I like ReactiveUI's code-based binding mechanisms. However, there are times when you need to use XAML bindings. In these cases, the DataContext needs to be set up properly between the View and ViewModel. I’ve been doing this in the View constructor:
public MyView()
{
InitializeComponent();
this.WhenActivated(disposables =>
{
this.DataContext = this.ViewModel;
...
});
}
This works, but I get errors in the output window at runtime:
System.Windows.Data Error: 40 : BindingExpression path error: ...
I'm using ReactiveUserControls, ViewModelViewHosts, and registering the View/ViewModel mappings in the Locator and letting ReactiveUI resolve them. I think I'm setting the DataContext as early as I can. So when I need to use XAML bindings - is there an alternative way of setting up the DataContext to avoid the phantom debug output errors?
The issue with what you have is you'll never adaptively get new versions of the ViewModel and you might get delayed subscriptions.
You are better off considering using the WhenAnyValue() operator eg:
this.WhenAnyValue(x => x.ViewModel).Bindto(this, x => x.DataContext);
Consider still playing it inside your WhenActivated() since that will avoid memory leaks, otherwise keep a reference to the IDisposable and disposing when your View has been closed.
Related
I have upgraded to the latest version of MvvmCross (6.4.1) from 4.2.3. I and using Xamarin Android not Xamarin forms
In the view which initiates the dialog I do the following
Create dialog fragment derived from MvxDialogFragment
Assign a view model to it
Then call ShowView on the fragment
However when I rotate the device it fails in OnCreate with the message
Your fragment is not generic and it does not have MvxFragmentPresentationAttribute attribute set!
This did not happen in 4.2.3. The reason I create dialog this way is that I want it to use different view models depending on where I need this dialog. For example I want to show a different list of data, but in the same format in the dialog.
It seems this will only work if we apply the MvxFragmentPresentationAttribute which needs the type of view model to be defined at design time rather than run time.
Is there anything I can do to achieve this
Any help will be appreciated
If you somehow need to specify the ViewModel type at runtime, you can instead of decorating the class with the MvxFragmentPresentationAttribute let it implement, IMvxOverridePresentationAttribute and return it there with the appropriate ViewModel to be presented in.
Something like:
public class MyDialog : MvxDialogFragment, IMvxOverridePresentationAttribute
{
public MvxBasePresentationAttribute PresentationAttribute(MvxViewModelRequest request)
{
return new MvxFragmentPresentationAttribute
{
ActivityHostViewModelType = myDynamicType
};
}
}
Where you implement some kind of logic to get the myDynamicType somewhere.
However, you should be able to use MvxDialogFragmentPresentationAttribute instead though and the presenter will attempt to use the topmost Android Activity to present it in if you provide a null ref as the ActivityHostViewModelType.
We have a couple of Fragments that we use as common controls:
MyCommonHeaderA
MyCommonHeaderB
In our common View class we call base.OnCreate(bundle) and once that has returned we fish out the fragment instances and set their ViewModels
var commonHeaderAFragment = (MyCommonHeaderA)this.SupportFragmentManager.FindFragmentById(Resource.Id.header1banner);
if (commonHeaderAFragment != null)
{
commonHeaderAFragment.ViewModel = this.ViewModel;
}
var commonHeaderBFragment = (MyCommonHeaderB)this.SupportFragmentManager.FindFragmentById(Resource.Id.header2banner);
if (commonHeaderBFragment != null)
{
commonHeaderBFragment.ViewModel = this.ViewModel;
}
Until recently this has been working with no problem. Recently we have upgraded Xamarin and MVVMCross.
Now whenever we rotate the device OnCreate is called and the execution path ends up in MvxFragmentExtensions.OnCreate where it tries to lookup a type for the Fragment using FindAssociatedViewModelTypeOrNull. There is no associated ViewModel type for the Fragment. We never needed to, should we have associated a type?
I did try MvxViewForAttribute and concrete typed ViewModel property but neither of those worked as they wanted to create new VM instances.
I have a solution which is that in the base OnCreate, if we have a bundle try and find the Fragments and set their ViewModel property before base.onCreate and when there is no bundle we set the ViewModel property after OnCreate. It is clunky but works. I just wanted to check if we should have been setting up our Fragments differently so that we would not have hit this issue
There is an example available that uses the MvxCachingFragmentActivity: https://github.com/MvvmCross/MvvmCross-AndroidSupport/tree/master/Samples
In there you don't need to worry about those kind of problems anymore.
I have a WPF application that is utilizing the reporting tools included with Visual Studio 2010. I've had some other problems that I've solved by creating a graph of objects that are all marked as serializable, etc., as mentioned on various other web pages.
The ReportViewer control is contained in a WindowsFormsHost. I'm handling the SubreportProcessing event of the ReportViewer.LocalReport object to provide the data for the sub report.
