In my MVVM Light application I do a search in a customer list. The search narrows the customer list which are displayed in a master/detail view with a datagrid (the master CustomerSearchResultView) and a separately defined usercontrol with FirstName, Lastname, Address etc, etc (the detail - CustomerSearchDetailView). Here are the main content of the master/detail view:
<StackPanel MinWidth="150" >
<TextBlock Text="Customer Search Result List" />
<Grid>
<DataGrid Name="CustomerList" ItemsSource="{Binding SearchResult}" SelectedItem="{Binding SelectedRow, Mode=TwoWay}" >
.....
</DataGrid>
</Grid>
<Grid Grid.Column="2">
<TextBlock Text="Customer Details" Style="{StaticResource Heading2}" Margin="30,-23,0,0"/>
<content:CustomerSearchDetail DataContext="{Binding SelectedRow}" />
</Grid>
</Grid>
</StackPanel>
Both have their corresponding ViewModels. Please remark the DC for the CustomerSearchDetail, SelectedRow - it is a property on the CustomerSearchResultViewModel and is defined like this:
private Customer _selectedRow;
...
public Customer SelectedRow
{
get { return _selectedRow; }
set
{
_selectedRow = value;
RaisePropertyChanged("SelectedRow");
}
}
...
Because of this I have not defined any DC on the CustomerSearchDetailView - it is set in the Binding on the "Master" view (as shown above) and it seems to work ok.
In my Model folder I have created the Customer class that is in use here. It implements ObservableObject and IDataErrorInfo and have public properties that raisepropertychanged events.
I run the application and everything seems to be ok. Note: the ViewModel for the CustomerSearchDetailView (that is CustomerSearchDetailViewModel.cs) is at this stage just an empty shell and not in use (as far as I can see ... the constructor is never accessed)
Now I want to add Save/Update functionality to my customer in the detail view. Ok, I add a Save button to the CustomerSearchDetailView like this:
<Button Content="Save" Command="{Binding Path = SaveCommand}" Width="80" Margin="0,0,15,0"/>
I create my "SaveCommand" RelayCommand property in my CustomerSearchDetailViewModel - but it is never accessed.
Hmmmmm ... well after some googling back and forth I come up with this:
<Button Content="Save" Command="{Binding Source={StaticResource MyCustDetails}, Path = SaveCommand}" Width="80" Margin="0,0,15,0"/>
I defined the "MyCustDetails" as a resource in this view pointing to the CustomerSearchDetailViewModel. And voila! I now hit the method when debugging ... but alas, my customer was of course "null". (In fact I spent 2 hours implementing the CommandParameter here and binding it to the "SelectedRow" Property on the master view - but the customer was still "null").
More googling and searching for mvvm examples, and I implemented my "SaveCommand" on the Customer class (the model object). And guess what? The edited customer got passed along - I could send it to my EF layer and everything seems to be ok ....
And - If you are still with me - here comes my questions:
1.) I would like - and thought that was the "proper MVVM way" of doing things - to have my CRUD/Repository accessing in the ViewModel. How can I do that in my scenario?
2.) Now that I have my CRUD in place via the Model class (Customer) - should i bother with question 1? In fact I have deleted the CustomerSearchDetailViewModel and everything runs ok. I feel I have invented the View - Model (MV) framework ... :-P
I would very much like feedback on this - and I apologize for this "wall of text".
Assuming DC means DataContext
Just my opinion:
First question is are you doing anything special with SelectedRow in CustomerSearchResultViewModel?
If the answer is no, just get rid of that property and have your CustomSearchDetailView bind directly to the DataGrid using {Binding ElementName=CustomerList, Path=SelectedItem}
Now your Save / update Commands need to be used by Button's in CustomerSearchDetailView. So instantly I'd be inclined to using a separate VM for that View and have these Command's defined there.
Now you mentioned these Commands were not accessed. Well the answer for that is because in your program you're never actually creating the CustomerSearchDetailViewModel.
