I have a view bound to a viewmodel that references a datacontext (linq to sql). The view renders great and displays the values that are in the database. Making changes to the values in the view correctly updates the values in the viewmodel, but when I attempt to submit the changes, navigate back from the page and then back to it, my changes are gone. Any ideas? Here's the code for my viewmodel:
public class WidgetViewModel : BaseViewModel
{
private Widget _widget;
public Widget Widget { get { return _widget; } set { _widget = value; NotifyPropertyChanged("Widget"); } }
public WidgetViewModel(int id)
{
Widget = Context.Widgets.Single(m => m.Id == id);
}
public void Save()
{
Context.SubmitChanges();
}
}
Related
I have the below (simplified) model to display and return Chats. On the UI I want to show New Chats and Existing chats separately. When I first load the chats, it works, but if I update a Chat object, the UI doesn't update.
In the Xamarin Forms UI I have a 1) CollectionView binding to NewChats and 2) CollectionView binding to Existing Chats.
I update the IsNew flag programmatically, but that is not reflecting in the UI.
Any thoughts on how to approach this?
public class Chat
{
public string UserId {get;set;}
public bool IsNew {get;set;}
}
private ObservableCollection<Chat> _chats;
public ObservableCollection<Chat> Chats
{
get
{
return _chats;
}
set
{
this._chats= value;
OnPropertyChanged(nameof(Chats));
OnPropertyChanged(nameof(NewChats));
OnPropertyChanged(nameof(ExistingChats));
}
}
public ObservableCollection<Chat> NewChats
{
get
{
if (_chats!= null)
{
return new ObservableCollection<Chat>(_chats.Where(x => x.isNew);
}
return new ObservableCollection<Chat>();
}
}
public ObservableCollection<Chat> ExistingChats
{
get
{
if (_chat!= null)
{
return new ObservableCollection<Chat>(_chats.Where(x => !x.isNew);
}
return new ObservableCollection<Chat>();
}
}
As the comment above says For change notification to occur in a binding between a bound client and a data source, your bound type should either:
Implement the INotifyPropertyChanged interface (preferred).
Provide a change event for each property of the bound type.
you can check here to get offical sample:https://learn.microsoft.com/en-us/dotnet/api/system.componentmodel.inotifypropertychanged?view=net-6.0#definition
I have Label in view, I need that Label's view in my ViewModel. I am using Dependency Service to set focus on Controls for Accessibility service, DS requires view as a param.
This is my Label
<Label
AutomationProperties.IsInAccessibleTree="{Binding ShowNoResults}"
IsVisible="{Binding ShowNoResults}"
Text="{Binding ResultsHeader}"/>
I tried Command but Label doesn't support command. Below code also not working
var view = GetView() as HomeworkView;
I am getting view always null. How can I fix this?
I am not quite sure what are you trying to achieve, but you can't access the View elements from you view model.
If you want to do something with the control, you can use the messaging center to do it, here is an example
in your ViewModel
MessagingCenter.Send(this, "your message here");
then in your page, you need to subscribe to this message from that view model and do the desired action
MessagingCenter.Instance.Unsubscribe<ViewModelClassNamedel>(this, "your message here");
MessagingCenter.Instance.Subscribe<ViewModelClassName>(this, "your message here", (data) =>
{
this.youControlName.Focus();
});
More detail added to Mohammad's answer.
Message Center doc.
In your ViewModel (with class name "YourViewModel"):
// Here we use control name directly.
// OR could make an "enum" with a value for each control.
string controlName = ...;
MessagingCenter.Send<YourViewModel>(this, "focus", controlName);
then in your page, subscribe to this message and do the desired action
.xaml.cs:
protected override void OnAppearing() {
{
base.OnAppearing();
// Unsubscribe before Subscribe ensures you don't Subscribe twice, if the page is shown again.
MessagingCenter.Instance.Unsubscribe<YourViewModel>(this, "focus");
MessagingCenter.Instance.Subscribe<YourViewModel>(this, "focus", (controlName) =>
{
View v = null;
switch (controlName) {
case "name1":
v = this.name1;
break;
case "name2":
v = this.name2;
break;
}
if (v != null) {
//v.Focus();
// Tell VM to use v as view.
