My viewModel contains some variables such as how many cardViews should be created in the recyclerView. Therefore, I am looking for a way to access the same viewModel object in my adapter class. Is there a way to do so or a better alternative? My code is in kotlin
class RecyclerAdapter : RecyclerView.Adapter<RecyclerAdapter.ViewHolder>() {
private val gameViewModel: GameViewModel by activityViewModels()
Since you have provided only 2 lines of code it is hard to know exactly what you are doing wrong.
Normally you would retrieve a ViewModel in an Activity class or a Fragment class like this
class MyActivity /* other stuff */ {
// this line produces/retrieves an instance of GameViewModel
// where its owner is MyActivity
private val gameViewModel: GameViewModel by viewModels()
}
Then somewhere else inside your activity class, you instantiate your RecycleAdapter class. There you would pass the gameViewModel to it. Of course, to be able to do that your RecyclerAdapter would either have to accept a GameViewModel as a constructor parameter, or through a setter, or some other function call.
Here is an example through a constructor parameter. Your RecyclerAdapter class would have to be defined something like this (note that this is Kotlin concise syntax for declaring properties and initializing them from the primary constructor)
class RecyclerAdapter(
private val gameViewModel: GameViewModel,
// add more constructor parameters/class properties here if needed
) : RecyclerView.Adapter<RecyclerAdapter.ViewHolder>() {
// other class properties that you don't want to initialize
// through the primary constructor
// ...
// the class body where you implement RecyclerView.Adapter<> methods
// ...
override fun onBindViewHolder(holder: ViewHolder, position: Int) {
// gameViewModel can be used here
gameViewModel.doSomething()
}
}
And as a last step, modify the line in your code, where you create your RecyclerAdapter instance
// here we create a new RecyclerAdapter and pass the gameViewModel to it
val adapter = RecyclerAdapter(gameViewModel)
recyclerView.adapter = adapter
Your Fragment's views should have a shorter lifecycle than your associated ViewModel, so it should be OK to pass it in as a constructor parameter.
class RecyclerAdapter(private val gameViewModel: GameViewModel) :
RecyclerView.Adapter<RecyclerAdapter.ViewHolder>() {
//...
}
Then pass the view model reference in from the Fragment when you instantiate the adapter in onViewCreated().
Personally, I wouldn't do this because presumably your ViewModel has lots of stuff in it that is irrelevant to the Adapter. Separation of concerns. I would make parameters only for the properties that are needed and let the Fragment pass them along from the ViewModel.
I have a problem and I searched a solution about it. Lucky, I red lot of post about it but I'm lost with the explaination I found. The initale problem is coming from a personal project about the polyline of the Xamarin.Forms.Map where the initialization is realized by a binding from the XAML part..
Let me be clear by an example :
I have an object CustomMap.cs which inherit from Xamarin.Forms.Map (This file is in the PCL part -> CustomControl/CustomMap.cs)
public class CustomMap : Map, INotifyPropertyChanged
{
public static readonly BindableProperty PolylineAddressPointsProperty =
BindableProperty.Create(nameof(PolylineAddressPoints), typeof(List<string>), typeof(CustomMap), null);
public List<string> PolylineAddressPoints
{
get { return (List<string>)GetValue(PolylineAddressPointsProperty); }
set
{
SetValue(PolylineAddressPointsProperty, value);
this.GeneratePolylineCoordinatesInner();
}
}
// ...
}
So the Xaml part of the page, where the control is called, looks like that:
<?xml version="1.0" encoding="utf-8" ?>
<ContentPage xmlns="http://xamarin.com/schemas/2014/forms"
xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
xmlns:control="clr-namespace:MapPolylineProject.CustomControl;assembly=MapPolylineProject"
x:Class="MapPolylineProject.Page.MainPage">
<ContentPage.Content>
<control:CustomMap x:Name="MapTest" PolylineAddressPoints="{Binding AddressPointList}"
VerticalOptions="Fill" HorizontalOptions="Fill"/>
</ContentPage.Content>
</ContentPage>
The Csharp part:
public partial class MainPage : ContentPage
{
public List<string> AddressPointList { get; set; }
public MainPage()
{
base.BindingContext = this;
AddressPointList = new List<string>()
{
"72230 Ruaudin, France",
"72100 Le Mans, France",
"77500 Chelles, France"
};
InitializeComponent();
//MapTest.PolylineAddressPoints = AddressPointList;
}
}
So, everything is fine if I edit the PolylineAddressPoints from the object instance (if the commented part isnt' commented..), but if I init the value from the XAML (from the InitializeComponent();), it doesn't work, the SetValue, in the Set {}, isn't called..
I then searched on the web about it and get something about the Dependency Properties? or something like that. So I tried some solutions but, from WPF, so some methods, such as DependencyProperty.Register();. So yeah, I can't find the way to solve my problem..
I also though about something, if DependencyProperty.Register(); would exists in Xamarin.Forms, then it means I would have to do it for each values? Because, if every value has to be set by a XAML binding logic, it would not work, I would have to register every value, doesn't it?