Normal operation is your View's DataContext is it's VM(If it requires one. In your case you do imo cos you need it to hold your Commands)
looking at your code I'd guess your using MVVM Light. So in ViewModelLocator you have your Main property and in your Main View, you got the DataContext set using that Main property and Source={StaticResource Locator} where Locator is the ViewModelLocator created in App.xaml Resources. This thereby creates that ViewModel for that view defining that DataContext. You can ofcourse do the same in code-behind but let's not go off topic.
So in your case you got the DataContext set as SelectedRow which is of type Customer and Binding's are resolved using DataContext and that's why when your command's are defined in Customer it works fine but when it's in the VM it did not.
So why did it work when you had the commands in your VM and used
<Button Content="Save" Command="{Binding Source={StaticResource MyCustDetails}, Path = SaveCommand}" Width="80" Margin="0,0,15,0"/>
^^ That worked because the DataContext was not used since Source has been specified explicitly. and where-ever MyCustDetails was defined in resources, there the VM got created.
So it worked what's wrong with that?
Well it's quite a big mess. Also just like you mentioned Customer details in that VM was null. Well I hope you can guess why that was by now. It's because your VM was created in resources via x:Key="MyCustDetails" but nothing in it was ever used or set apart from when the Binding's referred to it explicitly
In this system we got commands that refer either to the Model which is plain wrong or the VM which is created as a resource just for this purpose. The DataContext is heavily linked to the "SearchResults" view making it not so easy for future extensions or layout updates.
If we keep the View <-> VM a 1 <-> 1 relattion we can avoid all this confusion. So in summary we can answer both your question's together. While this works, please don't let your code be like this and tweak it to better help expansion for future and comply with some basic guidelines.
So how do we do that?
Approach 1:
In your CustomerSearchDetail View, add a DependencyProperty of type Customer lets call this say SelectedCustomer.
Now replace DataContext="{Binding SelectedRow}" with SelectedCustomer="{Binding SelectedRow}" in CustomerSearchResultView
Now set the DataContext of your CustomerSerachDetailView as it's VM similar to how CustomerSerachResultsView links to it's VM(guessing through DataContext Binding in xaml using the ViewModelLocator)
Now you can have your commands in Button's of CustomerSerachDetailView just as <Button Command="{Binding SaveCommand}" ...
Finally because SelectedRow is no longer the DataContext of the CustomerSerachDetailsView, your Bindings for FirstName, Lastname, Address will all appear to stop working.
We got plenty of options to address this.
First is to in each Binding use a RelativeSource FindAncestor binding pointing to CustomerSerachDetailsView and there via the CurrentCustomer DP(DependencyProperty) we created before get the appropriate field.
eg:
<TextBlock Text={Binding RelativeSource={RelativeSource FindAncestor, AncestorType={x:Type local:CustomerDetailsView}}, Path=CurrentCustomer.FirstName}" />
now if you have multiple properties this is gonna soon start getting annoying to type. So then pick a common ancestor(say 3 of these TextBlocks are grouped under a StackPanel) and apply it's DataContext as the CurrentCustomer element via a similar binding to ^^. Now the StackPanel's children DataContext will be the Customer element so in each of their binding's you don't have to do the whole RelativeSource thing and can just mention {Binding Path=FirstName} and so on.
That's it. Now you got two view's with their own respective VM and a Model(Customer) and each have their respective tasks.
Great, we done? err not quite yet.
While Approach 1 is better than what we started with it's still just "meh". We could do better.
Approach 2
MVVMLight has a Messenger class that will allow you to communicate between different classes in a weak dependent format. You need to look into this if you haven't already.
So what do we do with Messenger?
pretty simple:
In the setter of SelectedRow in CustomerSearchResultsViewModel we'll send a message with the new incoming value to CustomerSearchDetailsViewModel.
Now in CustomerSearchResultsViewModel we'll add a property CurrentCustomer and assign it this incoming value.
In the CustomerSerachDetailsView we no longer create a DP. Which means we no longer set SelectedRow to anything(DataContext or DP) in the CustomerSerachDetailsView from CustomerSearchResultsView ( sweet less work :) )
As for the way we assign DataContext of CustomerSerachDetailsView or way we bind the Button.Command - They remain same as Approach 1
Finally the actual "FirstName" and so Binding's. Well now CurrentCustomer is a property of the CustomerSearchDetailsViewModel. So binding to it just like how the Button bind's to it's commands
^^ this works fine now cos DataContext for the TextBlock is the VM and the property CurrentCustomer exists in it.
