't forget to register them if the user attempts to open the new.
//The viewmodel initialization is lazy by default so this comes at no costs.
SimpleIoc.Default.Register<CustomerMasterViewModel>();
}
}
ViewModel
public class CustomerMasterViewModel : ViewModelBase
{
public CustomerMasterViewModel()
{
Messenger.Default.Register<Message>(this, this.MessageReceived);
}
#region messages
private void MessageReceived(Message obj)
{
//do something
}
#endregion
#region helper methods
public override void Cleanup()
{
base.Cleanup();
ViewModelLocator.Cleanup();
}
#endregion
}
In Short:
1) As far as i understood clean up is necessary after you're done.
2) Yes, calling the Cleanup() method in GalaSoft.MvvmLight.ViewModelBase will unregister all messages for this viewmodel.
3) No, see above.
4) Completely means it will unregister ALL registered messages.

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