I'm struggling to find the best implementation.
I'm using Prism and I have a View (ParentView), which has a small region within it. Depending on the item in a ddl, another smaller view (ChildView) gets injected into the region of the ParentView.
The ChildView will just have some properties which I would like to access from the ParentView.
So I realize I can use a Publish/Subscribe method to move data between viewmodels, but the issue is I have nothing to hang the Publish on. The view is made up of TextBoxes and no event triggers. The ChildView can be vastly different based on the selection of the ddl. I like the clean separation of each ChildView being it's own view injected inside the ParentView.
What is the best way to achieve this?
One solution can be to implement the interface INavigationAware in your viewmodels. After that you can use the methods onNavigatedFrom(), onNavigatedTo() and onNavigatingTo() to register your event.
EDIT:
If you want launch the event when a field in the child is changed you can do something like this:
private string _yourField;
public string YourField
{
get { return _yourField; }
set { SetProperty(ref _yourField, value);
//Here you can launch the event
}
}
In this case when YourField change the event is launched.
I tried a few implementations, but the one that worked was creating a singleton instance of the ChildView (childviewmodel) and then gaining access to the properties through the instance. It may not be pretty, but it works.
private static ChildViewModel _instance = new ChildViewModel ();
public static ChildViewModel Instance { get { return _instance; } }
#region Properties
private ChildModel _childModel= new ChildModel ();
public ChildModel _childModel
{
get { return _instance._childModel; }
set
{
SetProperty(ref _instance._childModel, value);
}
}
private string _childProperty1;
public string ChildProperty1
{
get { return _childProperty1; }
set
{
SetProperty(ref _childProperty1, value);
ChildModel.ChildProperty1= _childProperty1;
}
}
In reality - there were many childproperties. I only listed one for demo. And then I call it in ParentView
var _instance = ChildViewModel.Instance;
var _cm = _instance.ChildModel;
_parentModel = new ParentModel
{
Property1= ParentViewProperty1,
Property2= _cm.ChildProperty1,
};
Hope that helps someone else.
Related
I would like to ask about bindings. What is the best approach to bind some actions in listview items in ios and android using xamarin in mvvm world. As I understand, we have few approaches.
1.
For every list item we have some Model, and to this model we have to add some Commands.
For example:
public class ItemModel
{
public string MyName { get; set; }
public ICommand RemoveCommand { get; set; }
}
Where in ViewModel we have SomeInitMethod
public class ViewModel
{
public ObservableCollection<ItemModel> Items {get;set;}
public async Task SomeInitMethod
{
Items = new ObservableCollection(await _myApiService.FetchItemsAsync());
foreach(var item in Items)
{
item.Command = new RelayCommand(RemoveItem);
}
}
public void RemoveItem(ItemModel item)
{
Items.Remove(item);
}
}
But I see a drawback in SomeInitMethod where we should set RemoveCommand. What if we should to set 2 or even more commands than we duplicate code in ListItemView(somehow we need to bind all these commands)?
Next approach is somehow handle events of remove/toggle buttons and others in Listview and then delegate this commands directly to ViewModel.
Example:
ContactsListView.ItemRemoveClicked += (ItemModel model) => ViewModel.RemoveItem
Advantages is: we no longer need to handle commands in ViewModel
Drawback is: we need every time to write custom ListView and support event handling in code-behind.
The last approach is to send ViewModel to ListItem to set Commands.
Example
somewhere we have method CreateListViewItem on the view, let's say on iOS.
private void InitTableView() {
TableView.RegisterNibForCellReuse(ItemViewCell.Nib, ItemViewCell.Key);
var source = new ObservableTableViewSource <ItemModel>
{
DataSource = ViewModel.Items,
BindCellDelegate = (cell, viewModel, index) =>
{
if (cell is ItemModel memberCell)
{
memberCell.BindViewModel(viewModel);
memberCell.RemoveItem = (item) => ViewModel.RemoveItem;
}
}
};
TableView.Source = source;
}
Advantages: we no longer need to have Commands in Model, and we don't need to setup this Commands in ViewModel.