Related
I want to call controls inside view like button and item template inside viewmodel. Please tell how can I do that. My view contains following
<ItemsControl Name="cDetails"
Width="395"
ItemTemplate="{DynamicResource Test}"
ItemsSource="{Binding ViewModels}"
Visibility="{Binding IsLoaded,
Converter={StaticResource visibilityConverter}}">
<Button Name="btnComplete"
Grid.Column="1"
HorizontalAlignment="Center"
Command="{Binding AuditCommand}"
CommandParameter="1">
Complete
</Button>
Please tell how can I call these items in my viewmodel using vb.net.
Thanks
Accessing your view components from inside your viewmodel is not the way to do things in MVVM. Because it is specifically not designed to work this way, you will have to go out of your way to make it work. You should probably investigate how to accomplish your goals using MVVM properly, or forego using MVVM at all and do the work in your code-behind.
Since you have not described what your goal is, it is hard to provide specific recommendations. In general when using MVVM, you manipulate things in your viewmodel and set properties. Your view binds to these properties so that it updates appropriately as they are being set. Your viewmodel does not directly manipulate the views themselves, only the viewmodel properties that they are bound to.
For example, let's say you are updating the text on a TextBlock. You could do something like this in xaml:
<TextBlock Text="{Binding SomeText}" />
Then, your viewmodel (which should implement the INotifyPropertyChanged interface) defines this property and sets it as desired.
public string SomeText
{
get { return _someText; }
set
{
if (_someText != value)
{
_someText = value;
NotifyPropertyChanged("SomeText");
}
}
}
private string _someText;
...
// At any time, you can set the property, and the
// binding will update the text in the control for you.
SomeText = "Some text";
If you absolutely need to manipulate your views from code (or if you are not using MVVM), the appropriate place for that sort of code is the "xaml.cs" file next to your view (the code-behind). You can assign a name to anything in your xaml using syntax like <TextBlock x:Name="SomeTextBlock" /> and then access it from the code-behind as a member variable with the same name. For example, you could do SomeTextBlock.Text = "Some text". However, this is not usually necessary for the vast majority of use cases if you are using MVVM.
You shouldn't try to access controls directly from the ViewModel. The ViewModel must not know about the View implementation.
Instead, in WPF we connect View and ViewModel through Bindings. Bindings connect Properties of controls in the View, with Properties in the ViewModel.
Commands are a special type of Property that can be bound as actions for controls like Button.
In your example, you would need to have these properties in your ViewModel:
A collection named ViewModels
A boolean named IsLoaded
And an ICommand named AuditCommand
By managing those properties, you should be able to control what's shown in your View and its behavior.
If you need more control, create more Bindings to other properties, or create some events in your ViewModel and manage them from your View's code-behind.
I have a WPF application with the following XAML in my MainWindow.xaml.
I don't understand why the DxTaskList constructor is called when I make a call to OnPropertyChanged("Sequences");. As you see below, my tab control is bound to a Sequences list. In the related view model class, I have a Sequences property that I modify, so naturally I need to let the view know, so I make the call to the OnPropertyChanged("Sequences") but I'm trying to understand how WPF works.
Does the entire visual tree get rebuilt when you refresh the binding of a parent? How does that work? Please note that my app uses Prism, so I'm not sure if this makes a difference.
<dxdo:LayoutPanel Caption="TaskList">
<dx:DXTabControl x:Name="TabControl"
ItemsSource="{Binding Sequences}"
SelectionChanged="TabControl_OnSelectionChanged">
<dx:DXTabControl.View>
<dx:TabControlMultiLineView HeaderLocation="Bottom"/>
</dx:DXTabControl.View>
<dx:DXTabControl.ItemTemplate>
<DataTemplate>
<Grid>
<TextBlock Text="{Binding Name}"></TextBlock>
<views:DxTaskList x:Name="Tasklst"/>
</Grid>
</DataTemplate>
</dx:DXTabControl.ItemTemplate>
</dx:DXTabControl>
</dxdo:LayoutPanel>
Everything appears to be working as it should.
