We can not create ViewModel on our own. We need ViewModelProviders utility provided by Android to create ViewModels. But ViewModelProviders can only instantiate ViewModels with no arg constructor. So if I have a ViewModel with multiple arguments, then I need to use a Factory that I can pass to ViewModelProviders to use when an instance … Read more
I had a similar problem using Dagger 2 that was caused by having different instances of the Dao, one for updating/inserting data, and a different instance providing the LiveData for observing. Once I configured Dagger to manage a singleton instance of the Dao, then I could insert data in the background (in my case in … Read more
Basically, Should everything be done through the viewmodel or is it Ok to also incorporate viewbag? Everything should be done inside a view model. That’s what a view model is. A class that you specifically define to meet the requirements of your view. Don’t mix ViewBags with ViewModels. It is no longer clear for the … Read more
You can create multiple ObservableCollections and then bind your ItemsSource to a CompositeCollection which joins those collections. Then in your XAML you can create different DataTemplates for the respective types using the DataType property which like styles gets automatically applied if it is placed in the resources. (You can also create the composite in XAML … Read more
It’s worth nothing to use MVVM frameworks if you don’t know what’s going on inside. So let’s go step by step and build your own ViewModelBase class. ViewModelBase is class common for all your viewmodels. Let’s move all common logic to this class. Your ViewModels should implement INotifyPropertyChanged (do you understand why?) public abstract class … Read more
AndroidViewModel provides Application context If you need to use context inside your Viewmodel you should use AndroidViewModel (AVM), because it contains the application context. To retrieve the context call getApplication(), otherwise use the regular ViewModel (VM). AndroidViewModel has application context. We all know having static context instance is evil as it can cause memory leaks!! … Read more
The preferred way since .Net 4.5 to implement data validation is to let your view model implement INotifyDataErrorInfo (example from Technet, example from MSDN (Silverlight)). Note: INotifyDataErrorInfo replaces the obsolete IDataErrorInfo. The new framework infrastructure related to the INotifyDataErrorInfo interface provides many advantages like support of multiple errors per property custom error objects and customization … Read more
Absolutely, positively NO Your UserControls should NOT have ViewModels designed specifically for them. This is, in fact, a code smell. It doesn’t break your application immediately, but it will cause you pain as you work with it. A UserControl is simply an easy way to create a Control using composition. UserControls are still Controls, and … Read more
You need to have a factory class for your ViewModel. public class MyViewModelFactory implements ViewModelProvider.Factory { private Application mApplication; private String mParam; public MyViewModelFactory(Application application, String param) { mApplication = application; mParam = param; } @Override public <T extends ViewModel> T create(Class<T> modelClass) { return (T) new MyViewModel(mApplication, mParam); } } And when instantiating the … Read more
“View Model” is just a pattern. There’s nothing magical about the name, but generally any class being passed to a view (whether for simply displaying data or for the purposes of form submissions) is referred to as a “view model” and given a name like FooViewModel or FooVM to indicate that it’s part of that … Read more