Develop Reference

Usb List Devices in Xamarin.Forms

I try to do list of usb devices, connected by serial odt with smartphone, within xamarin.forms.
To do that I use this project https://github.com/anotherlab/UsbSerialForAndroid
How to do listview in shared project with devices from Project.Droid.MainActivity? I tried to do that with dependency service:
This is my Page1(where I want to have listview):
public partial class Page1 : ContentPage {
public Page1()
{
InitializeComponent();
DependencyService.Get<Interface1>().moj();
}
}
My interface:
namespace SensDxMobileApp.Views.MainWindow {
public interface Interface1 {
void moj();
}
}
And MyActivity(Droid project):
[assembly: Xamarin.Forms.Dependency(typeofProject.Droid.MainActivity))]
namespace Project.Droid {
public class MainActivity: Interface
{
protected override void OnCreate(Bundle bundle)
{
base.OnCreate(bundle);
listView = new Android.Widget.ListView;
}
public async void moj()
{
adapter = new UsbSerialPortAdapter(this);
listview.Adapter = adapter;
listView.ItemClick += async (sender, e) =>
{
await OnItemClick(sender, e);
};
await PopulateListAsync();
detachedReceiver = new UsbDeviceDetachedReceiver(this);
RegisterReceiver(detachedReceiver, new IntentFilter(UsbManager.ActionUsbDeviceDetached));
}
}
But I have an error "Object reference not set to an instance of an object.", on " DependencyService.Get().moj()" in Page1();
Did someone do something similar? Thanks

If you going the DependencyService route, then you'll want to create a separate class that implements Interface1 and registering that as a dependency service. I don't think you can register the MainActivity as a DependencyService implementation.
One problem that you are going to hit this is mostly async code and callbacks.
You also shouldn't be newing up an Android ListView as a DependencyService call. That would be better suited as a custom renderer. As a DependencyService implementation, you would want the moj() method to return data that can be consumed by the Xamarin.Forms code. You would need more than just that method. You would need code to initialize the UsbSerialPort class, code to query the list of devices, and then invoke a callback that sends back that list, In theory anyway. I never tested that library with Forms.

Refresh data with RxJava and Retrofit

I'm using Retrofit with RxJava2 to obtain some data from a Rest API. I want to use a SwipeRefreshLayout to update the view and I'm using a ViewModel to handle the API call, so I want to implement a method in there to refresh the data programmatically.
I want to obtain something like this https://stackoverflow.com/a/34276564/6787552 but instead of having a periodic trigger, I want to do that programmatically when the user pull to refresh.
That's the ViewModel:
public class DashboardViewModel extends ViewModel {
public final Single<Dashboard> dashboard;
public DashboardViewModel() {
dashboard = Api.getDashboard();
refresh();
}
public void refresh() {
// Refresh data
}
}
And in the DashboardFragment:
#Override
public View onCreateView(...) {
...
viewModel.dashboard
.observeOn(AndroidSchedulers.mainThread())
.subscribe(dashboard -> {
binding.setDashboard(dashboard);
binding.swipeRefreshLayout.setRefreshing(false);
});
binding.swipeRefreshLayout.setOnRefreshListener(() -> viewModel.refresh());
...
}
Thank you in advance!
EDIT:
That's what I ended up doing:
public class DashboardViewModel extends ViewModel {
private final BehaviorSubject<Dashboard> dashboard;
public DashboardViewModel() {
dashboard = BehaviorSubject.createDefault(Api.getDashboard());
}
public void refresh() {
// I use a Object because null values are not supported
dashboard.onNext(Api.getDashboard());
}
public Observable<Dashboard> getDashboard(){
return dashboard;
}
}
And then in the DashboardFragment just subscribe to viewModel.getDashbaord()

I'm not 100% sure that I understood what you want to do but if I got the question right, you can do something like this:
put a subject inside the model (probably a BehaviorSubject?)
expose it as an observable to the
view and subscribe to it (instead of subscribing to the single)
in the model, when you
receive a new call to refresh() from the ui, do something like
subject.onNext(Api.getDashboard())
in this way, each call to refresh will cause the emission of a new dashboard, and that will be properly bound by the subscription in the view.