If your Sequences property is an ObservableCollection it will fire NotifyPropertyChanged on any change to the size of the collection. If Sequences is not an ObservableCollection, but is some other IEnumerable then your OnPropertyChanged("Sequences") will do it.
Back to your question:
Let's assume it is not an ObservableCollection. You insert an item and then fire OnPropertyChanged("Sequences"), a new DxTaskList is created (ctor is called) and gets inserted into the visual tree.
Try calling OnPropertyChanged("Sequences") twice in a row, I bet you are only hitting the constructor after the first, and not on the second. If not, it could be because of how the DxTabControl is internally implemented.
It's a good thing I don't mind feeling stupid.
I'm trying to bind to an ObservableCollection on my view model. The data hierarchy looks like: Parent -contains list of- Child objects. Nothing complicated.
At the outermost grid of my Xaml tree I establish a link to the view model with:
<Grid DataContext="{StaticResource src}">
Yes, src does reference the view model and the two dozen bindings before the problem textbox work fine. There is not another DataContext in my Xaml tree. Now I come to a simple textbox. I want to bind Textbox text to a child.property.
This works:
<TextBlock
DataContext="{Binding Parent}"
Text="{Binding Path=Child.Property}"
Style="{StaticResource headerMajor}"
/>
This doesn't work:
<TextBlock
Text="{Binding Source=Parent,Path=Child.Property}"
Style="{StaticResource headerMajor}"
/>
I thought they were two ways of saying the same thing. Ordinarily I wonder for a moment and then keep on coding. However, some advice I've read mentioned that DataContext attributes buried in Xaml controls can lead to hard to find bugs.
Please explain why one works and the other does not. This will help my grasp on the whole binding topic.
Jim
Source is a property which holds an object used as source for the binding, it does not resolve to a property. Hence your binding is looking for the property path Child.Property on the string "Parent", see the problem?
first of all I would like to thank you for the many good posts that i read in this forum. Unluckily I could not find anything of help for my current problem (either here or anywhere else).
What I'm trying to do sounds quite simple, but I have no clue how to get it to work ... perhaps I'm still to new to wpf or I don't thing wpfy enough :)
I'm designing a front end for a part in an automated manufacturing:
I have a quantity of places where pallets can be put (but it can be empty as well).
Each pallet has up to 3 places where parts can be mounted
Everything is created dynamically of a database and is reacting to changes.
The position of the parts on the pallet comes from the database as well and should be visualized
What I would like to have is:
An overview over the pallet-places with a preview of the pallet
When I select a place I want to see a detail view of the place
When I click on a part on the pallet of the detailed pallet I want to see details to the part
The first two points are quite simple and work nicely:
I have got a DataTemplate for every component (part, pallet, pallet-place). Actually those are UserControls that are imported as Datatemplates
the overview is a ListBox with the places as DataContext
for the detail-place-view I use the UserControl and bound it to the SelectedItem of the Listbox
I tried to bind the Text of a Textblock to the ID of the selected Part ... and fail.
Probably I could use some global variables in the code behind - but that sound very ugly.
Can anybody help?
I have got a solution ... it is not nice but works.
I created an event in the pallet, that triggers, when the selected part-place changes
I handle the event in the pallet-place and create a new one
And finally I handle it in the overview and change the detailview accordingly
Most likely there are much nicer solutions, but it will suffice.
Perhaps try an ElementName binding?
<TextBlock Text="{Binding ElementName=Name_of_your_Listbox, Path=SelectedItem.ID" />
Can you post a bit more code of your TextBlock and your Binding?
Context is important, if i use a ContentControl and bind its content to the SelectedItem like this:
<ContentControl Content="{Binding SelectedItem, ElementName=mylistbox}">
I can bind to the ID of the selected item in the DataTemplate like this:
<ContentControl.ContentTemplate>
<DataTemplate>
<TextBlock Text="{Binding ID}" />
</DataTemplate>
</ContentControl.ContentTemplate>
That is because setting the Content of the ContentControl automatically sets the DataContext as well, and this binding is relative to the DataContext since no source (ElementName, RelativeSource, Source) has been specified.