I'm sorry if I'm not clear, but I'm so lost about this problem.. Please, do not hesitate to ask for more details, thank in advance !
I finaly got a solution just over here => Ignore the Binding initialization
Copy paste from Stackoverflow. This following answer was given by Stephane Delcroix, thank to him !
There are multiple questions in this:
Why is the property setter never called when using Xaml ?
Am I properly defining my BindableProperty ?
Why is my binding failing ?
Let me answer them in a different order.
Am I properly defining my BindableProperty ?
The BindableProperty declaration is right, but could be improved by using an IList<string>:
public static readonly BindableProperty PolylineAddressPointsProperty =
BindableProperty.Create(nameof(PolylineAddressPoints), typeof(IList<string>), typeof(CustomMap), null);
but the property accessor is wrong, and should only contains this:
public IList<string> PolylineAddressPoints
{
get { return (IList<string>)GetValue(PolylineAddressPointsProperty); }
set { SetValue(PolylineAddressPointsProperty, value); }
}
I'll tell you why while answering the next question. But you want to invoke a method when the property has changed. In order to do that, you have to reference a propertyChanged delegate to CreateBindableProperty, like this:
public static readonly BindableProperty PolylineAddressPointsProperty =
BindableProperty.Create(nameof(PolylineAddressPoints), typeof(IList<string>), typeof(CustomMap), null,
propertyChanged: OnPolyLineAddressPointsPropertyChanged);
And you have to declare that method too:
static void OnPolyLineAddressPointsPropertyChanged(BindableObject bindable, object oldValue, object newValue)
{
((CustomMap)bindable).OnPolyLineAddressPointsPropertyChanged((IList<string>)oldValue, (IList<string>)newValue);
}
void OnPolyLineAddressPointsPropertyChanged(IList<string> oldValue, IList<string> newValue)
{
GeneratePolylineCoordinatesInner();
}
Why is the property setter never called when using Xaml ?
The property, and the property accessors, are only meant to be invoked when accessing the property by code. C# code.
When setting a property with a BindablePrperty backing store from Xaml, the property accessors are bypassed and SetValue() is used directly.
When defining a Binding, both from code or from Xaml, property accessors are again bypassed and SetValue() is used when the property needs to be modified. And when SetValue() is invoked, the propertyChanged delegate is executed after the property has changed (to be complete here, propertyChanging is invoked before the property change).
You might wonder why bother defining the property if the bindable property is only used by xaml, or used in the context of Binding. Well, I said the property accessors weren't invoked, but they are used in the context of Xaml and XamlC:
a [TypeConverter] attribute can be defined on the property, and will be used
with XamlC on, the property signature can be used to infer, at compile time, the Type of the BindableProperty.
So it's a good habit to always declare property accessors for public BindableProperties. ALWAYS.
Why is my binding failing ?
As you're using CustomMap as bot View and ViewModel (I won't tell the Mvvm Police), doing this in your constructor should be enough:
BindingContext = this; //no need to prefix it with base.
As you're doing it already, your Binding should work once you've modified the BindableProperty declaration in the way I explained earlier.
I would like to know if I can access the constructor of the base class in its derived classes in C#. If yes please let me know how could we make it. Thanks in advance.
You can call the base class constructor as part of the execution of the derived class constructor
public MyBase
{
public MyBase() { }
}
public Derived
{
public Derived() : base() { }
}
When using this pattern, you are said to be using the base class initializer.
For more background, see the base keyword and instance constructors on MSDN.
Consider the following class:
class MyContext : DbContext
{
public DbSet<Order> Orders { get; set; }
}
and instantiating a new object:
var mycontext = new MyContext();
Why mycontext.Orders is not null? When it was initialized? Who has initialized it? I'm really confused because the base class (DbConetxt) cannot access the derived class properties so it is not possible that the automatic property was initialized in the base object.
From looking at the reflected code, when the DbContext (the base class) is constructed it makes a call to the DbSetDiscoveryService (an internal clasS) - which essentially uses reflection to find all properties on the DbContext, and then initializes those that need initializing.
So in short - using reflection in the constructor.
I have an app (written using MonoTouch and currently working) that I want to add landscape orientation to. I am using a UITabBarController.
I don't see how to create a controller that will allow me to override the "ShouldAutorotate..." method. Can anybody point me to an example using a UITabBarController in MonoTouch?
TweetStation contains a sample precisely for this setup, and propagates the rotation down all of the nested view controllers.
Are you subclassing UITabBarController?
You are probably non subclassing and just adding a vanilla controller in Interface Builder. You have to subclass to override that property.
First make a new class like this:
//Test this, it's off the top of my head
[Register("YourTabController")]
public class YourTabController : UITabBarController
{
public YourTabController (IntPtr handle) : base (handle) { }
[Export("initWithCoder:")]
public YourTabController (NSCoder coder) : base (coder) { }
//Override should rotate
public bool ShouldAutoRotateToInterfaceOrientation(UIInterfaceOrientation o)
{ return true; }
}
Then, if you already have a UITabBarController in IB, there is a 'Class' property that you set to the name of your new class.