Possibly, drawback is that we somehow need to have ViewModel reference.
In WPF or UWP you have DataContext, you can binding directly to ViewModel.
Which approach you use, maybe I miss something, and it would be perfect if you provide some examples or thoughts.
Thanks.
I am trying to find a way to be able to set from the View to what ViewModel I have to navigate. This is to be able to change the navigation flow without changing the core project.
I thought the easier way would be creating an interface, setting the target ViewModel there and injecting the interface into the ViewModel to then perform the navigation.
public interface IModelMapping
{
MvxViewModel ViewModelToNavigate();
}
public class MyViewModel : MvxViewModel
{
readonly IMvxNavigationService navigationService;
readonly IModelMapping modelMapping;
public MyViewModel(IMvxNavigationService navigationService, IModelMapping modelMapping)
{
this.navigationService = navigationService;
this.modelMapping = modelMapping;
}
public IMvxAsyncCommand GoContent
{
get
{
IMvxViewModel vm = modelMapping.ViewModelToNavigate();
IMvxAsyncCommand navigateCommand = new MvxAsyncCommand(() => navigationService.Navigate<vm>());
return navigteCommand;
}
}
}
The problem with this code is I am getting an error setting the navigationService.Navigate(). The error is 'vm is a variable but it is used like a type'
What about using the URI navigation together with the facade? See also https://www.mvvmcross.com/documentation/fundamentals/navigation#uri-navigation
Say you are building a task app and depending on the type of task you want to show a different view. This is where NavigationFacades come in handy (there is only so much regular expressions can do for you).
mvx://task/?id=00 <– this task is done, show read-only view (ViewModelA)
mvx://task/?id=01 <– this task isn’t, go straight to edit view (ViewModelB)
[assembly: MvxRouting(typeof(SimpleNavigationFacade), #"mvx://task/\?id=(?<id>[A-Z0-9]{32})$")]
namespace *.NavigationFacades
{
public class SimpleNavigationFacade
: IMvxNavigationFacade
{
public Task<MvxViewModelRequest> BuildViewModelRequest(string url,
IDictionary<string, string> currentParameters, MvxRequestedBy requestedBy)
{
// you can load data from a database etc.
// try not to do a lot of work here, as the user is waiting for the UI to do something ;)
var viewModelType = currentParameters["id"] == Guid.Empty.ToString("N") ? typeof(ViewModelA) : typeof(ViewModelB);
return Task.FromResult(new MvxViewModelRequest(viewModelType, new MvxBundle(), null, requestedBy));
}
}
}
I currently have a view model looking like this:
public class PhrasesFrameViewModel : ObservableProperty
{
bool customPointsSwitch;
public PhrasesFrameViewModel()
{
var aButtonClickedCommand = new Command(() =>
{
App.DB.IncrementScore(App.cfs, App.phrase, (int)App.aBtn);
App.correctButtonPressed = (int)App.aBtn;
ResetTimer2();
});
var wordGridClickedCommand = new Command(() =>
{
if (App.Timer1Running)
ResetTimer1();
else
ResetTimer2();
});
}
private static void ResetTimer1()
{
if (App.tokenSource1 != null)
{
App.Timer1Seconds = 0;
App.tokenSource1.Cancel();
}
}
private static void ResetTimer2()
{
if (App.tokenSource2 != null)
{
App.Timer2Seconds = 0;
App.tokenSource2.Cancel();
}
}
public bool CustomPointsSwitch
{
get
{
return customPointsSwitch;
}
set
{
if (value != customPointsSwitch)
{
customPointsSwitch = value;
NotifyPropertyChanged("CustomPointsSwitch");
App.DB.UpdateBoolSetting(Settings.Cp, customPointsSwitch);
}
}
}
I believe most view models would have code similar to that for CustomPointsSwitch but how about the code for the gesture recognizers and the commands plus the small method for reset (used by a few other methods in the view model). Does that all belong in the view model or should it be in another class?
Short answer: In this particular case, as per code shared in question, they belong in view model.
Long answer:
It depends. If your command handler needs to interact with UI - it should stay in view; If it is anything else, i.e. presentation or business logic - it should be defined in view model.