Xamarin Android passing data between views

I am new to Xamarin Android. I am looking to create a multi-step registration form i.e. wizard process.
I am seeking for some best practises on a approach that can save each step state in order to build up a model and then submit the model (Builder design pattern).
I am not sure how to save a state when going to another view. From research, would i pass a view model as part of the intent by calling PutExtra to pass data ?
I am not sure what is the good way doing this. I am using MVVMCross for building up the viewmodel. I hope the community can assist me on the right direction please

In MvvmCross you don't pass data between views, but all navigation is done between ViewModels. Extensive documentation about this can be found at: https://www.mvvmcross.com/documentation/fundamentals/navigation
An example is:
public class MyViewModel : MvxViewModel
{
private readonly IMvxNavigationService _navigationService;
public MyViewModel(IMvxNavigationService navigationService)
{
_navigationService = navigationService;
}
public override void Prepare()
{
//Do anything before navigating to the view
}
public async Task SomeMethod()
{
await _navigationService.Navigate<NextViewModel, MyObject>(new MyObject());
}
}
public class NextViewModel : MvxViewModel<MyObject>
{
public override void Prepare(MyObject parameter)
{
//Do anything before navigating to the view
//Save the parameter to a property if you want to use it later
}
public override async Task Initialize()
{
//Do heavy work and data loading here
}
}

ListView not updating after async call

I have a weird / unique situation with my ListView. This is the scenario:
I'm making use of the MVP design pattern. As the Activity starts, it raises an event to notify the presenter to fetch some data from a web service. The web service call is an Async call. Once the web service Completed event is raised, I take the result and push it into a property (which is of type Array) that resides within my View / Activity.
Everything I mentioned works just fine, but as soon as the device is rotated, some interesting developments take place.
The async call resumes as normal and provides the property (Array) with a value. So nothing wrong there... (And yes there is data in the collection) I then set the ListView Adapter and call the notifyDataSetChanged, but nothing happens. The UI is not updated or anything?? If I re-enter the Activity the data is visible again ??
I even tried calling invalidateViews and invalidate on the ListView - this didn't do anything.
Could someone please assist me in this matter?
Many thanks in advance!
[Update]
I would like to stress the fact that I am making use of C# (Xamarin) and not Java (:sigh: - yes I know). Furthermore, I am not making use of the ASyncTask class, instead I'm making use of the async methods created within the proxy classes generated by Visual Studio. Pretty straight forward, but this is the code that populates the ListView - the property is set from the presenter
Presenter
Where View is of type IContactsView
protected override void OnCollectData(System.Collections.IEnumerable data, Type typeOfData)
{
if (data != null && typeOfData != null && typeOfData.Equals(typeof(UserContact)))
{
this.View.UserInformationCollection = data.Cast<UserContact>().ToArray();
}
}
Activity
The activity implements IContactsView
public UserContact[] UserInformationCollection
{
get
{
return this._userInformationCollection;
}
set
{
this.RunOnUiThread(() =>
{
this._userInformationCollection = value;
ListView listview = this.FindViewById<ListView>(Resource.Id.userLV);
if (listview != null)
{
UserContact[] subsidiesList = this.GetIndexedContacts(this._userInformationCollection);
listview.Adapter = new ContactsAdapter(this, subsidiesList.ToList());
((ContactsAdapter)listview.Adapter).NotifyDataSetChanged();
}
});
}
}
[/Update]

Found a much better solution! So please ignore the static variable idea!
Activity:
Override the OnRetainNonConfigurationInstance and return the presenter
public override Java.Lang.Object OnRetainNonConfigurationInstance()
{
return this._presenter;
}
Within the OnCreate check the LastNonConfigurationInstance and get the presenter - if it isn't null:
protected override void OnCreate(Bundle bundle)
{
...
if (this.LastNonConfigurationInstance != null)
{
this._presenter = this.LastNonConfigurationInstance as ContactsPresenter;
this._presenter.RefreshView(this);
}
else
{
// create a new presenter
this._presenter = new ContactsPresenter(this);
}
...
}
So maybe, you saw what I did in the previous code sample? Yes, I send the new instance of the activity to the presenter - have a look at the RefreshView
Presenter:
So within my base presenter I have the following method:
public class Presenter<T> : Java.Lang.Object, IPresenter where T : IView
{
/// <param name="view">The view.</param>
public void RefreshView(T view)
{
this.View = view;
}
}
The above code helps my presenter say with the creation of new activities - so when it returns data after the async call it will have the latest and greatest instance of the activity!
Hope this helps!
Kind regards,