I do not know how your UserControl handles the context, if the DataContext is not affected such bindings will not work. You would need to bind directly then:
<uc:MyDetailsView Data="{Binding SelectedItem, ElementName=mylistbox}">
<!-- ... -->
<TextBlock Text="{Binding SelectedItem.ID, ElementName=mylistbox}" />
This of course defeats the purpose of having the binding on the UserControl itself in the first place. But unless you post some relevant code it's quite hard to tell what is wrong.
Also check the Output window in VisualStudio, binding errors will show up there and might provide valuable information as to what went wrong.
I'm working on a set of controls that has a number of DependencyProperties. The properties are themselves DependencyObjects and created during the get method of the properties. During the Get method, they are also set back to the propertybag using the SetValue() method, so they are in fact valid in Xaml and their properties can be storyboarded without having to explicitly created in the the visual tree.
These DependencyObjects has all its properties as DependencyProperties as well, for supporting DataBinding. They are as mentioned above possible to use in Storyboards.
At the same time I'm developing special designtime support for Blend 3 for these properties and have created an InlineEditorTemplate in the form of a Control. I create the template and set it for the PropertyValueEditor like this:
var vectorEditControl = new FrameworkElementFactory(typeof (VectorEditorControl));
var dataTemplate = new DataTemplate {VisualTree = vectorEditControl};
InlineEditorTemplate = dataTemplate;
In the Control I have the following:
<Grid DataContext="{Binding Value}">
<StackPanel Orientation="Horizontal">
<TextBox Text="{Binding Path=X, Mode=TwoWay}"/>
<TextBox Text="{Binding Path=Y, Mode=TwoWay}"/>
<TextBox Text="{Binding Path=Z, Mode=TwoWay}"/>
</StackPanel>
</Grid>
The editor shows up and I can edit the data. And even while debugging, I see that it actually sets the data back to the DependencyProperties on the DependencyObjects, but nothing happens to the Xaml. So the data is actually not persisted in any way in the Xaml and lost when I close the Xaml file and open it again.
Is there anything I need to do specifically for it to actually get into the Xaml? I was under the impression that this would happen automatically?
Excellent Question!
The core issue you're running into a misunderstanding as to what PropertyEditors in Blend/Cider end up databinding to.
Consider this object graph:
- MyControl
-- MyControl.MyProperty
--- FooClass
---- FooClass.BarProperty
Let's look at a scenario where we have a PropertyEditor (of any type: Inline, Dialog or Extended) to property MyControl.MyProperty.
When inside MyPropertyPropertyEditor you'd expect to get a fully settable copy of FooClass and be able to manipulate it's members.
That's a good assumption, but the wrong one.
The core issue is that Blend/Cider have elaborate data structures that represent your model at design time. There's about 3-5 levels of abstraction in how Blend/Cider interact with an actual control.
Creating those levels of abstraction allows Expression Blend / Visual Studio designers to be leveraged between framewroks (Silverlight / WPF) and support advanced scenarios (like Property transactions and property chaining).
So, the value you actually get to DataBind to is just one of those levels of abstraction.
Don't believe me? In your custom PropertyEditor, register for this.DataContextChanged event and checkout the type in this.DataContext. You'll end up getting the PropertyValue class (or one of it's friends).
Every single property change you want persisted to XAML (and shown on the design surface) should go through those abstraction layers.
the question you have to ask yourself is "Where do I get one of these absteaction classes for my PropertyValue.Value property instance?".
Well, what I'd do if I were you is create a ModelItem around MyControl.MyProperty and set that as your PropertyEditor.DataContext.
We've shipped an example of using ModelFactory.CreateItem in the Silverlight Toolkit as part of the Chart DefaultInitializer: Source Code, Ning Zhang (Awesome Design Time Dev) explains about ModelItem
If you've got follow-up questions I'd consider pinging PeteBl or UnniR through the Silverlight Insiders mailing list.
Sincerely,
-- Justin
It partly solves my problem. I'm having a dialog with UnniR for a followup.
I couldn't see how I could use this together with the PropertyValueEditor, but for default values this is brilliant and something I'll implement ASAP.
Thanks.