((YourViewModel)BindingContext).SetFocus(v);
}
});
}
protected override void OnDisappearing() {
MessagingCenter.Instance.Unsubscribe<YourViewModel>(this, "focus");
base.OnDisappearing();
}
If need to pass View v back to VM, because that has the logic to use it:
public class YourViewModel
{
public void SetFocus(View view)
{
... your code that needs label's view ...
}
}
Not tested. Might need some slight changes. Might need
...(this, "focus", (sender, controlName) =>
instead of
...(this, "focus", (controlName) =>
UPDATE
Simple approach, if there is only ONE View that is needed in VM.
public class YourViewModel
{
public View ViewToFocus { get; set; }
// The method that needs ViewToFocus.
void SomeMethod()
{
...
if (ViewToFocus != null)
... do something with it ...
}
}
public class YourView
{
public YourView()
{
InitializeComponent();
...
// After BindingContext is set.
((YourViewModel)BindingContext).ViewToFocus = this.yourLabelThatShouldBeFocused;
}
}
ALTERNATIVE: It might be cleaner/more robust to set ViewToFocus in page's OnAppearing, and clear it in OnDisappearing. This ensures it is never used while the page is not visible (or in some delayed action after the page has gone away). I would probably do it this way.
protected override void OnAppearing()
{
base.OnAppearing();
((YourViewModel)BindingContext).ViewToFocus = this.yourLabelThatShouldBeFocused;
}
protected override void OnDisappearing()
{
((YourViewModel)BindingContext).ViewToFocus = null;
base.OnDisappearing();
}
I am trying to create my first Xamarin.Forms custom user control named LocationSelector. It has an Entry and when the user enters something, a list with selectable locations is shown (similary to the selection in Google Maps).
The selected location is the important 'return value' of the control.
My plan is to catch the ItemSelected event of the list and set the SelectedLocation property. LocationSelector is designed as MVVM and since everything is working so far here just the Code-Behind (which I think is enough to describe the problem):
public partial class LocationSelector : StackLayout
{
public static readonly BindableProperty SelectedLocationProperty =
BindableProperty.Create<LocationSelector, LocationModel>(s => s.SelectedLocation, new LocationModel(), BindingMode.TwoWay);
public LocationSelector()
{
InitializeComponent();
var model = new LocationSelectorModel();
BindingContext = model;
_listView.ItemSelected += (sender, args) =>
{
SelectedLocation = model.SelectedLocation;
};
}
public LocationModel SelectedLocation
{
get { return (LocationModel)GetValue(SelectedLocationProperty); }
set { SetValue(SelectedLocationProperty, value); }
}
}
Now I want to use this control on a search view where the BindingContext is set to the SearchViewModel:
<ContentPage x:Class="Application.App.Views.SearchView" ...>
<c:LocationSelector SelectedLocation="{Binding Location}"/>
</ContentPage>
public class SearchViewModel : ViewModel
{
private LocationModel _location;
public LocationModel Location
{
get { return _location; }
set { SetProperty(ref _location, value); }
}
}
Unfortunately this is not working. The output throws a binding warning:
Binding: 'Location' property not found on 'Application.App.CustomControls.LocationSelectorModel', target property: 'Application.App.CustomControls.LocationSelector.SelectedLocation'
Why points the binding to a property in the ViewModel that is used 'within' my custom control and not to the property in the BindingContext of the view?
The problem is setting the BindingContext to the view model of the user control:
public LocationSelector()
{
var model = new LocationSelectorModel();
BindingContext = model; // this causes the problem
// ...
}
In this post I found the solution. Setting the BindingContext to each child elements rather than the whole user control is doing the job:
public LocationSelector()
{
var model = new LocationSelectorModel();
foreach (var child in Children)
{
child.BindingContext = model;
}
}
I have an ASP.NET MVC 3 application that uses custom attributes to create select controls for model properties that can be populated from external data sources at runtime. The issue is that my EditorTemplate output appear to be cached at the application level, so my drop down lists are not updated when their data source changes until the Application Pool is recycled.