Commands are integral part of MVVM pattern - as they allow for decoupling of view model from view, which in turn makes it easier to unit test, maintain and extend. They are the recommended channel for communication between view and view-model (other than data-binding).
In most of the cases, the command interface is used when a user-interaction/event in view needs to trigger various actions in the view-model - and in these cases command is defined in view-model itself and exposed as a property.
I might be missing something simple, so bear with me.
I have a ViewModel that contains the following:
public ObservableCollection<Person> PersonCollection
{
get { return personCollection; }
set
{
if (personCollection != value)
{
personCollection = value;
RaisePropertyChanged("PersonCollection");
}
}
}
Then in another ViewModel I have:
public ObservableCollection<Person> PersonCollection
{
get
{
PersonViewModel vm = (App.Current.Resources["Locator"] as ViewModelLocator).PersonViewModel;
return vm.PersonCollection;
}
}
public PersonViewModel PersonViewModel
{
get
{
return ((App.Current.Resources["Locator"] as ViewModelLocator).PersonViewModel)
}
}
In my XAML if I bind to PersonCollection then updates don't happen on my view, but if I bind to PersonViewModel.PersonCollection it does. so is this the "proper" way to do it or is there a way for the view to detect the notifications using the first approach?
Change your binding to {Binding PersonViewModel.PersonCollection}
Your wrapped PersonCollection property has no change notifications, so the view doesn't know that the property has changed (it certainly has no way of knowing it originally came from PersonViewModel in order to get change notifications from it)
I'm just switching a project across to mvvmlight and trying to do things "the right way"
I've got a simple app with a listbox
When an item is selected in the listbox, then I've hooked up a RelayCommand
This RelayCommand causes a call on an INavigationService (http://geekswithblogs.net/lbugnion/archive/2011/01/06/navigation-in-a-wp7-application-with-mvvm-light.aspx) which navigates to a url like "/DetailPage.xaml?DetailId=12"
The DetailPage.xaml is then loaded and ... this is where I'm a bit unsure...
how should the DetailPage get hooked up to a DetailView with DetailId of 12?
should I do this in Xaml somehow using a property on the ViewLocator?
should I do this in the NavigatedTo method?
Please feel free to point me to a full sample - sure this has been done a (hundred) thousand times before, but all the blogs and tutorials seem to be skipping this last trivial detail (focussing instead on the messaging and on the ioc on on the navigationservice)
Thanks!
The only place you can retrieve the URL parameter is in the view. So since your view is likely depending on it, you should fetch it in the OnNavigatedTo method.
Then, you should pass it along to your viewmodel, either using messaging (to expensive if you ask me), or by referring to your datacontext (which is the viewmodel I presume), and execeuting a method on that.
private AddTilePageViewModel ViewModel
{
get
{
return DataContext as AddTilePageViewModel;
}
}
protected override void OnNavigatedTo(NavigationEventArgs e)
{
var postalCode = NavigationContext.TryGetKey("PostalCode");
var country = NavigationContext.TryGetStringKey("Country");
if (postalCode.HasValue && string.IsNullOrEmpty(country) == false)
{
ViewModel.LoadCity(postalCode.Value, country);
}
base.OnNavigatedTo(e);
}
I'm using some special extensions for the NavigationContext to make it easier.
namespace System.Windows.Navigation
{
public static class NavigationExtensions
{
public static int? TryGetKey(this NavigationContext source, string key)
{
if (source.QueryString.ContainsKey(key))
{
string value = source.QueryString[key];
int result = 0;
if (int.TryParse(value, out result))
{
return result;
}
}
return null;
}
public static string TryGetStringKey(this NavigationContext source, string key)
{
if (source.QueryString.ContainsKey(key))
{
return source.QueryString[key];
}
return null;
}
}
}
Create a new WindowsPhoneDataBound application, it has an example of how to handle navigation between views. Basically you handle the navigation part in your view, then set the view's DataContext accord to the query string. I think it plays nicely with the MVVM pattern since your ViewModels don't have to know anything about navigation (which IMO should be handled at the UI level).