Common.Xamarin.ViewModel
Common Xamarin ViewModel to Help with Property Relationships
Purpose
This library was created to eliminate some of the boilerplating that is needed to have a useful viewmodel in Xamarin (or WPF).
Basic Usage
Create a viewmodel that extends BaseViewModel to have access to the built in features of the library.
public class ExampleViewModel : Common.Xamarin.ViewModel {
//Basic Examples
public string Title {
get => GetValue<string>();
set => SetValue(value);
}
public int Count {
get => GetValue<int>();
set => SetValue(value);
}
}Extended Usage (Relationships)
using Common.Xamarin.ViewModel.Attributes;
//...
public class RelationshipViewModel : Common.Xamarin.ViewModel {
//Basic Examples
//This value affects IsValid
public string Title {
get => GetValue<string>();
set => SetValue(value);
}
//This Value Affects both Message and IsValid
[Affects(nameof(Message))]
public int Count {
get => GetValue<int>();
set => SetValue(value);
}
[AffectedBy(nameof(Title), nameof(Count))]
public bool IsValid => Count > 0 && !string.IsNullOrEmpty(Title);
//This one could also be AffectedBy Count.
public string Message => $"The total count is: {Count}";
}ShellAware ViewModels * New *
Package:Common.Xamarin.ViewModel.ShellAware Shell Aware View Models simplify the process of navigating through shell as well as passing and receiving values between viewmodels. This is an extension of the common BaseViewModels but specifically for Xamarin.
ShellAwareViewModel:
Class that automates a lot of the process of transmitting information between viewmodels as navigation occurs. Initially I intended to extend the process allowed
via QueryPropertyAttribute usage, but Xamarin already does some auto setting via this parameter which do not allow for complex data transmission. It extends
BaseViewModel so the same utilities for getting and setting are still available. There are 2 attributes which simplify communication:
QueryParameterAttribute
These are examples of the usage of the parameters passed via query between shell viewmodels
[QueryParameter("queryid")] //Receive value stored in shell parameter queryId
public string QueryId { get => GetValue<string>(); set => SetValue(value); }
[QueryParameter] //Receive value stored in shell parameter of the same name as the property
public int Id { get => GetValue<string>(); set => SetValue(value); }NavigationParameterAttribute
These are examples of the usage. This attribute inherits from QueryParameterAttribute so it automatically populates the value on navigating to the viewmodel with this
attribute, and will automatically populate the query on navigation. The parameters are the same as QueryParameterAttribute.
[NavigationParameter("queryid")] //Set or use value into query parameter 'queryid'
public string QueryId { get => GetValue<string>(); set => SetValue(value); }
[NavigationParameter] //Use value stored in shell parameter of the same name as the property
public int Id { get => GetValue<string>(); set => SetValue(value); }Automating Communication
The ShellAwareViewModel has a method GoToAsync that wraps around the Shell.Current.GoToAsync method, but adds the logic to serialize data transmitted between viewmodels.
await GoToAsync("NewRoute"); //Will serialize any navigation properties into the Query
await GoToAsync("NewRoute", new Dictionary<string, object> {
{ "Test", 1 }
});Additional
Triggers on Change
Technically you can implement triggers in a variety of ways including subscribing to the INotifyPropertyChanged Event, but that can usually be a little messy. Instead you can implement Method Calls in either the getter or the setter of the properties to handle if you want to manipulate the old or new data.
Example:
///...
public string Title {
get {
var value = GetValue<string>();
//Takes the newly stored value. This getter is accessed on each Notified Change
//so it would execute after a change
CallMethodWithNewValue(value);
return value;
}
set {
//Optional if you need to compare the values or something along those lines
var oldValue = GetValue<string>();
//Called everytime there is a change in the value of this particular property.
CallMethodWithBothValues(oldValue, value);
SetValue(value);
}
}Implementing Events
If you want to enable the INavigationAware, IResetOnNavigation events than in the Class that extends your shell you can modify it like so:
// Autogenerated code for Xamarin Forms Shell Navigation
public partial class AppShell : Xamarin.Forms.Shell
{
public AppShell()
{
InitializeComponent();
Routing.RegisterRoute(nameof(ItemDetailPage), typeof(ItemDetailPage));
Routing.RegisterRoute(nameof(NewItemPage), typeof(NewItemPage));
}
private async void OnMenuItemClicked(object sender, EventArgs e)
{
await Current.GoToAsync("//LoginPage");
}
// ADD THE FOLLOWING CODE
#region Common.Xamarin.ViewModel Extensions to Shell
private Page _lastPage;
protected override void OnNavigating(ShellNavigatingEventArgs args)
{
_lastPage = CurrentPage;
base.OnNavigating(args);
}
protected override void OnNavigated(ShellNavigatedEventArgs args)
{
if (_lastPage?.BindingContext is IResetOnNavigation iR) iR.Reset();
if (_lastPage?.BindingContext is INavigationAware iNA) iNA.OnNavigatedFrom();
base.OnNavigated(args);
if (CurrentPage?.BindingContext is INavigationAware NA) NA.OnNavigatedTo();
}
internal Page CurrentPage =>
(Current?.CurrentItem?.CurrentItem as IShellSectionController)?.PresentedPage;
#end region
}