I also have output the contents of the MVC 3 ActionCache that is bound to the ViewContext.HttpContext object as shown in the MVC 3 source code in System.Web.Mvc.Html.TemplateHelpers.cs:95.
Action Cache GUID: adf284af-01f1-46c8-ba15-ca2387aaa8c4:
Action Cache Collection Type: System.Collections.Generic.Dictionary``2[System.String,System.Web.Mvc.Html.TemplateHelpers+ActionCacheItem]
Action Cache Dictionary Keys: EditorTemplates/Select
So it appears that the Select editor template is definitely being cached, which would result in the TemplateHelper.ExecuteTemplate method to always return the cached value instead of calling ViewEngineResult.View.Render a second time.
Is there any way to clear the MVC ActionCache or otherwise force the Razor view engine to always re-render certain templates?
For reference, Here are the relevant framework components:
public interface ISelectProvider
{
IEnumerable<SelectListItem> GetSelectList();
}
public class SelectAttribute : Attribute, IMetadataAware
{
private readonly ISelectProvider _provider;
public SelectAttribute(Type type)
{
_provider = DependencyResolver.Current.GetService(type) as ISelectProvider;
}
public void OnMetadataCreated(ModelMetadata modelMetadata)
{
modelMetadata.TemplateHint = "Select";
modelMetadata.AdditionalValues.Add("SelectListItems", SelectList);
}
public IEnumerable<SelectListItem> SelectList
{
get
{
return _provider.GetSelectList();
}
}
}
Next, there is a custom editor template in ~\Views\Shared\EditorTemplates\Select.cshtml.
#model object
#{
var selectList = (IEnumerable<SelectListItem>)ViewData.ModelMetadata.AdditionalValues["SelectListItems"];
foreach (var item in selectList)
{
item.Selected = (item != null && Model != null && item.Value.ToString() == Model.ToString());
}
}
#Html.DropDownListFor(s => s, selectList)
Finally, I have a view model, select provider class and a simple view.
/** Providers/MySelectProvider.cs **/
public class MySelectProvider : ISelectProvider
{
public IEnumerable<SelectListItem> GetSelectList()
{
foreach (var item in System.IO.File.ReadAllLines(#"C:\Test.txt"))
{
yield return new SelectListItem() { Text = item, Value = item };
}
}
}
/** Models/ViewModel.cs **/
public class ViewModel
{
[Select(typeof(MySelectProvider))]
public string MyProperty { get; set; }
}
/** Views/Controller/MyView.cshtml **/
#model ViewModel
#using (Html.BeginForm())
{
#Html.EditorForModel()
<input type="submit" value="Submit" />
}
** EDIT **
Based on suggestions in the comment, I started to look more closely at the ObjectContext lifecycle. While there were some minor issues, the issue appears to be isolated to an odd behavior involving a callback within a LINQ expression in the SelectProvider implementation.
Here is the relevant code.
public abstract class SelectProvider<R, T> : ISelectProvider
where R : class, IQueryableRepository<T>
{
protected readonly R repository;
public SelectProvider(R repository)
{
this.repository = repository;
}
public virtual IEnumerable<SelectListItem> GetSelectList(Func<T, SelectListItem> func, Func<T, bool> predicate)
{
var ret = new List<SelectListItem>();
foreach (T entity in repository.Table.Where(predicate).ToList())
{
ret.Add(func(entity));
}
return ret;
}
public abstract IEnumerable<SelectListItem> GetSelectList();
}
public class PrinterSelectProvider : SelectProvider<IMyRepository, MyEntityItem>
{
public PrinterSelectProvider()
: base(DependencyResolver.Current.GetService<IMyRepository>())
{
}
public override IEnumerable<SelectListItem> GetSelectList()
{
// Create a sorted list of items (this returns stale data)
var allItems = GetSelectList(
x => new SelectListItem()
{
Text = x.DisplayName,
Value = x.Id.ToString()
},
x => x.Enabled
).OrderBy(x => x.Text);
// Do the same query, but without the callback
var otherItems = repository.Table.Where(x => x.Enabled).ToList().Select(x => new SelectListItem()
{
Text = x.DisplayName,
Value = x.Id.ToString()
}).OrderBy(x => x.Text);
System.Diagnostics.Trace.WriteLine(string.Format("Query 1: {0} items", allItems.Count()));
System.Diagnostics.Trace.WriteLine(string.Format("Query 2: {0} items", otherItems.Count()));
return allItems;
}
}
And, the captured output from the System.Diagnostics.Trace is
Query 1: 2 items
Query 2: 3 items
I'm not sure what could be going wrong here. I considered that the Select may need an Expressions, but I just double-checked and the LINQ Select method only takes Func objects.