Got it working by doing the following:
declare a static variable of the activity:
private static ContactsActivity _cachedActivity = null;
Overrode the OnResume within the activity and set the variable:
protected override void OnResume()
{
base.OnResume();
_cachedActivity = this;
}
Override the OnCreate within the activity and set the variable:
protected override void OnCreate(Bundle bundle)
{
...
_cachedActivity = this;
...
}
Lastly I changed the property mentioned earlier:
public USBUserContact[] UserInformationCollection
{
get
{
return this._userInformationCollection;
}
set
{
_cachedActivity.RunOnUiThread(() =>
{
_cachedActivity._userInformationCollection = value;
ListView listview = _cachedActivity.FindViewById<ListView>(Resource.Id.userLV);
if (listview != null)
{
UserContact[] subsidiesList = _cachedActivity.GetIndexedContacts(_cachedActivity._userInformationCollection);
listview.Adapter = new ContactsAdapter(_cachedActivity, subsidiesList.ToList());
((ContactsAdapter)listview.Adapter).NotifyDataSetChanged();
}
});
}
}
Kind regards,

How can I call a function in my main Activity class from a custom Gallery view in Android?

I have a custom gallery view in which I am overriding some methods. I would like to be able to call a function in my main activity from this class. How do I make a reference back to my main class?
I thought I'd just push the class reference into CustomGallery by creating a setter function ---> g.setBaseClass(this);
CustomGallery g = (CustomGallery) findViewById(R.id.playSelectionGallery);
g.setSpacing(10);
g.setCallbackDuringFling(false);
g.setAdapter(new ImageAdapter(this));
g.setSelection(1);
registerForContextMenu(g);
g.setBaseClass(this);
Problem is this is of type Context and someFunctionToCall() will result in a not a member of this class error. In my custom class I have:
public void setBaseClass(Context baseClass)
{
_baseClass = baseClass;
}
private void callSomeFuntionOnMyMainActivityClass()
{
_baseClass.someFunctionToCall();
}
All I want to do is call back to my main class, called ViewFlipperDemo. This would be easy in As3. Any thoughts? Hopefully I'm missing something really simple.

That's actually not a good idea... but you can do it this way:
private void callSomeFuntionOnMyMainActivityClass()
{
((ViewFlipperDemo)_baseClass).someFunctionToCall();
}
What you should do instead is implementing a simple observer which allows you to notify the Activity that something happened. That's one of the main OO principles, your custom class shouldn't know anything about your activity class.
Observer pattern example
The Observer interface:
// TheObserver.java
public interface TheObserver{
void callback();
}
Your custom view:
public class CustomGallery{
private TheObserver mObserver;
// the rest of your class
// this is to set the observer
public void setObserver(TheObserver observer){
mObserver = observer;
}
// here be the magic
private void callSomeFuntionOnMyMainActivityClass(){
if( mObserver != null ){
mObserver.callback();
}
}
// actually, callSomeFuntionOnMyMainActivityClass
// is not a good name... but it will work for the example
}
This is the activity that will benefit of the observer (notice that now you can use your custom view on different activities not just one, that's one of the key reasons to implement it this way):
public class YourActivity extends Activity{
// your normal stuff bla blah
public void someMethod(){
CustomGallery g=(CustomGallery)findViewById(R.id.playSelectionGallery);
g.setObserver(new TheObserver(){
public void callback(){
// here you call something inside your activity, for instance
methodOnYourActivity();
}
});
}
}
You will notice that this design pattern (observer) is widely used in Java and Android... almost any kind of UI event is implemented using observers (OnClickListener, OnKeyListener, etc.). By the way, I didn't test the code, but it should work.