The object graph that I'm reporting on is generated in my viewmodel, and that viewmodel holds a reference to it. The SubreportProcessing handler is in my code behind of my window (may not be the best place - but I simply want to get the ReportViewer working at this point).
Here's the problem: In my event handler, I'm attempting to get a reference to my viewmodel using the following code:
var vm = DataContext as FailedAssemblyReportViewModel;
When the handler is called, this line throws an InvalidOperationException with the message The calling thread cannot access this object because a different thread owns it.
I didn't realize the handler might be called on a different thread. How can I resolve this?
I attempted some searches, but all I've come up with is in regards to updating the UI from another thread using the Dispatcher, but that won't work in this case...
I solved this problem using something I believe is a hack, by adding the following function:
public object GetDataContext() {
return DataContext;
}
And then replacing the line of code from my question with:
object dc = Dispatcher.Invoke(new Func<object>(GetDataContext), null);
var vm = dc as FailedAssemblyReportViewModel;
However, this seems like a hack, and I might be circumventing some sort of safety check the CLR is doing. Please let me know if this is an incorrect way to accomplish this.
That's a nasty problem you have there.
Why don't you use in the view a content presenter which you bind to a windows form host?
And in the view model you would have a property of type of type WindowsFormsHost. Also,in the view model's constructor you could set the windows form's host Child property with the report viewer.
After that is smooth sailing, you could use your report viewer anywhere in your code. Something like this:
View:
<ContentPresenter Content="{Binding Path=FormHost}"/>
ViewModel:
private ReportViewer report = new ReportViewer();
private WindowsFormsHost host = new WindowsFormsHost();
public WindowsFormsHost FormHost
{
get {return this.host;}
set
{
if(this.host!=value)
{
this.host = value;
OnPropertyChanged("FormHost");
}
}
}
public ViewModel() //constructor
{
this.host.Child = this.report;
}
After that happy coding. Hope it helps.
I have a ViewModel which contains a Boolean property which tells you if the user has authenticated or not.
I have a WrapPanel which is bound to a collection of profiles. The DataTemplate for these profiles has an icon - a closed padlock for when the user is not authenticated and an open one for when the user is authenticated. Ideally these would be bound to the Boolean on the ViewModel but the DataContext for the templates is the individual profile objects.
I have tried,
Setting the Source selector in the binding as specified here although it appear Windows Phone 7 does not support x:Reference
I tried also the Inversion of Control(?) method detailed here (but containerLocator was not found on my object)
I tried applying a Style.Trigger but these are not supported in Windows Phone 7
I also tried accessing the XAML elements in the code behind and updating programmatically on event triggers, however I could not get a handle on the Image element inside the DataTemplate
Edit after comment: WP7 does not support style triggers. But if anyone is looking for this answer on following versions I let the reply below:
I would use a Style Trigger as seen here to update the icon Source property on the fly - as part of the style of your DataTemplate so you would get a hold of your Image.
One way I found that works based on an answer by Damian Antonowicz but does not implement the full inversion of control method that he uses, is as follows,
Create a partial class which resolves to your view-model instance under your view-model namespace, e.g.
public partial class ViewModelInstanceLocator
{
public AppViewModel AppViewModel // Or whatever the type of your view-model is ...
{
get
{
return App.VM; // Or wherever your view model instance is ...
}
}
}
Define the other half of the class in your XAML page as a resource so that it can be referred to as a static resource, I did this in my App.xaml so that it could be referred to everywhere,
<ResourceDictionary>
<viewmodel:ViewModelInstanceLocator x:Key="ViewModelInstanceLocator" />
...
</ResourceDictionary>
You may need to include the relevant namespace if there is not already a reference to your view-model namespace e.g. at the top,
xmlns:viewmodel="clr-namespace:MyAppNamespace.ViewModel"
Finally to bind to the view-model as follows,
{Binding AppViewModel.SomeProperty, Source={StaticResource ViewModelInstanceLocator}}
The binding updates as usual just as if the view-model instance had been referred to through the DataContext. However, it does not work with design-time data.
I'm starting to use the MVVMLight framework and have a question about binding to properties in the ViewModel. I found that I have to call the RaisePropertyChanged method in the setter for the property in order for the View to be updated. And I have to call RaisePropertyChanged from through the dispatcher otherwise I get a thread access error.
public string Lat { get { return _lat; } set
{
_lat = value;
Deployment.Current.Dispatcher.BeginInvoke(() => RaisePropertyChanged("Lat"));
} }
This works but its a lot of code to get auto binding properties. Is there a helper to handle this more cleanly?
Raising PropertyChanged events is mandatory when you want to bind UI elements to properties on your model classes, independently of whether you're using MVVM Light or not. In fact it's easier with MVVM Light as it provides the RaisePropertyChanged method, which you would otherwise have to code yourself. :)
Using Dispatcher.BeginInvoke() is only needed if the set accessor of your property can be invoked from a thread different from the UI thread. Otherwise it's OK to call RaisePropertyChanged directly.