Any additional suggetions?
Problem Solved!
I finally had a chance to re-visit this issue. The root cause had nothing to do with LINQ, the ActionCache, or the ObjectContext, rather it was related to when attribute constructors are called.
As shown, my custom SelectAttribute class calls DependencyResolver.Current.GetService in its constructor to create an instance of the ISelectProvider class. However, the ASP.NET MVC framework scans the assemblies for custom metadata attributes once and keeps a reference to them in the application scope. As explained in the linked question, accessing a Attribute triggers its constructor.
So, the constructor was run only once, rather than on each request, as I had assumed. This meant that there was actually only one, cached instance of the PrinterSelectProvider class instantiated that was shared across all requests.
I solved the problem by changing the SelectAttribute class like this:
public class SelectAttribute : Attribute, IMetadataAware
{
private readonly Type type;
public SelectAttribute(Type type)
{
this.type = type;
}
public void OnMetadataCreated(ModelMetadata metadata)
{
// Instantiate the select provider on-demand
ISelectProvider provider = DependencyResolver.Current.GetService(type) as ISelectProvider;
modelMetadata.TemplateHint = "Select";
modelMetadata.AdditionalValues.Add("SelectListItems", provider.GetSelectList());
}
}
Tricky problem indeed!
(couldn't think of a better title, sorry)
So I've got my layout page, on this page there is a searchbar + options. Choosing whatever, should take you through to the search page, with the results etc. Fairly standard. What I've done to get this working is to create a MasterModel class, with a SearchDataModel class member on it. This SearchDataModel contains the various parameters for the search (search term, what fields to search on etc).
I've then strongly typed my layout page to the MasterModel class, and using a Html.BeginForm... I've constructed the search form for it. However all the checkboxes relating to the fields aren't checked by default, even though the default value for all the fields is true (via a private getter/setter setup).
Yet when I submit the form to the SearchController, all the checkboxes are set to true. So I'm a bit confused as to why it knows they should be true, yet not set the checkboxes to be checked?
Putting breakpoints in key places seems to show that the model isn't insantiated on the get requests, only the post to the Search controller?
I may be going about this all wrong, so if so, pointers as to the right way always appreciated.
public class MasterModel {
public SearchDataModel SearchModel { get; set; }
}
public class SearchDataModel{
private bool _OnTags = true;
private bool _OnManufacturers = true;
private bool _OnCountries = true;
[Display(Name= "Tags")]
public bool OnTags {
get { return _OnTags; }
set { _OnTags = value; }
}
[Display(Name= "Manufacturers")]
public bool OnManufacturers {
get { return _OnManufacturers; }
set { _OnManufacturers = value; }
}
[Display(Name= "Countries")]
public bool OnCountries {
get { return _OnCountries; }
set { _OnCountries = value; }
}
[Required]
[Display(Name="Search Term:")]
public string SearchTerm { get; set; }
}
Then in the _layout page:
#Html.CheckBoxFor(m => m.SearchModel.OnTags, new { #class="ddlCheckbox", #id="inpCheckboxTag" })
#Html.LabelFor(m =>m.SearchModel.OnTags)
Make sure you return a MasterModel with initialized SearchModel from your views:
public ActionResult Index()
{
var model = new MasterModel
{
SearchModel = new SearchDataModel()
};
return View(model);
}
Another possibility to implement this functionality than strongly typing your master layout to a view model is yo use Html.Action as shown by Phil Haack in his blog post.